what is a

ventilated cavity?

Righto, ventilated

cavities.

Okay, um, so,

I can't believe we've got

Dr.

Cameron Munro stumped.

No, I'm

just trying to stick it

about where to start here

because I'd forgotten what

we're even talking about.

ventilated cavities.

Ventilator cavities.

I don't know what we're

talking about now.

I'm just getting

my head around it.

Alright.

Okay.

You didn't realize

Cameron that none of this

is getting edited out.

This is the beginning

of this episode.

terrific.

Great.

, okay.

So if we start with a

heating dominated climate

like a Melbourne or a

Hobart or a Canberra, the

water vapour for most of

the year, particularly

when it's cold, wants

to dry from or move

from the interior of the

building to the exterior.

And so in order to

allow that drying to

occur, we've got to

consider the buildups of

each layer within that

wall or roof system to

have increasing vapour

permeance as we move out.

So we have that ability

for that vapour to drive

towards the exterior.

And then when it hits the

exterior in a traditional

construction, we would

often have, say, a sarking

layer, which now would be,

need to be vapour permeable

in most of these climates.

But then we'd often just

fix our cladding directly

onto that sarking.

And that's where

the problems arise,

because we've just run

completely contrary to

our intention here of

having increasing vapour

permeance as we move out.

So we're allowing the water

vapour to dry towards the

exterior, but then we're

trapping it on the back

side of the cladding.

Whether that might be

like a fibre cement sheet

of some description, or

even weatherboards, or

potentially worst of all,

some sort of metal based

cladding like a standing

seam or something, where

completely trapped it.

And so a ventilated

cavity is where we try

and offset the cladding,

from that membrane,

that sarking layer.

So we can allow the wall

system to dry towards the

exterior so that vapour

that moves out towards the

outside of the building

gets into an air cavity

and then we've got air

movement through that air

cavity that allows the

vapour to be carried away.

So that's the fundamental

of a ventilated cavity.

, so for a number of years

recently, everyone was

sort of complaining

about, oh, weatherboards,

they just don't last as

long as they used to.

Is that because we

essentially changed the

way we build our walls?

Like we've changed

that drying potential

by to the wall.

We've got the

metal sarking, it's

wedged on hard.

Where if you look at my

house at the moment, it's

old hardwood studs, no

insulation, weatherboards

waxed stride on.

Is that why they're

not lasting?

Cause they're the ones

soaking in the water.

There's got no room for

it to dry out they're

just being eaten alive.

There's probably two

factors there isn't there?

I mean it's true in a way.

The timber that we've

used has changed.

It's no longer

very old hardwoods.

It's often softwoods or

plantation grown timbers

now that grow much more

quickly and therefore

can absorb more moisture

and are less tolerant

to high moisture levels.

But also as you point out

Matt, because we've moved

to a building technique

now where we're emphasising

thermal performance

and so we're putting

insulation in the walls.

Therefore, we're reducing

that heat movement through

the wall and reducing

the drying potential of

every element within that

wall, and particularly

those on the cold

side of construction,

which is the cladding.

So yes, we are, making

it more important that we

consider, what materials

we use on our cladding and

make sure they're durable

enough to high moisture.

So what we're probably

talking about here, if

you picture a building and

you've got your membrane

externally, that first

thing, if you've got a

vertical cladding, you'd

have your vertical, Counter

batten, we call it your

vertical batten, which

is typically we're using

a 19 mil batten Hamish.

I don't know if you're

using something similar.

We've, actually, switched

to using a 35 mil,

so 35 and 35, and the

reasons for that are

just more to standardize

our window reveals,

a good idea.

yeah,

we're going overall 70

mil because then we're

trying to fix our window

reveal prices with a

company at the moment.

So design in that

kind of reveal detail.

So it's standardized

across all our projects,

but minimum 20 mil.

So if you had a vertical

cladding, you didn't have

your cross batten, which

is a way typically using

a 90, in some cases.

, so that's creating that

sort of cross layer.

If you had a horizontal

cladding, you'd only need

your vertical cladding,

your vertical batten.

All right, so water

can drain down.

Now, the important part

though is that the cavity's

there for two reasons,

Cam, and one of them is

to allow air to circulate

through and dry up, but

the second is for moisture.

now.

Okay.

They both require

different thicknesses

for what they require.

, from my understanding

with drainage of water,

you only need a few mil,

technically, but the

air you need a lot more.

Do you want to maybe

explain that and why

that's important?

Yeah.

Okay.

So, uh, probably the two

words here I'd use would be

ventilation and drainage.

So, ventilation is

about the water vapor.

So, the gaseous water

that's in the air that

we can't see and this

the drainage is about

the liquid water.

So, this is the water

primarily that comes

in beyond the cladding.

So, Joe Starbrick has

famously always says,

there's the claddings

that have leaked and

the claddings that

haven't leaked yet.

You know, you've always

got to assume that your

cladding is going to have

a failure in it at some

point, and indeed when

we do Wolfie hygrothermal

analyses, we're required

by the standards that

we test those to, to

assume that 1 percent of

the rain water that hits

that wall, makes its way

in beyond the cladding.

And, in a real building,

that's probably going to

happen at your windowsills

probably is your first,

most worrying point.

So you're going to get

water, liquid water coming

in beyond the cladding.

And so you don't want

that hard up against your

sarking or your vapour

permeable membrane on the

exterior of your building.

because that liquid water

can potentially will

find any weak point.

, this is where capillary

action of liquid

water comes in.

As we know, it can readily

find at the smallest

hole any, layer and

leak in beyond that.

And then if it accumulates

on our moisture sensitive

materials like our timber,

then we start having rot

and moisture problems.

So if you think about

this cavity, it's kind

of like the traditional

brick veneer, where they

drilled their timber frame,

they'd have the gap and

then they have the brick.

But for me, what I get

confused about, and I've

always not understood,

is why We thought about

this with brick, but no

other cladding, because

the weep holes were kind

of the ability to drain

out and you had those

vents that were allowed

to suck air through to

drain it to dry everything

out behind there, but

every other cladding

it just got ignored.

Yeah, so one of the reasons

why I think we've always

done the cavity with brick

veneer is because brick

is kind of like a sponge.

If you don't render or coat

that brick with something,

every time the rain hits

that brick it just gets

absorbed into the brick and

that can be fine if the sun

can then comes out and it

dries towards the exterior.

But often what will

happen when the sun comes

out, it actually dries

towards the interior.

And so you've got this

high grit buffering

capacity, this ability for

this brick to absorb vast

quantities of water and

then you're not quite sure

where it's going to go.

Depending on the

atmospheric conditions and

the conditions within the

house, it could go towards

the exterior or interior

or a combination of both.

And so you just need

to provide that cavity

to allow a bit of

weak ventilation.

But bear in mind that

in that, our traditional

brick veneer, you've got

very weak ventilation

within that cavity.

You've got 40 or 50 ml

of, of space, so any

liquid water that should

happen to get in there,

say through the mortar

joint somewhere or at a

window sill, can drain down

and out the weep holes.

That's fine, but you've

only got comparatively

weak ventilation because

you've only got those

weeps at the bottom to

allow air movement through.

Didn't you prove on

our passive house in

Williamstown though that

there's a high amount of

air exchange through there?

Because weren't they

arguing there was only

about eight, eight air

exchanges through those

weep holes but you

were able to prove that

was maybe around 40?

It

anything, Matt.

was enough to potentially

get certified.

let's just jump for a

second to talk about the

depths of the cavity.

You know, how deep

should that cavity be?

And let's differentiate

here between the two

functions, drainage

and ventilation.

Let's deal with the

drainage one first.

So this is about

liquid water, yeah?

So the liquid water that

whacks into our cladding

when it rains gets

beyond that cladding.

We need that to

drip down and away.

And all you really need

is in the order of two

to five millimetres for

that drainage occur.

If you've got one

millimetre, then you

imagine a drop of water.

and you put like a sheet

of glass, like maybe

put a drop of water on

your kitchen aisle and

benchtop or something like

that, and then put a bit

of glass on top of that

and just hold it like a

millimeter above, there's

this, tension within that

water droplet that can

allow it to pull itself

up and can move vertically

within that, cavity.

And that's how you can

get things like rising

damp, because the water,

droplets within that.

, masonry or brick or

whatever it is can

move against gravity,

it can move up.

And so we need to break

the ability for that

capillary tension , to

move that water up.

And all you really need

is a couple of mil.

in order to do that.

And so if you look at many

of the external cladding

systems, like some of the

foam based cladding boards

or something like rock

wool or mineral wool type

product, they've often

got these grooves in the

back face of those boards.

And when you put those

onto your wall, those

grooves, you hopefully you

put the right orientation.

So the grooves are

running up and down.

So any water that gets into

that space, can then drain

through those very minimal

grooves, you know, they're

only two or three mil deep.

, so if the drainage is

relatively easily solved,

provide two or three

mil of cavity, and sure

obviously you've got a

drainage at the bottom

for that water to get

away, problem solved.

The trickier bit is

ventilation, and for

ventilation you tend to

need a lot more than two

or three mil, but how

much do you really need?

And much will depend

obviously firstly how much

water vapour is driving

through your assembly.

So if you've got a

conventional building and

you don't have an internal

air and vapour control

membrane and you don't have

HRV then potentially you've

got quite a large moisture

load building up in the

interior of that building.

And if you live in a very

cold climate, say in Falls

Creek or Mount Buller or

Mansfield or somewhere

like that, as opposed to

a more benign climate, You

know, , New South Wales

coast or even the Victorian

coast, then the vapour

drive is much larger in

those very cold climates.

And so all else being

equal, you're going to

need a bigger cavity

to allow sufficient air

movement through to carry

away that water vapour.

What we're trying to ensure

here is that we carry

a lot more vapour away

than we can accumulate.

It's always about rates.

And so the rate at which

air moves through that

cavity is a function of

a whole heap of things.

The climate, the

exposed wind, , the

color of the cladding.

So darker color claddings

will get warmer and

therefore drive more

stack effect, more

buoyancy through that

cavity and move more air.

The orientation

of the wall.

If it's south facing,

it gets less sun and

therefore is cooler and

drives less air movement

than a north facing wall.

And of course then

things like, have you got

shadowing on that wall?

You know, if you've got

big trees and vegetation

and a garden right next

to the north facing wall,

then the effect of the

sun on heating that space

and driving that chimney

effectively reduces.

And then the other

part to this is what

are you doing at the

bottom and at the top?

Because even if you say,

for argument's sake, look

I've got a house where

I can put a 45mm batten

or even maybe put like a

70mm batten on edge and

create a very, very deep

cavity, if you then go

and constrain air movement

getting in and out of that

cavity with a bug screen

at bottom and top, then it

probably doesn't matter.

that you've gone through

a very deep cavity.

You might have be able

to get away with a 35 or

45 given how throttle,

your constraint is at

the inlet and outlet.

Three questions.

First question.

And it's going back

to something you said

before and may or may

not be quite relevant.

But I'm interested

to know the answer.

You said on a brick wall

when it's absorbing that

water or that rain, how

and why does the sun drive

it into the building?

What's happening there?

Yeah, so it's about the

vapor pressure and the

radiation from the sun.

you're heating outside

surface, So it's, taking

it to the least vapor

pressure, which in

that situation will be

towards the interior.

And it might only happen

for a short period of

time, but often we'll be

confronted with situations

where we get that storm

comes through, the wall

gets whacked, and then the

storm clears and the sun

comes out, particularly

this time of year around

December, and the sun is

very strong, you know,

it's coming into summer

and so you can potentially

get quite a lot of, that

vapour driving into the

cavity behind the cladding.

That was a big conversation

in our Williamstown

project, Cameron.

That exact issue.

a really difficult problem.

It's like a lot of

these things, you've

always got multiple

factors at play and

you're never quite sure.

On balance, which of these

is going to dominate, or

where things are going

to land, and this is

why we sort of need to

be conservative in our

thinking, I think, about

a lot of this, and say,

well, let's assume all

else being equal, the

worst case, to give us

best chance of, not having

issues down the track.

I've got another question

and it's relevant for a

project where I've got

coming up where we've

got rockwall externally

and then we have,

cladding battens over

the top of that rockwall.

Now, immediately my mind

goes to wind washing.

Is there something in the

makeup of that rockwall,

mineral wall, whatever,

that is not as impacted

as much by wind washing?

Wind washing may or

may not be the right

term, but obviously you

want to encourage, air

movement in that cavity.

Is that, , detracting

from the insulation

value of that rockwool?

washing is about the

movement of air past our

insulation and essentially

scouring the insulation

and taking some of those

the air that's otherwise

caught in the cavities

of our insulation and

degrading the performance

of the insulation and the

analogy I use is Standing

on the top of a mountain,

got a woolen jumper on.

If it's a still day in the

middle of winter, you're

perfectly warm and fine.

But if the wind picks up,

then the air is penetrating

right through that woolen

jumper, scouring away

the thermal resistance

that's providing and

therefore you feel cold.

So how do you respond?

You put a Gore Tex jacket

on top, but the Gore Tex

jacket has no thermal

resistance whatsoever.

but it's preventing air

movement and it's helping

the insulation, the

woolen jumper work best.

Now if we take that analogy

a bit further, if I use

a very loosely knitted

woolen jumper, I'm going

to be more susceptible

than if I use a very

tightly knitted jumper

or one of these more sort

of synthetic fibers that

create a very dense weave

within that insulation.

and insulation in our

buildings is the same.

So your high density

wall, glass wall

insulation is maybe 20

to 30 kilogram per cubic

meter density and that

can be quite susceptible

to wind washing.

And which is why we

would have that external

membrane on that.

When you move to something

like a semi rigid or a

rigid type insulation,

like a rock wool, a mineral

wool, they're usually

north of 60 kilograms

per cubic meter density.

So two to three times the

density of your typical

wall, glass wool bat.

And when you get to that

level of density, you're

far less susceptible

to wind washing.

So there's some good,

, experimental studies that

were done, uh, in Canada

by a consulting company

called RDH, so people

can Google that if they

want, looking for that.

, looked at the wind washing

impact, in Canada they use

a lot of this mineral wall

insulation or exterior,

externally clad like

high rise buildings, like

commercial buildings,

which are obviously far

more exposed to wind, and

in a place like Vancouver,

they you know, it's going

to be pretty heavily

exposed to wind and rain.

And they found that when

you get to that sort of 60,

80 kilogram density, the

impact of wind washing is

very small to negligible.

And so you save yourself

having to go for that,

another membrane on

the top of that, on the

exterior face of that.

Awesome.

And one last question.

We were talking before

about the impacts of

different claddings on

walls and then different

cladding types on different

elevations, you're

talking about some impact,

moisture movement or

air movement than others.

Should we have

different cavity batten

thicknesses or depths

on different elevations?

do you mean like the pitch

of the roof sort of thing?

Like it would a five degree

roof mean like

a bigger cavity?

yeah, or like you got

monument, tin cladding on

a Western elevation and you

got monuments in planning

on the Southern elevation.

Should that cavity

be different on the

West or the South?

Oh, I like this question.

I hope the answer is not

yes, we should, because

that's just a head's fuck

when we go to construction.

We want to standardize

to make it easier, but

I'm, curious to know the

reasons why it might make a

But you potentially might

be able to, you might be

able to just make it the

largest and then you're

safe in all situations.

And that's probably

where we get to in the

real world, isn't it?

Because in theory, if we

had perfect knowledge about

the boundary conditions

here, we knew everything

about the climate, we knew

everything about what the

occupants do, like how

long are their showers,

how much moisture is

being generated in that

building, exactly how

airtight it is, then you

could probably calculate

down to the millimetre

how thick that batten is.

In that, your ventilated

cavity needs to be on each

of those walls and roof.

In practice we don't,

and so we need a level of

conservative assumption,

and of course we can

only buy battens in given

increments of thickness.

so we probably come to

the question of, you know,

how deep a ventilation

batten do I need for

a particular scenario.

And as I alluded to before,

there's so many variables

at play here that there

is no generic answer.

And so I would say, as

a guide, I think we can

only work off general

first principles here.

If you're building a

building in Mildura, say,

which is a relatively

warm Victorian climate,

except in the winter it

can get quite cool there

too, and you're building

this building in Monument,

which is probably not

the best idea for a, you

know, otherwise mostly

hot climate like Mildura,

and you're building it in

a field where there's no

trees or shading around,

and you've got a HRV in

the building, and you've

got an internal air and

vapor control membrane.

Then, you know, 20

mil batten, you'd

probably go intuitively,

yeah, I feel pretty

comfortable with that.

And if you're building

in Melbourne, you'd say,

well, that's a bit cooler.

It's got a bit more rain.

Then maybe I'm looking

more like a 35 batten.

And now I'm building

on top of Melbourne.

a very cold climate or

maybe in Canberra, then

perhaps I'm pushing towards

a 45 batten, particularly

if the client also wants

his house to be surf mist

or some very light colour.

look, we can model this,

you can model this in

hygrothermal modelling,

it's something like ORPHE,

and I think that's often a

good idea if it's something

unusual or you're trying to

push a boundary somewhere.

But not as a means of

giving you an answer,

it's okay or it's not.

So coming back to

Matt's point before,

we proved in modelling

that this was okay.

We never prove

anything in modelling.

Ever.

Because we can't.

There's just too many

input assumptions to

allow us to have 100

percent confidence that

this is going to be okay.

All we can do is use the

modelling as a guide.

To say, well, with, these

good aspects of this build,

you know, it's being a

dark color and not ex

fully exposed in a mild

climate, as opposed to

these negative aspects.

There's no HRV or

air and vapor control

layer on balance.

The modeling would seem to

suggest it's more likely

to be okay than not.

We are playing a

probabilities game and

we're trying to minimize

the probability of failure.

Maximize the probability

of success, but We

never have a hundred

percent confidence this

is never going to fail,

and we never have zero

percent confidence that

this is going to fail.

What percentage

do we have though?

What would you roughly say?

impossible to say, right?

You know, you look at

any risk stuff like,

so back in my aerospace

days, you know, you're

looking at, it was

always ten to the nines,

Is this when you're at

NASA, building rockets?

I was not doing it.

Never.

But you know, you're

looking at a failure

rate of how many million

hours can this aircraft

fly before it hits a

level of turbulence

that is so great.

So beyond anything that

we've designed this

aircraft for that it's

going to structurally fail.

You know, you're looking

for the likelihood of

that happening being so

minusculely small because

of course, the consequences

of failure are Uh, in

building is not quite that

extreme, but it's still

fairly extreme because of

course the last thing we

want is to have demolish a

building or, have mold and

rot forming in a building.

So we're trying to

minimize the risks, but

also minimize the cost.

So how do we get

that balance right?

And this is where I just

would bang on about saying

modeling is a tool to

help inform a decision.

But you can't just

blindly go into the

modeling and say, look,

give me an answer here.

I don't even want

to think about this.

Just tell me the answer.

Is it yes or no?

You know, that's

not how it works.

just coming back to that

sort of depth of the

cavity, this is so many

parameters at play here.

And this is where I

think testing it within

something like Wolfi and

saying, well, with this

combination of things,

Here's a hierarchy.

Here's the things

that are more likely

to be problematic.

And so we can talk with

the builder, with the

architect, with the

client and say, well, we

realize there's a client,

you're cost constrained

and you're trying to do

the minimum required.

We think you should

spend your money doing

X, Y and Z over these

other, other things

to minimize the risks.

we're trying to identify

the main levers that are

going to reduce our risk.

So, I think one thing

that we need to get in

our heads a lot of the

time, and most people are

starting to do a really

good job of wrapping the

external wall of a house.

And then putting in some

form of a baton on whether

they're doing it right or

not and restricting that

access of air to flow

through from the bottom

is a whole and maybe

different conversation.

But a wall is also

a roof just tilted.

what I see a lot of the

time is people go spend all

this money on all these.

Awesome Rats is what,

which is where we need

to spend our money on a

build, but then they go

whack Anticon on the roof.

And I feel like it's

kind of defeating the

purpose because all

that a roof should be

is that wall tilted on

whatever angle you want.

Now, it does change once

you get under 10 degrees

quite significantly

and definitely when

you get under 5.

you maybe explain why

that is the case that

a wall is a roof?

Yeah, so the vapor drive

in a heating dominated

client from in to out

obviously applies both

on a wall and on a roof.

It's more problematic on

a roof because in a house

you've got stack effect,

the hot air rises, heat

doesn't rise but hot air

rises and as that hot air

rises there's a pressure,

an air pressure that's

trying to drive that air

carrying moisture into

your roof and if you've

got 50 down lights in

that ceiling and the

plasterboard is acting

as your air tightness

then you've got all this

moisture laden air moving

into that roof cavity.

So you've got the

potential, you've got

all this moisture there

plus in the middle of

winter, that roof sheet is

exposed to the night sky.

So when it's a clear,

still overnight in the

middle of June or July

in Canberra, Melbourne,

Hobart, wherever, then

that roof sheet will get

very much colder than the

ambient air temperature.

So even if the ambient

air temperature only

gets down to one, five

degrees, something like

that, the roof sheet

can be minus five.

It can be below freezing,

which is why you'll get

frost on that roof sheet.

Or you get

condensation on it.

And so you've got a

combination of moisture

from inside the building

getting into the

roof, plus you've got

a much colder roof.

And it's that combination

of moisture and cold that

creates the problems.

So roofs are even

more problematic.

Plus we're not getting

the stack effects to the

same level on a roof as

we do on the walls simply

because of orientation,

the wall being vertical,

the roof being pitched.

So then, we have

our counter battens

on our roof.

And then we've got

the same system.

So typically you're 19

and you're 35 Hamish was

talking about before.

And we usually have

our membrane on the

underside of that.

So any condensation can

hit it, drain down and

away from the building.

On top of that crock

counter batten, if

condensation is such an

issue on that roof sheet

in those colder months,

why then don't we put

Anticon on top of that

roof batten as well to

mean that the condensation

is happening on the other

side and we're keeping that

section of the roof warm?

And that's what, how we've

traditionally dealt with is

something like a foil face

blanket, like an anticon.

And it sort of works

like it, it captures

any moisture that

forms immediately

under the roof sheet.

It prevents air moving

on the underside of

that roof sheet, which

allows condensation.

And so in a lot of

sort of our high

performance passive house

buildups, we're getting

rid of the anticon.

And we're just having a

free, freely ventilated

cavity, counter batten,

batten, roof sheet.

And we've got air coming

in from underneath.

So we've got a much

higher volume of air

that is passing by that

cold roof sheet and

therefore a higher level

of condensation risk.

And so intuitively we

go, well that's actually

counterproductive,

isn't it?

Well, not necessarily,

because one of the concerns

we have with some of

these fall face blankets

is their durability.

what happens over, six

years, ten years, twenty

or thirty years plus.

A lot of us have probably

pulled off roof sheets

and seen that anticon, or

that foil faced blanket,

essentially, fall into bits

because roofs are insanely

difficult environments.

they're like the

climate multiplied

by a factor of 10.

They get much colder

overnight in winter and

they get much, much hotter.

So it's, you know, even on

a spring day in Melbourne,

a light colored roof,

like a surf mist type

roof will get well above

60 degrees immediately

under the roof sheet.

I think they've got

recordings of 90 to

100 degrees between

that cavity, like

it's quite high.

And it will depend again

on what the color of the

roof sheet is and what the

climate is and how well

ventilated that space is.

you know, a material,

materials don't like going

through hot, cold cycles.

You take any material,

particularly a metallic

material like aluminium,

which expands and

contracts and you make it

hot, make it cold, make

it hot, make it cold.

Eventually it

starts to fail.

so,

so, key taking out of

that though is Anticon

should not be on a roof.

Yeah.

If you wrap consistently

externally with a

ventilated cavity on your

roof system, is there any

situation where you might

still run the anticon

or an air cell product?

Or are we just best

allowing it to dry out and

creating a thicker cavity?

Or potentially looking at

an adhered product on our

roof, so it's a peel and

stick membrane instead.

Yeah, so there's a

lot of things in that.

So my primary concern

with any of these

foil faced products is

about that durability.

And so, once that starts

to fail, if my, you expect,

accept my hypothesis

that it'll fail, then

it's bits of it start

dropping down into the

ventilated cavity below.

So I'm starting to impede

the ability air to move

through that cavity and

therefore allow drying.

And if I'm impeding

drying, then I risk

getting an accumulation

of moisture in my roof

and building failure.

And so I want, because I'm

not expecting the occupant

to lift the roof sheet

every five years and check

their Anticon or their

foil based membranes we

need something that's gonna

last for a very long time

, and we want to keep that

ventilator cavity clear.

But they've got insulation

values too, so how

do we now compensate?

Because I think there's

also a misconception

that these wraps

also hold an R value.

and then I've also seen

a lot of people talk

about saying,

Hey, well, now,

the wraps or the

antique on that.

well, no, well, I know

you don't like NatHerzCam,

they'll put in the anti con

as, oh, we need the extra

R value in the ceiling

through the anti con.

So there's, that's

probably part one.

Part two is people still

have a misconception

that these membranes

is weather resistant

membranes have an R value.

It's nothing meaningful.

And then two, I think

there's also this

misconception is now that

we've got this huge cavity,

we've got say in Hamish's

case is 70, 35 and 35.

We've now got a 75

mil cavity in there.

Well, isn't air an

insulation value?

So we're getting,

we're getting

insulation to that too.

I think there's a few

misconceptions that occur

in our industry around

that, breakdown and that

buildup of that, that

roof or wall section.

so a foil phased bat.

has an R value, it has a

thermal resistance, because

it has a bulk insulation

there, there is glass

wool, you know, 50 mil

potentially of glass wool.

Now if you go and highly

compress it under your

purlins, then that's

reduced, but let's just

accept for a moment

that it's at full

loft, then there's no

question that there

is a material R value,

but does it contribute

at the assembly level?

Now if you put your

Anticon underneath the

roof sheet and you don't

have a ventilated cavity,

So your traditional system

may be with a warm roof

where you've got insulation

batts and essentially hard

up against the anticon.

Then yes, the anticon

or the foil faced

bat will contribute

thermal resistance.

Absolutely.

No question about it.

Now, I have concerns about

that assembly because it's

not allowing outward drive.

You know, we're trapping

the, vapour that's

diffusing towards the

exterior or the air

that's moving through

that assembly will touch

the interior face of

our foil faced bat and

it can't get through.

And then we risk getting

condensation on that

when it gets very, very

But it's got

holes poked in it

The holes are something

of a misnomer, I think.

You know, I keep trying

to bang on about this.

If you want to avoid

condensation, you've

got to stop water vapor

touching cold surfaces.

And something like

your foil faced bat is

presenting a cold surface

to just on the resistance

of air cavities, a still

air cavity does indeed

have thermal resistance.

But the creed bit here

is still, so you've got

to trap an air void.

Now, insulation is

fundamentally about

still air gaps.

That's exactly what we're

trying to do with every

insulation product, be

it a foam, or being it

something like glass wool,

or rock wool, or whatever.

We're just trying to

create all of these,

still air gaps.

When you have a film,

like a 25, or a 35, or

whatever cavity, If you

trap the air within that

cavity, then yes, that

can contribute, although

as it gets thicker, you

get convection, you get

little eddy currents of

air moving within that

space, which reduces

its thermal resistance.

But when we're trying to

do ventilated and drained

cavities, of course,

fundamentally, we're trying

to actually encourage air

to move through that space.

And so, if we come back to

our roof build up, where

we've put our rafters or

our truss down, we've then

put a vapour permeable

membrane on top, and

then we've put a batten,

put our roof sheet on,

and maybe we put a foil

faced bat underneath

there, the foil faced

bat is not contributing

any material R value

to the total assembly,

because we've got this

ventilated space between,

which is exactly what

we've designed it to do.

But we can't then double

count and say that,

oh, hang on, I've got

a foil faced bat here

that's contributing R1.

3 or whatever.

this is the other thing

that we do is, we've got

this foil facing an air

gap and we say that has

thermal resistance because

of the radiation foil.

And again, that in

theory works if it's

a still air gap.

But if it's a highly

ventilated cavity, which

is what we're trying to do

for moisture management,

it's then you can't

make that claim anymore.

But that still air gap

can only be certain

thickness, can't it?

Is it over 20mm

that it starts to

become practically

It still has some

resistance, but it

diminishes because

of that convection,

that eddy current sort

of idea that forms.

It's the same with, you

know, a double glazed

unit depending on whether

it's air or argon or

whatever fill need sort of

a minimum level, sort of

say 10, 12 mil, but beyond

sort of 20 or so mil.

it diminishes because

of that, that impact

of convection and air

movement within the,

within the cavity.

So I guess in summary,

like 20 mil good.

Less than 20 mil bad in

terms of cavity batten.

70 mil okay, not too bad.

Bigger in some

cases are better.

I know I touched on this

before, but what we're

trying to do on our

sites is to standardize

and simplify things.

So 70 mil overall cavity

from the, face of our

WRB to the back of the

cladding is something that

we're working towards.

Because that means you

could then turn a 70 mil.

Yeah, yeah,

Cam made a really

good point about it's

restricting that air flow

at the bottom or the top.

So if we just go whack on

these cavity closers at the

bottom, say for example,

you know those James Hardy

strips that have those drip

edges with little slots?

Like that's kind

of starting to

defeat the purpose.

Like it needs to be

something like an aluminium

mesh that still allows

air to come through.

That stops the bugs.

Like it's, we can't

just start closing

these up either with

these plastic PVC

materials or an aluminum

trim.

top, top and bottom too

though, correct Cam?

and even at transitions

of where your, wall might

hit an eave overhang

or something like that.

Like, there's all these

things that you want

to consider, like just.

this is what was running

into my head before, like

there's so many things to

think about when you're

building a home and trying

to put these, I would call

performance construction

elements in your home.

And I know on our

sites, we're not always

getting it right.

And we're always having

to pick up the phone

and ask the brains

trust or ask cam.

I guess this is what I love

about getting you on cam.

Cause I think every time

I've had, we've had you

on, I've come away with

a bit of an aha moment.

I'll just jump back

to this whole thing

about how you deal with

those cavity closures.

, because I think this is,

is the key bit to this,

where we sort of get

a bit, infatuated with

the idea of how deep

should my batten be.

and then dealing with

the question of the,

the entry point and the

exit point later on.

And a lot of those

proprietary cavity closes,

as Matt alluded to, are

sort of slots, and I

would hypothesize, I don't

know, but it strikes me

when I look at those,

this looks like it's

intended for drainage,

not for ventilation.

Like when my water drips,

drops, drip down, they can

escape through that slot.

Great, no problem.

But it's severely

restricting the air flow.

through that space.

So even if I've got a 20mm,

35mm, whatever it is, if

I've only got like a 3mm

wide slot running in 100mm

lengths or something,

I've only got an opening

area of maybe 5 percent of

the theoretical maximum.

You know, it's tiny.

And so I do prefer going

for a mesh solution, but

coming back to our fire

thing, if we're building

in a bell area where

we've got to go with 2mm

mesh, Then that's having

a significant impact too.

I mean, the rule of thumb,

you know, and an analogy

is when you put a fly

screen on a door and you're

trying to create a stack

of it, you're trying to

flush out your building

in the summer by opening

the door once cool change

comes through, you're

reducing air movement

across that opening by

about 40 percent by putting

the fly screen there.

Which is, you know,

quite impactful,

quite significant.

So even though, you know,

you're going to have

to put a mesh, no more

Well, maybe we need

to change the way we

build though, and not

use claddings that need

ventilated cavity and

look at things like say

a brick or a wood fiber

externally and render it

or something like that.

So maybe we need to

actually start to change

the way we construct

in these areas.

Well, I don't mind, I

think we didn't say this

earlier, but if anything,

probably the drainage

function of that cavity

is more important than

the ventilation function.

The building's going to

fail an awful lot faster

if it's got a leak.

than if it's got a,

a vapour problem.

, so you would fibre

with your render.

Yeah, that's okay.

I can see how that

can, in theory, get

away without the

ventilator cavity there.

But it does severely

restrict your ability to do

any other form of cladding.

and brick still needs,

you know, there's no

question you need a

cavity with brick.

I don't see a problem

with putting in a

ventilated cavity.

We do that fairly

regularly now.

It does surprise me when

you see details where the

cavity batten is actually

running horizontal.

It kind of suggests to

me that you've maybe

misunderstood what the

purpose of this cavity is.

when you're doing that

or maybe you're putting

on like two mil packers

or five mil packers or

something yeah okay that

theoretically will solve

the drainage part of the

problem as we've talked

about but it does very

little for your ventilation

I think it's just

to make the cladding

easier to install.

I don't think anyone that's

doing just a horizontal

batten to put, they're not

thinking about air movement

or the drainage at all.

They're simply just

thinking about how do

I fix my cladding on?

that's why we always come

back to first principles

isn't it like on one

hand as an engineer I say

you've got to model it

but on the other hand as

I will Think about where

the vapor and the water,

liquid water is moving

through this assembly.

Have I encouraged and

supported drying through

this construction?

And as soon as you do that

and think that through, you

go, well, hang on a minute.

Horizontal button?

Yeah, maybe not.

Yeah, so I think from

now on, the terminology

should be ventilated and

drainage cavity, I know

we've been sort of calling

it a ventilated cavity.

With the assumption that

it drains, but I think

we just need to be more

explicit, because then

it paints a different

picture in your brain,

because you're, dealing

with the ventilation, but

also the drainage, which

is probably, as you said,

the most important part.

That's actually good.

I like that, Haym.

sitting here thinking

before that I'm, I'm

really glad that I.

At the very minimum get

to listen to this twice

because I always listen

to it when it comes

out because there's

been so much that I've

like taken in today

and let's make that

another episode because

I think there's enough to

digest in this episode.

I've got two more

questions quickly

for you though, Cam.

Probably by the time this

podcast gets released,

we're in a whole new year.

What are you most

excited for in 2025?

uh, I don't know.

You should have asked

me, uh, given me

a lead up on that.

well, there's a

sort of a follow up

question in a sense.

Is, have you got like

a 2025 project that

you're most excited

to see get to site?

And it can't be the one

Hamish just said, or

is it one you really

want to see finished?

Or there's something

you're working on and

you're like, no, no,

this is really good.

And it's challenging me.

There's a very large

project, I'm doing

residential project with

an awful lot of steel

in it, which has caused

me any, any number of,

any amount of grief.

It'll be nice to see the

back end of that for the

grief side of it, but also

to actually see it come

to fruition given the.

level of detailing that

has been required on that.

Certification level,

a certification is a,

client requirement.

So it's, it's a

non negotiable.

and you couldn't just

put glulam beams in,

uh, nope.

this probably does go back

to, you know, for those

listening, you may be

keen on pursuing passive

houses, get your team lined

up from the outset, get

everyone on board with what

you're trying to do here.

be challenging.

I mean, there's

multiple constraints.

It's always building.

It's always that

competitive tension

between different

disciplines from my thermal

engineering point of view.

2025 model.

Get your team

together early.

It's so important.

It really is, yeah.

it comes even at the

fundamental geometric

design of the building.

You know, you're

architect, you're building

designer, you're draftee.

You know, we could talk

about this some other time,

but your flat roofs, your

parapets, your box gutters.

How many times do

we have to have

these conversations?

I know they're

architecturally great, and

they, you know, might be

required for your res code

in a certain place, but

man, they create headaches.

Thanks.

it does come back

to this debate about

probability and risk.

You know, you can't have

zero risk but as soon

as you if you're doing

things like a flat roof,

you are locking in a

much higher level of risk

than you otherwise would

and we cannot entirely

mitigate that risk no

matter how hard we try.

I think we leave it

there because Dr.

Evil is probably about to

piss off more architects

saying that you can't

have flat roofs and

they should be banned.

So no flat roofs with

a fireplace on them.

I didn't

say bad.

Thank you,

Thanks, Cam.

Thanks,

See you, Maddy.