Hey, welcome Cam.

It's been a few weeks.

How you doing?

Not too bad.

Thanks ish.

Good to see you, Matt.

Hello Cameron.

So this is going to work

in sort of two parts.

So in a few weeks on

the 18th of October.

If you're in Melbourne,

we're co hosting an event

with Sustainable Builders

Alliance, is that right?

we call it a mini

conference and just

allowing people to dip

their toes into some

basic understanding

of building science.

All right, so we'll go

into detail on why we

build the way we do.

This is really aimed

at anyone that wants to

come where you can be a

builder, architect, person

that wants to know more

about building, you could

be a potential client.

this isn't just tradies

or our listeners.

All right.

So, we have a number of

speakers on the day and

we're going to start with

the smartest person in

Australia when it's talked

about building science.

And that's our friend

here, Cameron Munroe,

and he's going to talk

about the perfect wall,

which is also make up part

of today's discussion.

And then we're followed

by our good friend, who's

been on our podcaster

for Airboss Dan.

And we're going to

talk about flat roofs

and the importance of

creating a well managed

sort of membrane and

cavity through that.

And what we need to

look at because flat

roofs are notorious

for leaking and damage.

And then we have

a host panel.

I'm going to be asking

questions to Selena

Edwards from Envirotexture,

Rob Petruzzi from

Hip V Hype, and Joel

Segrin from Fantech.

And we're going to throw

anything we can at them

to Give people confidence

to get themselves into

this side of the building.

You kind of want to dispel

the myth that this is

more expensive, that we

just need to do things

right from the start.

So come along.

Tickets are available

through the SBA podcast.

We'll also show, throw

them in the show notes.

It's also on both Hamish

and I's social media.

through our businesses,

plus the SBA Instagram.

There are many ways

to get a ticket.

there's a really good deal.

If for your whole

team, I think it's

a hundred dollars

for your whole team.

You can bring,

110,

for

four people.

Yeah, it says

four people, but

if you're listening to

this podcast, I'm going

to override that and you

can bring your whole team,

the more the merrier.

And we'll put a promo

code, the mindful builder

podcast in there, if

you want to do that.

So the whole idea

has come along.

we want to really change

the way we build and be

active in creating change.

So if you have any

questions you also want to

ask, just shoot us a DM.

We'll do our best to

sneak them in and throw

some curveball questions.

But we will start now with

the start of sort of Cam's

presentation and why we

need to understand this.

And it's perfect wall.

So Cam, what is

the perfect wall?

So the perfect wall is

a term that supposedly

started, was introduced

by a bloke by the

name of Joe Steinberg.

An American who ran a

business called Building

Science the.com in the

us which listeners would

probably be familiar with.

And they publish.

really great free

material on their website.

So if you get into building

science, I really recommend

having a look on that.

There's a lot there.

The perfect wall is

really a concept.

It's not actually

a physical wall

of itself at all.

It's really a mechanism

to try and understand

how building science

works on our assemblies.

So imagine we have a

wall, on the interior we

have some sort of lining

board, something like

plasterboard, typically.

Then beyond that,

as we move out, we

have our structure.

And so it's usually a

timber frame structure,

it might be concrete

precast panel, it

might be a steel frame.

So that's our structural

layer of our building.

And then moving out

beyond that, we have an

air, vapor, and water

control layer, or layers.

They can be three different

layers, or they can be

one layer in one function.

And that might be a

membrane, typically, and

sort of a plastic material.

And then beyond that,

you have insulation.

And then beyond that,

you have a cavity.

And beyond that,

your cladding.

So it's hard to deal with

that sort of only orderly

rather than with a picture.

But imagine that as a

fairly typical wall.

But the key point is

that we're separating

out our control layers.

So, importantly,

our structural layer

is separate from

our insulation.

Our insulation is to the

exterior, to the outside.

of all of our water

sensitive materials.

So our timber frame wall,

for example, and we have

these other layers in the

middle there, our air vapor

control layer and our water

control layer, just to act

to prevent any water that

gets in behind our cladding

to get to get into that

water sensitive material.

it's a sort of a,

conceptual tool to

understand how we

should build assemblies.

And what's the whole

purpose of this?

Like, why do we need

to build, a perfect

wall or a proper wall?

yeah, so that's

a good question.

Right?

So what's the objective

of, of a wall or

a roof or a floor?

It's obviously to

hold something up.

So we need that

structural layer.

Increasingly it's to

accommodate insulation.

We're trying to manage

the heat flow across

our, our building.

So we need some

insulation there.

But as soon as we do

things like add insulation

to our buildings, we're

changing the heat flow

across the building and

so we're changing the

ability of each layer

within that wall to dry.

we know that there's

water vapor all around

us, everywhere, all

the time, in the air.

We can't see it, but we

know it's there because

when we take that glass

out of the fridge and put

it on the kitchen island,

we get beads of water on

the outside of that glass.

So we've got water vapour

floating around in the

air that it can't see.

When it gets to a cold

surface, it condenses.

And that's really our

absolutely fundamental

element of building

science, or when we think

about buildings, is you're

trying to stop water vapour

touching cold surfaces.

so by thinking about a

perfect wall, we're trying

to avoid cold surfaces

because we're putting the

insulation on the exterior

of just about everything.

So on the exterior

of our structure.

Okay, we keep our nice

timber frame dry and warm.

Perfect.

so just to summarize

Cam just so I've

got this correct.

So we've got plaster,

structure or frame,

insulation, air control,

water barrier, and then

cladding on the outside.

Is that right?

the only bit that's,

you've got that wrong

way around there Hamish

is the control layers.

So the air vapor and

water control layer will

be between the structure

and the insulation.

Okay, cool.

So, I have heard about

this perfect wall before,

and I guess I'm trying to

relate it back to how we

build on our projects and

it is different to that.

So we've got plaster,

cavity batten, air control

or intelligent membrane,

frame, insulation within

that frame, external

of that we've got

our weather resistant

barrier, then we've

got a cavity cladding.

So if this is the kind of

go to method that we've

adopted here in Australia,

Why is it that we're not

doing this perfect wall?

I said, it's

not a real wall.

And the term is

a bit strong.

I mean, in terms of the

engineering of it, it is

perfect because it manages

the heat and moisture in

an ideal way in a practical

real world wall though.

We have cost constraints

and we have to accommodate

our services and our

windows and everything

else that goes into it.

And those complexities

are where we end up with

something like what you've

described there, Hamish,

a realistic Australian

wall that we would build.

But we still, we can use

the concept or the idea

of a perfect wall to sort

of extrapolate and link

those elements to what

you've just talked about.

So where Hamish and Matt

build is obviously in

Victoria, so heating

dominated climates.

So the Consideration

in a heating dominated

climate from a moisture

point of view is that

for most of the year, the

water vapor wants to move

from the interior to the

exterior of the building.

And so, as we've discussed

before, there's two

ways vapor can move.

The primary way by

which is air movement.

So you've got, put

PowerPoints into your,

plasterboard, air moves

in through there, moves

out across the insulation,

typically glass wool

insulation, and then if

it hits a surface, like

maybe we put sarking on

the exterior of our wall

assembly, traditionally

that was a, a metal foil.

So what have we got?

We've got water vapour

touching a cold surface.

And what's the number one

thing we've got to avoid in

building stop water vapour

touching cold surfaces.

So really the whole idea

of this wall is protect

the structure from water.

that's exactly what

we're doing, so, well,

as well as meeting those

other criteria like

insulating our buildings

to keep the heat in.

We're trying to keep

the heat in, we're

trying to stop the vapor

moving across that wall.

If it does, we're trying

to allow it to dry

towards the exterior.

And this is where, what

Hamish described with those

two layers of membrane.

A membrane towards

the interior of the

construction, that's

a vapour retarder,

that's trying to slow

the movement of water

vapour across the wall.

So that moisture that's

floating around inside our

building, we're putting an

airtight layer, so we're

taping that membrane at

all the seams, to stop

air getting into our wall.

The characteristics of

that material, usually a

plastic sort of material,

are such that they

stop vapor diffusion.

They prevent the

water molecules moving

across that plastic.

And then towards

the exterior, you've

got another membrane

that is vapor open

or vapor permeable.

So that does not stop

Vapor diffusion out.

you know, we've build

imperfect walls in the

real world, no wall

can be ever be perfect

because there's always

construction defects.

You're not building uh,

completely airtight

building, and so you've

got to assume that at

some point in the life

of that building, that

interior membrane will

have an issue somewhere.

There'll be a small

penetration of it.

Air movement could

get into that space.

And so if it does, we've

got to allow it to dry

out, remembering that

in a heating dominated

climate like Victoria,

or the ACT, or Tasmania.

The vapour drive, the

direction of water

vapour movement is

towards the exterior.

And so external membrane

has got to be more

vapour open than your

internal membrane.

why can't we just

whack up a wall?

a sort of a liberally

use of history.

once upon a time we

built walls without

any insulation in them.

We whacked weatherboards

on the exterior.

We whacked some

plasterboard or, they'll

lay the plaster on the

interior and everything

was pretty fine.

If you ran your heating

inside then the heat could

move readily through that

wall as could moisture,

but because the heat

could move through it

would allow drying, the

whole system would keep

dry because the heating

within the home would

help to dry that building.

But of course,

the weatherboards

occasionally would fail.

You get water, rain from

the exterior getting behind

the weatherboards that

gets onto the timber frame.

And so then you get rot and

mould on the timber frame.

So how did we solve that?

We introduced

sarking materials.

So we said what we need to

do is have a second line

of defence to that bulk

watering, that rainwater

behind the cladding.

And so we put these metal

foils on the exterior

of our stud frame and

we whacked weatherboards

straight up hard up

against them so no

cavity there behind that

kind of sort of worked

because you still had no

insulation in the wall.

The problem really arises

or becomes most acute when

you combine that exterior

sarking, the vapor barrier

with the insulation on

the interior face, because

all of a sudden I've

now got a cold surface.

I've got that metal

foil sitting on the

exterior of my wall.

I've got insulation between

it and the interior.

And so when the interior

is 20 degrees, because

I'm heating that, the

room, And the exterior

is five degrees.

The metal sarking is

now at five degrees.

And if I've got nothing

to stop water vapour from

the interior, remembering

that the water vapour

move, wants to move out, it

moves out, gets through the

plasterboard, through the

glass wall, hits that metal

surface, it's water vapour

touching cold surfaces

that causes condensation.

it's always that beer can

out of the fridge analogy.

That's what's actually

happening in the

wall structure.

You just can't see it.

that's exactly

the analogy here.

So how do we prevent

this happening?

Well there's really three

or four ways to do that.

One is you've either got

to stop the water vapour

getting there in the

first place and that's

why often, particularly

in high performance and

passive house construction,

we'll use an interior air

and vapour control layer.

So a membrane on the

interior face of our

frame to try and stop that

vapour moving both by air

movement primarily, but

secondarily through vapour

diffusion into that wall so

that it can never hit that

sarking on the exterior.

Another way is to

manage the moisture

internal to the building

in the first place.

So use your range hood when

you're cooking the pasta.

It should be externally

venting so you get that

vapour out of the building.

When you shower, use

the exhaust fan, and of

course it needs to be

externally vented as well.

But ideally, you

have something like

a HRV, so centralized

ventilation, that runs

24 7, that constantly

manages the moisture.

So source control.

Avoid that moisture

building up within

the building such that

there's a strong vapor

gradient or strong vapor

pressure between the

interior and exterior.

Now you can't avoid that

vapor generation because

of course we're sweating

and we are always doing

things in our buildings.

So we're creating some

water vapor invariably.

You can't completely

eliminate the source.

And you don't want to.

Yeah.

So just so I've got this

right, we have a rain

control layer, an air

control layer, a vapor

control layer, and a

thermal control layer.

so just want to go the

rain control layer because

that's the outside.

That's what the if anyone's

built a house, you'll see

at the moment, but both

Hamish and I use the pro

climber products, the

Exosun or externally the

Blue House what we're sort

of talking about here.

Now, the whole idea

for that is stop the

weather from getting

from inside to outside.

The whole idea of the house

is you want to separate

yourself from the elements

of inside to outside.

Now, with the rain

control layer, that is

not your cladding, is it?

No, it's not your cladding.

And usually we assume,

like when we do modeling,

hygrothermal modeling

using something like ORPHE,

we'll often assume that

1 percent of the rain

that touches the cladding

gets behind the cladding.

And that's a lot.

Like if you think in

Melbourne, it's about

700 mil of rain a year.

If you assume 1 percent

gets behind, that's 7

mil of rain of liquid

water getting onto

your timber frame.

So it's a

lot.

that's okay.

if it dries though

with the cavity.

Well, it's always a

balance of wetting

and drying, isn't it?

You've got to make

sure the drying exceeds

the wetting potential

of your structure.

And so, yes, if you heat

your house, so imagine

I've got a really 150

year old miner's cottage

in Ballarat, say, with no

insulation in the walls,

no external sarking.

If I run that heating,

I've got a heating system

inside, and I run it

flat chat all winter

long to make sure I keep

the house at 20 degrees.

It's going to cost me

a bomb, but my wall is

going to be nice and dry.

Now, the reality is that

most people can't afford

to run their heating system

like that because their

house is so inefficient.

And so the whole

building gets cold.

And so the drying

potential of the wall

And so you have this

balance happening where

you've got water getting

into your wall, you've

got to make sure it can

dry, you've got to create

enough heating for it so

it can dry, and who wins?

Ballarat, there's heaps

of like fallen trees and

that so you could just burn

the firewood inside and be

sort of free, wouldn't it?

that was a slide

about my views of wood

fires and how horrific

they are for, health.

Sorry, I'm not

advocating forward fires.

I just know that

it's a real pain

point for Cameron.

lemme just take a

slightly different angle

on this for a second.

So, remember with our

metal foils when we

introduced those and we

found that we had problems?

And then for a while,

we introduced breathers

breather, foils.

Where we punched little

holes in them, and

we said, well that

will fix the problem.

Because we recognised at

that point that we needed

to allow the vapour to

get out of these walls.

And the vapour was

touching these, cold

vapour impermeable

aluminium surfaces.

So let's punch little

holes in them to let the

air and the vapour out.

Now I don't know quite

where that came from, but

that doesn't seem to really

accord with the science.

Because firstly,

you've still got a

cold surface there.

So if I've got some water

molecules floating in my

air through my wall, 99.

9 percent of that aluminium

has not got a hole in it.

Okay, there's, the holes

are tiny little pinpricks

every, 300 ml or whatever.

But when you have a hole,

you have a leak though.

and that's the second point

we'll get to in a second.

you're still getting

the vapor condensing

on the interior face

of this aluminium foil.

But secondly, you've just

defeated the whole purpose

you put it there for in

the first place, which

is exactly Matt's point.

Why did we put

the sarking there?

Because we were worried

about water getting

in past our cladding.

Now what have we done?

We put holes

in our sarking.

It doesn't make any sense.

feel like I've been quite

quiet here because I've

been sitting here just

enjoying being educated

on this stuff and it's

certainly not things that

I haven't heard before or

that I understand, but.

I always just enjoy

listening to Cam, explain

these things in very

simple terms to us.

So, originally we talked

about a perfect wall.

Now, speaking, there's

restrictions around that

due to how we execute

that on site and cost.

How I explained our wall

buildups before, but

you maybe just touch

on that, because that

seems to be an approach

that a lot of us take,

particularly down here in

Victoria in our climate.

Can you talk about that

and maybe talk to some of

the imperfections in that

particular wall assembly?

And are we running into

any risks with that wall

assembly if it's not quote

unquote quite perfect?

So we're trying to

make that particular

assembly quite resilient.

So it's a good,

durable design.

So remember that our water

vapor is moving from the

interior to the exterior

for most of the year,

sort of eight, nine, 10

months of the year in a.

in the southern

Australian climate.

So we've got our vapor

retarder on the interior.

So we're trying to prevent

wetting in the first place.

We're trying to

prevent that moisture

getting into our wall

and causing wetting.

On the exterior, we

have a membrane that is

preventing any of that

rainwater gets past the

cladding, getting onto our

structure, and critically,

and very differently to

our aluminium sarkings

that we had in the past,

it's vapour permeable.

It's vapour open.

So we're trying to

maximise the drying

potential of the wall.

while minimizing the

wetting potential of them.

So it's a balancing

act we're trying to

get here where we

maximize the drying,

minimizing the wetting.

you've got your external

membrane, your weather

barrier, which is your,

outside control layer.

And then we just spoke

about having holes So

when we have weather

barriers, there's

obviously microporous

versus monolithic.

Aren't microporous barriers

just again, holes with

little holes in them?

Yeah.

bit of debate

about this I think.

I want to be a bit careful

here because obviously the

suppliers of the monolithic

membranes, obviously I

advocate for those as being

superior and a case to be

made in that direction.

So microporous, you're

essentially sort of

trying to create tiny

little pores, holes

within the, fabric.

But a liquid water

molecule is larger than

a, a water molecule.

And so won't get in

through the microporous

membranes that the issue

seems to be more about

the longevity and the

impact of pollutants,

chemicals, particularly

in some urban areas on

the characteristics and

the longevity of those

microporous membranes.

want to be a little bit

careful and being too

overly differentiating

between those two.

so we've got rain and

then we go into air and

essentially there's no

point worrying about air

if you're not worrying

about rain because

you've already, your

water's already get in.

Like, it's not like,

this isn't, it goes down

to this principle thing

that Cam hates talking

about is, we just can't

pick which ones we want.

It's like, oh, we only want

the vapor control layer.

doesn't work

again like that.

you're right, Matt, that

if you've got a leak in

your roof, don't worry

about where, how the

water vapour is moving

out through your wall.

You've got bigger fish

to fry, your cladding

system right first.

Then the second thing

you've got to worry

about is air transport.

So the movement of air

carrying that water vapour

out through your walls, up

through your roof, that's

by far and away, The second

most significant issue

does that also mean air

through a ventilated cavity

or just air through the

structure because I feel

somewhere along here, the

cavity and allowing air

to move through the cavity

to dry your clutting out

and allow your structure

to dry out is somewhat

getting missed or is that

full under air control?

well, that's somewhat

different because that's

unconditioned air.

So that's if we're talking

about say ventilated cavity

towards the exterior of

our wall assembly, we're

drawing in outside air.

to act as a drying carrier.

It's not internal

air that's carrying

all the moisture.

just a really critical

point here that really

does our heads in.

But when it's cold, say

five degrees outside, it

might have a very high

relative humidity, you

know, in the middle of

winter, you might see mist,

fog, whatever, outside.

So it might be 80 percent

plus relative humidity.

But guess what?

That air is actually dry.

It's dry in absolute terms.

There's very little

moisture holding

capability within that air.

So we talk about relative

humidity as a percent

because that's the amount

of moisture that we can

carry in that volume of

air at a given temperature.

that saturation?

kind of like dew point

saturation, it all

fits into this bigger

picture, but let's not

try to complicate it

with too many words

So if you, it's like when

you go to say you arrive

at Qatar airport and you

walk outside and that

humidity and moisture in

the air is very different

to that type of air.

well, it's can be

very hot in somewhere

like the Middle East.

It can also be very,

very dry there too, to be

fair, in both absolute and

relative humidity sense.

But point I'm trying to

get to is that with our

ventilated cavities and

our heating dominated

climates, like Victoria,

even though you've got

cold air outside, You

can't carry much moisture,

but it's relatively

dry in absolute terms.

And this is where vapour

pressure comes into the

equation, because warm air

can carry lots of moisture.

If you go to Cairns, it

can be very, very humid

and very, very hot.

And in absolute terms, if

you measure the amount of

grams of water floating

around in that volume

of air, it'll be huge.

Whereas in Melbourne,

in the middle of winter,

when it's five degrees.

That air won't be

able to carry very

much moisture at all.

The absolute humidity

within that air is very

low, which is why you

might see condensation in

the form of mist or fog.

talk about a perfect wall.

A perfect wall is also the

same as a roof tilted on

an angle and practically

the same as a floor.

And I think that's what

people get confused about

is that so many people try

to change or to refer to

deal with it differently.

It's still the same thing.

It's just on a

different angle.

fundamental

principles remain

the same, don't they?

Stop bulk water getting

past your cladding.

Assume that some will,

so have a second line

of defence, which is

your sarking in our

classic terminology.

And then ensure that

you're minimising the

transport of air, vapour

laden air from the

interior of your building

towards the exterior.

Assume though that some

will get through, so you've

got to have a vapour open

membrane on the exterior.

And you've got to have

a ventilated cavity

to allow that vapour

to get taken away.

Alright.

And this is the other bit

that we kind of muck up,

I think, is that we, we

now have to put class four

membranes on the exterior

of our, our buildings.

So vapor open membranes

in heating dominated

climates within Australia.

But then you can whack

cladding straight

on top of that.

I'm just punching a million

holes when you work that

cladding on, like imagine

like a weather techs

when you got to shoot all

these screws or nails in.

got that perhaps, but

just in terms of you

think of your material

buildups again, what

am I trying to do?

I'm trying to have

layering of my building

that has increasing vapor

permeability as I move out.

And so I've got a vapor

open membrane on the

exterior, and then I

put some sort of FC, you

Block if it's a

vapor blocker.

whatever it is straight

on top of that.

There's no way for the

system allow vapor out.

So

I'm going to fumble through

this question, Cam, and

I'll try ask it in, I hope,

in the most coherent way.

So when we're insulating

our structure, so if

we think about it, our

continuous insulation,

we've got more insulation

on the roof, typically,

or in the ceiling, than

we do on the walls.

And then generally on

the floor, the floor has

probably kind of similar

to what's on the walls.

Two part question,

why is that the case?

And secondly, does

that impact how

moisture moves out?

Do we see more moisture

moving or wanting to

move through the roof to

the walls or the floors?

That's the best question

you've ever asked, Hamish.

I love it.

You didn't fumble your

way through that at all.

So the first part of that

question, the first time

is about why is it that

we have more insulation

in the ceiling than the

walls and the floor.

To start in reverse,

the floor in a heating

dominated climate like

ours, in the middle

of winter, ground will

be warmer than the

ambient air temperature.

So in Melbourne in

the winter it's 20

30 percent down, it's

something like centimetres

under the ground.

It's pretty stable at

about 16 degrees in

winter, which is warmer

than the air temperature.

So the delta T, the change

in the temperature there

is less, so you need

less insulation ground

assuming you're on a slab

than you do elsewhere.

Now, why the roof

insulation higher than

the wall insulation?

People would say

because heat rises,

and I say that's wrong.

Heat doesn't rise,

hot air rises.

And so if you think

about how heat moves,

there's convection,

conduction, and radiation.

Conduction and

radiation don't care

about the orientation.

Alright, if I've got a,

burning a flame, the heat

that radiates from that

flame radiates equally

in all directions.

The hot air, so that

flame creates hot air,

Which then creates

convection, the movement

of air, and hot air rises.

And so as hot air

rises, it gets warmer

near the ceiling.

And hence we need

ceiling insulation is

more important relative

to wall insulation.

But then the other problem

that arises is that

because I've got this

convective air movement,

hot air buoyancy, trying

to drive air movement

up carrying vapor from

within the building.

If I've got 50 downlights

in my ceiling around which

that air can now escape,

then I'm going to get

a lot more risk of that

air transport, that air

movement of condensation

into my roof assembly.

Which is going to rot out

the roof really quick.

so with that air.

If we think about moisture

traveling in the air, we've

got a higher chance of

there being more moisture

moving through that

ceiling and roof space

than we do on the wall.

Which is why we have

a greater amount of

insulation in the roof.

Does that kind of work

the opposite too with

restricting heat or cold

or temperature coming

from the outside in?

Because typically we

have the sun belting

down onto the roof.

Is that another reason why

we have greater insulation

in that ceiling space?

Somewhat, but we need to

be a bit careful there

because the sun is, is

a radiant heat source.

It's generating radiation,

which then strikes the

earth, heats surfaces,

which then conduct, but

also create warm air,

which creates convection.

And so if your concern

is summer overheating

of a roof buildup, the

main thing you need to do

then is to actually have

that ventilated cavity

towards the exterior.

So you put your roof

sheet on battens, have

it well ventilated.

So you're effectively

creating an umbrella

on your building.

So you've got your bulk

insulation at your after

plane say, then you've

got a button with a clear

cavity, and then your roof

sheet beyond that, sun

strikes the roof sheet,

heats up the roof sheet,

which in turn transfers

into that air, within

that cavity underneath,

which then moves through.

It creates a stack effect,

it buoyants, and so it

moves up the pitch of your

roof and it drives out.

So essentially what you're

trying to do is put an

umbrella over the building

to prevent that heat

getting into the, to the

bulk insulation and the

structure of the building.

I just want to clarify

to the listeners there

who maybe not know what

a ventilator cavity is.

What we would typically do

on our projects is we would

put a vertical batten of

a certain thickness which

allows moisture to run down

our wall assembly and air

move up that wall assembly.

Now this is relevant

to the question I'm

about to ask Cam.

Is there perfect

batten size?

or minimum or maximum

batten size for that

ventilator cavity.

Or that batten

that we're using to

create that cavity.

no, it depends.

It's got to depend.

So it's got to depend

on where you live.

Are you in Sydney

or Melbourne?

And if we talk about

a roof for a moment.

What color is

the roof sheet?

if you're using a dark

color like Monument,

then you get much more

solar radiation absorbed

into that surface, so it

gets hotter, and so you

need a narrower batten

than you do if you go

with, know, a very light

colored roof sheet.

Then of course,

what orientation

is that roof at?

There's any number of

factors to come into play.

Now typically we'd be

talking anything from

what, minimum 19 mil

batten to 35 to 45 being

probably pretty common.

if you're building a

house, and you want to be

absolutely confident that

you are both ensuring the

moisture is taken away

from that, that cavity, and

that you're minimizing that

summer overheating risk,

which is actually probably

the one that drives the

minimum batten size then

the bigger, the better,

but we can't go over

engineering our buildings

ad infinitum, especially

in this cost environment.

We've got to do

what we need to do

and nothing more.

If we're trying to

get that cost optimum.

That's a great point.

one of the things that

we've been trying to do,

and we actually caught up

with a couple of people

down in Tassie a few weeks

ago, to try and develop

a whole bunch of standard

details, just so our window

reveals are always the

same, our wall buildups are

always the same, etc, etc.

well the motivation is it

all that window reveals?

We're just trying to

standardize that detail of

how to finish the window

reveals and cavity clothes

and all that kind of stuff.

And I guess,

fortuitously, we've

landed on 35mm as that

perfect counter batten.

So our overall cavity

is always 70mm.

So sometimes we're turning

a 70 35mm on edge, And

sometimes we're using 35

vertical and 35 horizontal.

And all of that

depends on what kind

of cutting we're using.

And that way it has

allowed us to standardize

all our window reveals.

So I'm glad you said 35,

but can we be too big?

Or is that just come

down to a cost thing?

I

As long as the air can

escape at the top though,

there's no point having

these cavities unless if

you block it all in the

right.

Yeah, and that was one of

the other things that we

were discussing there, when

we were in Tassie as well,

is making sure that air can

get out of the eaves and

at the rooftop, considering

you have eaves though.

well, yeah, or we also

talked about where it

terminates at like a

party wall or a box

gutter or a skillion roof.

So these are all the things

that we were sort of work

Isn't this the

architect's job?

I'm sorry, I'm going

to call it out.

Shouldn't they be

thinking through this?

A hundred percent.

this is certainly not poo

pooing on any architects

because we work with some

amazing building designers

and architects of core,

but, have been some

occasions where we've been

sat there on site trying

to work out like the

best way to approach it.

And like the whole exercise

about, trying to work out

or standardize our details

was so we could say, Hey,

this is how we do it.

And this is what works.

And these are the reasons

why, but it is nice to know

that that 35 mil, but it

is nice to know that that

35 mil, Cause forever in

a day, we're using 20 mil.

Now we've landed

on that 35 mil.

It's good to hear that that

is a good thickness to use.

Treat it a pine or

stand it a pine?

I was going to

ask that question.

So I'd love to hear Cam's

thinking on this one.

We've had some good chats

about this in the past few

days.

But before we get to

the, treated question

just on that, the

batten sizing, you,

your constraint quickly

becomes the cavity closes.

So if you said a 35 is

not enough, and I'm going

to be really crazy and

go 45, or put a 70 on

edge, or something like

that, create a really

deep cavity, beyond some

point constraint is the

airflow in either at the

eave or at the ridge.

And so you've got to

really, you know, if

you want to maximize

the airflow, you've

got to suddenly open up

more at the eave and,

you you know, if you're

above rated area, then

you've got a two mil mesh

aperture problem as well.

If you look at some of

the cavity closes that

you can get commercially,

they're just a little

slot in a bit of plastic.

And so the opening area

is actually sort of 20

percent or thereabouts.

Thanks.

Of the little extrusion

that you've got there.

And so those seem to

me to be designed more

to allow drainage, not

allow ventilation, which

is what we're really

trying to do here.

You know, we've got

to provide drainage.

That's the

number one thing.

to my mind, just in a

practical sense, anything

beyond 35, 45, sort of

batten heights probably

doesn't make sense unless

you can come up with a

really cunning solution

for your eve and ridge

ventilation as well.

Matt's point about treated

timber, jury's a bit out

on that to say the least.

Pine battens

are pretty good

current gut feel on

this, and this is only

gut feel, is that If

you've got somewhere

like a valley maybe a box

gutter or something, and

you've got battens where

water might conceivably

collect in, reasonable

quantities, then that might

sway you towards going

with a treated batten.

just in that area.

think the jury's out

here but that you know,

intuitively feels like

that's the riskier place.

So if you're going to go

treated battens anywhere,

you're probably going to

do it in those spots first.

Is it justified

everywhere else as well?

Again, hard, hard to say.

I mean, how's the

water getting in

Why can't you sit them

up on a packer like a

five mil packer off

your and you raise

And that might work to a

large extent too, Matt,

but bear in mind that

there's two ways in which

that water is getting

into that roof cavity.

One is that 1 percent

that, you're going to

get some leakage past the

roof sheet, but hopefully

that's pretty minimal to

negligible, but you are

going to get quite a lot.

In the winter on

that cold but sunny

winter's morning through

night sky radiation.

So remember what's going

to happen in that roof

is the roof sheet is

facing towards the sky.

No clouds in the sky.

So it's night sky

radiation is about.

The radiation to space,

space is at absolute

zero minus 273 degrees.

And so that roof sheet

will drop, five to 10

degrees colder than the

ambient air temperature.

So if it's five degrees

ambient air temperature

in Melbourne and on

that coolish winter

morning, it might be

minus five on the, on the

top of the roof sheet.

It's the same as your

windscreen on your car.

You go out in the morning,

the windscreen's got frost

on it, but there's no,

there's no it's above zero.

How can that be?

How can I have

frozen water?

Because the surface of

the windscreen has got

below zero and therefore

allowed frost formation.

I feel like treated

chemicals are a form of

laziness in some most

situations I get that can

never trump good building

practices and if you can

think through a design

where you're building

that up off the ridge or

like you're building the

packer off the ground and

trying to avoid it like

we've had for example

Like timber shingled roofs

were a thing they last

if it's allowed to dry

Yeah, but bear, bear

in mind here with our

ventilator cavity mat, by

design, We're encouraging

vapor laden air to get

into that, to circulate

behind the roof sheet,

which is super cold.

And we've got air that's

already cold, so with

very little moisture

carrying capacity.

And so it is inevitable.

It is a design feature of

what we have done here that

will get water droplets

forming on the underside

of that roof sheet.

which will then roll

down, drip off, possibly

onto the battens,

possibly not, roll down

that membrane, possibly

collect on the end grain

of one of those battens

and get absorbed in.

Couldn't you just use

like a small amount of

insulation a thin thin

amount That's going

to just break that

or some details, you see

you've got like a damp

proof course batten.

Yeah

There's probably some

options that you could

look at, but you're still

going to get some vapor

or some moisture forming

immediately underneath

your roof batten.

That's then going to drop

out down onto the batten.

Now we come back to

this balance between

wetting and drying.

at 6am on that

winter morning.

I've got some wooden

droplets dropping

onto the, the batten.

And then from 10am

onwards, the sun comes up.

It gets a little bit

warmer in this cavity,

particularly if I've got

a dark colored roof sheet,

like a monument that

heats it up that cavity

and allows that dry.

And in theory, as long as

that whatever droplets of

water are formed on that

batten can dry by the next

evening, then we don't have

a net accumulation water.

So drying potential exceeds

the wetting potential.

so quick yes or no,

because you did talk about

monument roofs and how

dark roofs can dry quicker.

this whole heat, urban

island effect thing

where we councils and

everyone's trying to

remove dark roofs.

Is that a bad thing?

got 10 seconds to

try explain that one.

Yes or no?

Yeah, no, I'm on the

fence on that one.

And again, if you're

in a heating dominated

climate in the winter, a

dark roof can be somewhat

beneficial because it helps

provide a warm boundary

about a kilowatt hour,

we found this about

a kilowatt hour on a

project of Passive House.

remember also, we're

often putting solar PV

on our roofs as well,

dark.

which is dark, but

it's also you know, a

highly ventilated cavity

because those panels are

sitting a good 60, 80 mil

proud of the roof

can,.

and Every time I chat

with Cam, whether it's

the podcast or outside of

the podcast, I always feel

like I come away smarter

and Cam, this is 100%.

One of those cases right

now, been doing this

for a few years now,

and I've learned a heap.

I've taken a whole

bunch of notes here.

And thank you so much

for your time, Cam.

Like, I feel like your

wealth of knowledge and

understanding of building

physics and science is so

valuable to our listeners

and, Selfishly for me too.

And I'm sure Matt

feels the same.

So thanks again.

I'm personally incredibly

excited about the event

that you're coming to chat

at and partner what you're

going to talk about with,

with what the other people

are going to talk about.

I think it's going to

be incredibly valuable

and I'm equally excited

to see the questions

that Matt is going to

throw at those panels.

Cause I know Talena,

Rob, Joel, if you're, if

you're listening um, hold

on, it's going to be fun.

Yeah, I'm going to ask

hard questions, but it's

also going to be fun.

But the whole idea is to

educate everyone here.

And I think that's

what everyone forgets.

It's not an us versus

them or we're better

than you're better.

And I think that's the way

the world is at the moment.

You're either on one

side or the other.

The whole idea is we

need to, we need to

bring our industry up

to a higher standard.

That's the point of this.

We want to give people

the confidence that there

are people doing this,

that it does work, that

it is cost effective

and it can be done.

The answers are

already there.

You don't have to

go search for them.

We can give them to you.

But it's just about

that willingness to

want to learn and

wanting to be better.

Probably.

Thanks, Ken.

thank you.

Thank you.

Thanks everyone.