Roof membrane

Hey, look! First post. This one is to consider the issue rather than me having a real problem.

Attachment 526062
This diagram is sourced here: Access Denied I got it from the Manuals / instructions section.

Paying attention to my red circle – roof membrane. It's positioned under the roof battens. How did it get there?

So, the rafters are in place. The diagram shows the membrane on top of the rafters. Okay, fine. Next goes the 'drainage space battens.' I presume they’re nailed to the rafters, and that would keep the membrane from being blown away if there's a wind about. Then the roof battens go on, followed by the roofing. That sounds reasonable. I'm just wondering how it's done.

I’m trying to visualise someone doing this. Roof battens are ladder-like – they provide somewhere to walk (carefully) while moving about the roof. So, how can anyone roll out the membrane without roof battens to stand on?

I'm also trying to figure out how the membrane would be taped to the next one without having anything to stand on. If the sheets meet up mid-rafter that could be a hell of a job, and if you don't tape adjacent sheets it seems to me there's little point putting a membrane in place. I mean, short of building something on the ceiling joists to stand on and hanging upside down from a crane like a trapeze artist.

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Originally Posted by ErrolFlynn

I’m trying to visualise someone doing this. Roof battens are ladder-like – they provide somewhere to walk (carefully) while moving about the roof. So, how can anyone roll out the membrane without roof battens to stand on?

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It's done all the time for tiled roofs. Roof Sarking | Roof Sarking in Melbourne Region, VIC

No idea why you'd bother for a metal roof. They usually just lay the sarking on top of the battens.

Not sure what country that diagram is sourced from but it is sort of correct in Australia but with a tiled roof

The typical installation is the membrane ( sarking) is rolled out across the rafters starting at the bottom of the roof so that it self flashes when you roll out the next run. The tile batterns are fixed on over the sarking. The roofers just walk on the rafters to carry out the procedure ( you don’t need the batterns to walk on for the experienced)
The drainage space batterns as they call it are to create a channel fir any water but I have never seen it done with a battern anymore but they use a foam pad that they call antiflap pads which create the same recess but also and their primary function is to stop the flapping sound of the sarking.

Go for a drive around a new housing development under construction and you will see it all happening and it looks quite effortless when you have the appropriate skills

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Originally Posted by ErrolFlynn

Hey, look! First post. This one is to consider the issue rather than me having a real problem.

Attachment 526062
This diagram is sourced here: Access Denied I got it from the Manuals / instructions section.

Paying attention to my red circle – roof membrane. It's positioned under the roof battens. How did it get there?

So, the rafters are in place. The diagram shows the membrane on top of the rafters. Okay, fine. Next goes the 'drainage space battens.' I presume they’re nailed to the rafters, and that would keep the membrane from being blown away if there's a wind about. Then the roof battens go on, followed by the roofing. That sounds reasonable. I'm just wondering how it's done.

I’m trying to visualise someone doing this. Roof battens are ladder-like – they provide somewhere to walk (carefully) while moving about the roof. So, how can anyone roll out the membrane without roof battens to stand on?

I'm also trying to figure out how the membrane would be taped to the next one without having anything to stand on. If the sheets meet up mid-rafter that could be a hell of a job, and if you don't tape adjacent sheets it seems to me there's little point putting a membrane in place. I mean, short of building something on the ceiling joists to stand on and hanging upside down from a crane like a trapeze artist.

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Ok my first reply here but I cannot access the pic as (it seems) my account has not been activated yet I can reply it seems.

The simple way is
roll out first run of membrane maybe standing on planks/trestles/scaffold
set lower batten then next batten then next batten (no idea of drainage channel you speak of)
roll out next run of membrane while standing on battens
set more battens and tape joint as you mention

Australian company with offices in Sydney and Melbourne
They specialise in eco environment passive design
The sketch is a conceptual render of various ways of achieving thermal comfort and efficiency

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Originally Posted by rwbuild

Australian company with offices in Sydney and Melbourne
They specialise in eco environment passive design
The sketch is a conceptual render of various ways of achieving thermal comfort and efficiency

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It is also a noisy option with no wool under the iron to deaden the sound :D

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Originally Posted by Beardy

It is also a noisy option with no wool under the iron to deaden the sound :D

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Being old school, I love the sound of rain on the roof, but also happy with the cooling effect of wool and siso.

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Originally Posted by jack620

It's done all the time for tiled roofs. Roof Sarking | Roof Sarking in Melbourne Region, VIC

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That’s an interesting link. I can see the sarking/membrane sheets running horizontally in the photo. I had assumed it would be rolled out vertically. Laying them horizontally allows the roof battens and tiles to be installed progressively. I didn’t consider that as an option. Though, it wouldn’t be possible with a steel roof.

This method doesn’t include the ‘roof drainage space battens’ but it probably doesn’t need them if the sheets are overlapping, because moisture drainage would be possible. It would still be a hell of a job trying to tape the sheets together.

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Originally Posted by jack620

No idea why you'd bother for a metal roof. They usually just lay the sarking on top of the battens.

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I’ve seen numerous videos with the foil/sarking being rolled out vertically, then followed up with the steel roofing sheet. It tends to sag between the battens. One of our forum members mentioned that he was advised that this was okay because the extra air gap can have the effect of increasing the R factor (this was with foil). Though, if the material is a semi-permeable membrane that allows water vapour to pass through and drain away, the foil sags between the battens would possibly capture moisture and leave it pooling if the roof, if the slope was only slight.

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Originally Posted by ErrolFlynn

That’s an interesting link. I can see the sarking/membrane sheets running horizontally in the photo. I had assumed it would be rolled out vertically. Laying them horizontally allows the roof battens and tiles to be installed progressively. I didn’t consider that as an option. Though, it wouldn’t be possible with a steel roof.

This method doesn’t include the ‘roof drainage space battens’ but it probably doesn’t need them if the sheets are overlapping, because moisture drainage would be possible. It would still be a hell of a job trying to tape the sheets together.


I’ve seen numerous videos with the foil/sarking being rolled out vertically, then followed up with the steel roofing sheet. It tends to sag between the battens. One of our forum members mentioned that he was advised that this was okay because the extra air gap can have the effect of increasing the R factor (this was with foil). Though, if the material is a semi-permeable membrane that allows water vapour to pass through and drain away, the foil sags between the battens would possibly capture moisture and leave it pooling if the roof, if the slope was only slight.

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Hop on this site

Home Page - THOR BUILDING PRODUCTS

and you should get a pop up regarding condensation requirements 2023, but if in doubt, give them a call and speak to their technical dept.
I have bought considerable amounts of insulation off them and they are very helpful.

Thanks for that. I'll check it out.

I must admit that I thought insulation was one of the simpler things in a building. However, the more I look into the subject the more I see how complex the technology actually is. And the differing viewpoints of everyone sometimes confound the issue rather than being enlightening, and that includes advice provided by the so-called experts.

try retrofitting insulation into the (lack of) roof space on a shouse (as my kids call it - a house that is a converted steel shed). so low pitch roof with gabled gyprock ceilings inside. thankfully, most of the place has been done already but.... i have started to reconfigure one large room into two bedrooms for my teenage sons. we had to remove the ceiling anyway as it was sagging as the original builder had not used enough battens. now with the ceiling out, (trim dek roofing sheets, 80-100mm thick insulation, and a couple of top hat battens are all that is above this spot) i have noticed a moisture line on the floor. its underneath a plaster bulkhead that was built to cover up the steel girder that runs along the top of the original shed wall. in this picture, the floor is the darker and the wall is the lighter coloured surfaces.

Attachment 526758

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Originally Posted by ErrolFlynn

Thanks for that. I'll check it out.

I must admit that I thought insulation was one of the simpler things in a building. However, the more I look into the subject the more I see how complex the technology actually is. And the differing viewpoints of everyone sometimes confound the issue rather than being enlightening, and that includes advice provided by the so-called experts.

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Put simply, insulation is simple, the internet has made it harder with so many opinions on insulation.

I have installed a lot of insulation and metal roofs and honestly never thought much about the insulation provided it was as tight as I could physically get it without tearing.
Now just this week I have been removing the almost flat trimdeck zincalume roof on the closed in car port and fitting 60mm wool and siso blanket.
Reason for this is the closed in car port is also the workshop and also the beer garden so it gets hot in summer (Qld)

I am half way so this morning I checked out condensation under the section of roof not insulated and no condensation.
I then checked out the open colorbond corro patio roof and it is literally dripping with condensation so,
my point is if the area being insulated is closed in, condensation should not be a problem.

Tile roofs IMO should have sarking under the tiles (sisilation but not insulation), and any insulation should be at ceiling level.
Reason for sarking is tile roofs allow water in during blows as it blows under the tiles and of course falls to the ceiling, not a good idea
Concrete tiles also become porous and after continual rain, start to drip, again not ideal
Zincalume/colorbond roofs don't need sarking but the home/building would benefit from wool siso blanket directly under the sheeting on top of the battens.

Keep in mind I am talking Qld where insulation is primarily for cooling in summer although some homes being built now days would even benefit with insulation in floors exposed to outdoors
eg bedroom/living areas above open patio.

When you refer to insulation and refer to it being tight I assume you mean reflective foil (single or double-sided). Before I took an interest in this subject I would have agreed with you that it should, as you say, be “as tight as I could physically get it without tearing.” I can see that being reasonable, but it doesn’t seem to be what others advocate. In fact, it’s commonly mentioned that leaving it to sag is the way to go. I believe that’s to create an air gap. Though you wouldn’t want it to sag to the extent that water pools in the saggy depression. They actually point out that if there is no air gap then there’s no point in having it as a radiant heat barrier.

I’m not quite getting your set up with your closed-in carport. Are you referring to there being three walls and a door like a garage sort of closed-in setup, or is it simply a roof (no walls) and if I were to look up when standing underneath I’d not see the roof sheeting because there’s plasterboard or some other kind of board up there as a ceiling? I’ll assume the latter.

You said you’ve been fitting 60mm wool and siso blanket. I presume you’re placing that over the rafters so that it’s in contact with the roof sheeting.

You say, “My point is if the area being insulated is closed in, condensation should not be a problem.” Presumably, this is the space between the roof sheeting and the ceiling. You’re saying that you found no condensation there and you put that down to it being sealed. I can see that because condensation will form when warm moist air hits the cold metal. And indeed, that’s what you found in the other section of the roof. It’s logical – if no warm moist air gets in there then there’s nothing to condense. The thing is, from what I’ve read, moist air will be there. Because you can’t see any condensation doesn’t mean it won’t happen, under the right conditions. From what I’ve read, the moist air will move through the ceiling (in a house), through the insulation if you have any and move up to the cold steel. This is why manufacturers talk about a membrane that can permit vapour to pass through on its way toward the roof sheeting, where it will condense, and when the droplets fall and drop onto the membrane they flow over it, down, and out into the gutter. (Water vapour can pass upward and through the material, but water cannot return back down.) At least, that’s what they say. Weird, I know.

Of course, the notion of warm moist air in buildings and cold roof sheeting is a particular issue in cool to cold climates, where you have the house shut up to keep the heat in. The moisture is coming from us as we breathe, cooking, and other living activities. If the moisture can’t get out of the house it’ll go through the ceiling. In your case, if your carport is open to the air on three sides I suspect the wind will be blowing most of the moist air away, and as you say, it doesn’t get a chance to move through the carport ceiling there will be no problem. And you’ve witnessed that. But would it be the same if you put up some walls around the carport to turn your carport into a fully enclosed room? I doubt it.

You make good points about the rain coming into the roof space with a tiled roof. Certainly, you can see daylight when in the roof space. If you can see out when inside the roof space then moisture can get in. Though, from what I’ve read, silvered sheeting is advocated in both tiled and steel roofs. The argument that I recall was that batt insulation on the ceiling will restrict heat movement through the ceiling and into the house, but if the heat continues long enough the heat will still get through. I just slows the process. But if you reduce the amount of heat getting into the roof space in the first place by using the shiny foil material then the bats don’t have to work as hard because the foil is making the job easier by preventing the radiant heat from warming the bats.

Enclosed car port = started life as a timber framed roof 10 m long x 4 m wide attached to a besser wall on one long side, open other side and tilta door at front, with battens at rear.
Now has roller door at front, tilta door from front moved to rear, and corro sheeting along side with one sheet corro plastic at side for light.
4 m of work benches with cupboards etc and most important the frig and the garage am fm stereo/vinyls player. Most important those 2.

Because it is (almost) fully enclosed (not totally sealed as a home would be) it is warmer so condensation is not forming as much as it does on the underside of the open patio roof in another area of the home.

It is physically impossible to stop a blanket of 60mm or even 50mm wool and sisilation from touching the underside of the roof when installed either over the battens or on top of the rafters considering the timber battens are 40mm (as opposed to 50/60 insulation) which means the insulation will be compressed by at least 10mm at the lowest point and of course getting closer to the underside of the roof and compressing more, as the insulation rises to go over the battens.

I have allowed the insulation to sag between battens only because it is impossible to install it without sagging unless I installed safety wire first and.
If you were to look up at any industrial/commercial building you will see the insulation is usually tight up to the underside of the roof usually because safety wire has been installed first.

Going back to the very first roof I insulated in 1965 we had clips which we hammered into the ribs of the cliplock roof from underneath and insulation batts were then lifted and the clips were bent under the insulation to hold the batts before the ceiling was installed.
As you can picture, the batts were in contact with the underside of the roof sheet.

In Qld I have never seen anyone attempt to allow the insulation to sag so it is not touching the underside of the sheet and honestly don't see any reason for it.

It's a bit overcast here today but if you wish, when the sun comes out I will take some pics of the insulation to hopefully give you a better idea.
Let me know .
This is one I took last week
Attachment 526953

In domestic construction having the insulation touch the underside of the roof is desirable as it reduces the sound transmission in rain. From an insulation point of view the whole premise of the insulation is to encapsulate air to prevent temperature transfer so I can’t see any practical differences if there was a sag in the blanket as the air is still trapped.
The reasons they put the wire on commercial jobs is firstly a Workcover safety issue plus typically the purlins are spaced much further apart with pan type roofing profiles than in domestic situations so the insulation needs to be supported by the wire.

Thanks for the update on the carport. It does seem enclosed from your description.

You mention, “If you were to look up at any industrial/commercial building you will see the insulation is usually tight.” Indeed so. Since my interest in insulation has peaked my eyes wander all over the place. Hardware stores, large sheds, etc. And yes, it seems to be drawn tight. However, what I can’t see is the roof sheeting (because, obviously the roof sheeting is obscured). So, here’s the million-dollar question: is there an air gap between the foil and the roof sheeting in those commercial buildings?

Manufacturers do emphasise having an air gap. I suspect that’s the reason for advocating a sag. There’s little choice in domestic applications on how the foil is installed. If you want an air gap then you need a sag. I wonder if in those industrial buildings, the trusses have been designed to support the foil in the ideal position that allows an air gap. It wouldn’t be too hard for the trusses to be built with the safety mesh and foil in mind so that when the safety mesh is attached to the trusses it rests at the perfect position so that there will be an air gap between it and the roofing sheets. Mounts could be welded in just the perfect positions on the trusses. And if the safety mesh has been attached to those fixing points that are in perfect alignment it will appear to be flat and when the foil is dropped on top it too will be seen to be flat.

Here's a photo of some foil being installed. Note the batten layout. Battens to support the tiles and others to provide an air gap. Any moisture that gets through the barrier will have no trouble flowing into the gutter.

I wouldn't be concerned about seeing the shiny side up. I've heard of it being done both ways, but more often with the shiny side down. It could also be the case that it's shiny on both sides.

Attachment 526954

Figure 5.1.2 provides some notes too on this link: https://cdn.dcs.bluescope.com.au/dow...llation-manual

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Originally Posted by ErrolFlynn

Thanks for the update on the carport. It does seem enclosed from your description.

You mention, “If you were to look up at any industrial/commercial building you will see the insulation is usually tight.” Indeed so. Since my interest in insulation has peaked my eyes wander all over the place. Hardware stores, large sheds, etc. And yes, it seems to be drawn tight. However, what I can’t see is the roof sheeting (because, obviously the roof sheeting is obscured). So, here’s the million-dollar question: is there an air gap between the foil and the roof sheeting in those commercial buildings?

Manufacturers do emphasise having an air gap. I suspect that’s the reason for advocating a sag. There’s little choice in domestic applications on how the foil is installed. If you want an air gap then you need a sag. I wonder if in those industrial buildings, the trusses have been designed to support the foil in the ideal position that allows an air gap. It wouldn’t be too hard for the trusses to be built with the safety mesh and foil in mind so that when the safety mesh is attached to the trusses it rests at the perfect position so that there will be an air gap between it and the roofing sheets. Mounts could be welded in just the perfect positions on the trusses. And if the safety mesh has been attached to those fixing points that are in perfect alignment it will appear to be flat and when the foil is dropped on top it too will be seen to be flat.

Here's a photo of some foil being installed. Note the batten layout. Battens to support the tiles and others to provide an air gap. Any moisture that gets through the barrier will have no trouble flowing into the gutter.

I wouldn't be concerned about seeing the shiny side up. I've heard of it being done both ways, but more often with the shiny side down. It could also be the case that it's shiny on both sides.

Attachment 526954

Figure 5.1.2 provides some notes too on this link: https://cdn.dcs.bluescope.com.au/dow...llation-manual

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The pic is overseas evident by batten size 75 or 100 x 25 or 30, valley gutter, and what appears to be left hand drive vehicle.
As to air gap in commercial/industrial buildings, there would an air gap because what you see is the siso but above that is the wool/fibreglass insulation usually 50-60mm thick
As to shiny side you are now referring to sisilation/building paper/condensate barrier/sarking only, not insulation as in wool/fibre blanket/batts.
Reason for shiny side usually down is to save installers eyes and bodies from what can often be uv rays
While siso does have some insulation properties, not much beats a siso/wool blanket under a metal roof especially in Qld where I have spent most of my life.
With sisilation only, I would agree to a gap between roof sheeting and siso, but again, maybe not practical on a 1 to 3 to 5 degree pitch metal roof.
The internet is, by and large, overseas principals, so in AU we need to be careful not to be hoodwinked into going with principals which may actually cause us more problems than they solve.