Air tightness question

Superdaddy

I had a meeting with an air tightness contractor on site. He told me that I need a cavity between airtight membrane and the plaster slab. Make up is warm roof so insulating the pitched roof. 100mm PIR on the 9x2 rafters, full fill of metac between rafters then airtight membrane then standard 12mm plaster slab. He says a 2x1 batten is needed between the membrane and slab. I question why. His answer was for services. I told him the house has been designed so that no service are in the pitched roof. He says it's still needed, though he can't give me a reason for it. It was basically a case of "it's just how we've always done it".

I can't see the need for it, it reduces roof height and is an increase of material and labour. Anybody have any views on this?

hexosan

No need for it

Rabbo

It's sole function would be to minimise/eliminate service penetrations into the AT layer. if you have no services fitted along the slope, there is no need for it.

galwaytt

No need.

I mean, that's like someone telling you that you have to have a door or a window that you don't need. If it's not needed, it's not needed, end of.

Superdaddy

Thanks lads, just as I thought so. Doesn't inspire me in confidence in his air tightness.

Calahonda52

Superdaddy wrote: »

Thanks lads, just as I thought so. Doesn't inspire me in confidence in his air tightness.

I have seen it used before as it makes it easier for fitting the slabs as the lathes are visible and therefore reduced risk of the AT being punctured

ferryman35

Apart from providing a service cavity, another reason for counter battening inside the air tightness membrane is to protect the membrane.

If you fix the plasterboard directly over the membrane, then every drywall screw will puncture the membrane, whereas if you batten over the membrane (with a suitably sized batten!), you'll be then be fixing the slabs to the batten.

Now, if its done right, the side of the batten in contact with the membrane will be taped with a special sealing / foam type tape that seals the considerably reduced number of punctures from nailing the battens to the rafters.

Similarly, the underside of the counter battens on the outside should be treated with the same tape where they are fixed to the rafters.

I'm not sure by reading your post whether its too late to ask.......but is that construction breathable (or sufficiently breathable). The airtightness membrane is a vapour barrier internally, so the roof has to be able to breath from the inside out.

galwaytt

The above is only true of the screws due the plasterboard are no longer than the depth of the batten - which they're not. 50 - 75 drywall screws is the norm, so they'll be going through the membrane behind the batten anyway.

Superdaddy

ferryman35 wrote: »

I'm not sure by reading your post whether its too late to ask.......but is that construction breathable (or sufficiently breathable). The airtightness membrane is a vapour barrier internally, so the roof has to be able to breath from the inside out.

According to the building regs part F I am fine and my engineer agrees. It's a warm roof build with vapour barrier on the warm side and breathable materials on the cold side.

How is the vapour barrier normally attached to the rafters, staples? Is an airtight mastic needed on the rafters for these penetrations?

ferryman35

We used a 44mm batten and with 12mm plasterboard, so the 40-50mm screws did I not go through the membrane, but even if they did the sealing tape was still there.

The tape is sticky on both sides and it stops the hole made by the penetration from stretching.

You've obviously checked the breathability question, but is pir board breathable. I heard a some stories about this detail being got wrong, with very expensive consequences.

Calahonda52

galwaytt wrote: »

The above is only true of the screws due the plasterboard are no longer than the depth of the batten - which they're not. 50 - 75 drywall screws is the norm, so they'll be going through the membrane behind the batten anyway.

50mm to 75mm on a 12 mm slab is the norm?

Madness

rampantbunny

Not to derail, but I think this is along the lines of some other thread content.

I'm looking at installing 50mm wool type insulation underneath my rafters. I'll need to fix battens and place the 50mm wool between these. This will give me a service cavity if needed, and I'm sure would be not unlike what the AT contractor advised the OP.

I can't imagine the 50mm wool staying in place for long enough to allow others come and fix plasterboard over. Anyone any ideas e.g. run some cheap tape across the battens as the insulation goes in?

Secondly, if installing battens, how many and how far apart should they be kept for plasterboard fixing?
Don't want to overdo it and pepper the AT membrane...

galwaytt

Calahonda52 wrote: »

50mm to 75mm on a 12 mm slab is the norm?

Madness

Is not a technical thing: it's to do with stocking as few variations in screws as possible. Is overkill in some cases certainly. But if a contractor is doing single slabbing one day, double on another, then they don't want to keep changing the screws in the magazines.
A 70mm screw for example works for double slabbing on a 50mm. batten..

cork2

galwaytt wrote: »

Is not a technical thing: it's to do with stocking as few variations in screws as possible. Is overkill in some cases certainly. But if a contractor is doing single slabbing one day, double on another, then they don't want to keep changing the screws in the magazines.
A 70mm screw for example works for double slabbing on a 50mm. batten..

But could you really justify using 70mm screws in a normal application?! They're expensive!! If my lads wasted 70mm screws on 12mm slabs I'd be telling them to bring a dozen boxes with them tomorrow seeing as there so cheap!! How long does it really take to reload the magazine!

mfceiling

cork2 wrote: »

But could you really justify using 70mm screws in a normal application?! They're expensive!! If my lads wasted 70mm screws on 12mm slabs I'd be telling them to bring a dozen boxes with them tomorrow seeing as there so cheap!! How long does it really take to reload the magazine!

If you've a regular screw gun it's only a matter of lifting a 38mm screw as opposed to a 32mm.

The magazine for the self feeding guns takes about 5 seconds to change the depth.

cork2

I know that, I have three self feeding guns. But I still wouldn't agree with wasting long screws. Look I'm not trying to start an argument at all, it'd be of no benefit we all have our own ways I suppose.

Superdaddy

I've been to a couple of houses and seen air tightness done by the professionals. They seem to be stapling the membrane to the rafter. I couldn't see any sort of sealant or double sided tape under the membrane on the rafter. It doesn't seem right to me. My thinking on this is that if you want air tightness you need some way of sealing where staples and screws penetrate into the wood.
Or will the slab and skim coat seal these?

cork2

Superdaddy wrote: »

I've been to a couple of houses and seen air tightness done by the professionals. They seem to be stapling the membrane to the rafter. I couldn't see any sort of sealant or double sided tape under the membrane on the rafter. It doesn't seem right to me. My thinking on this is that if you want air tightness you need some way of sealing where staples and screws penetrate into the wood.
Or will the slab and skim coat seal these?

The holes in the membrane made by the staples is so tiny it'd make no difference.

Superdaddy

cork2 wrote: »

The holes in the membrane made by the staples is so tiny it'd make no difference.

Is there any merit to using a double sided tape on the rafters?

hexosan

Isover's vario Xtra safe is fleece lined on the back and you use a double sided tape on the rafters.

I spoke with on of th -snip- before about the staples in the intello membrane and they said one project they worked on the client went back after and struck a piece of tape on each staple. They reckoned there's no need for it at all.

I assume since theses are the two largest Airtightness brands it's on to name them.

cork2

Superdaddy wrote: »

Is there any merit to using a double sided tape on the rafters?

I haven't seen it done with double sided tape, just with staples, but I've only seen two different contractors do it so I wouldn't be hugely experienced. Hexosan sounds to be more in the know than me tbh.

If in doubt contact customer care for the membrane manufacturer and request a data spec sheet and fixing instructions. Also recently most architects or engineers or whoever is designing and minding the project does an airtightness spec.

893bet

cork2 wrote: »

I haven't seen it done with double sided tape, just with staples, but I've only seen two different contractors do it so I wouldn't be hugely experienced. Hexosan sounds to be more in the know than me tbh.

If in doubt contact customer care for the membrane manufacturer and request a data spec sheet and fixing instructions. Also recently most architects or engineers or whoever is designing and minding the project does an airtightness spec.

I am using both.

Double sided tape and some staples aswell to ensure it stays in place before slabbing and the first blower test.