rafter insulation query regarding multi-foil

neilw · 29 Aug 2009

Hello,
In trying to save space I've been looking at multi-foil for the ceiling insulation on my loft conversion. Given TLX (thinsulate) and Gen-X have BBA approval I figured it must be ok. I'm trying to work out how deep it will actually go to compare against normal insulation but there are two installation diagrams and I'm not quite certain.

Both have 70mm PUR then 25mm air gap then multi-foil, followed by a 25mm air gap and finally the plasterboard.

However one has the PUR right next to the breather membrane below the tiles and the other has a 50mm gap between the breather and the PUR.

Can anyone tell me which I require, I always thought you needed an airgap, but I figured there must be a reason. If it helps, my current rafters are 100mm. Obviously I'd rather go for the smallest (i.e. no 50mm air gap), but if it's needed then I'll do it.

If it helps, the diagrams are here:

http://www.just-insulation.com/pdfs/multifoil/Thinsulex/Thinsulex_Silver_Install.pdf

(namely options A and C)

Thanks for any help.

jeds · 30 Aug 2009

The diagram with the 50mm gap is because the undertile membrane is not breathable. Therefore you need a ventilated gap. Using a breather membrane avoids that.

On your overall scheme. Personally I would upgrade the rafters to 125mm, fit 100mm PUR between rafters and 25mm beneath. This gives you a U-value of 0.20 and saves a lot of messing about. And without adding it up isn't that less overall space than the multifoil stuff?

neilw · 30 Aug 2009

I need to double check, but I believe the part L requirement for my area is 210mm of standard rockwool (if using rockwool). I chose the multi-foil to reduce this height but keep the same thermal and hopefully noise levels.

My existing loft membrane isn't breathable. Does that mean with your suggestion I'd need 50mm air gap, 100mm PUR and 25mm air gap?

Would that pass regulations and will 100mm PUR be better than a 70 PUR with a multi-foil? Some of the diagrams I'm looking at with the installations for multi-foil are using 80mm PUR so I can't see how 100 will be sufficient to pass? Are you talking about something like Celotex GA3000?

It's all quite confusing

btw, would I use PIR or PUR, I don't really know the difference, just the multi-foil has PUR.

Thanks.

^woody^ · 30 Aug 2009

If using just insulation board, then you need 75mm between the rafters, and 50 across the underside.

BCO will normally ask for a 50mm continuous air gap to the underside of the felt.

To fit foil insulation then it needs an air gap between the foil and the plasterboard, so that's 50-60mm below the underside of the rafters too.

If the BCO wont accept the foil on its own, then it will need some random extra insulation, and the BCO will have to tell you what is acceptable additional insulation.

So add all this up and it means that you are going to lose at least 60-75mm below the rafters. So clarify what the BCO wants in addition to the foil.

Or fit 100mm between the rafters (no air gap) and 35mm to the underside, then board and skim giving 50mm below the rafters which is probably as good as you will get.

But clarify your proposal with the BCO first

neilw · 30 Aug 2009

Thanks, that's a lot clearer. I'll go and see building control.

I doubt they'll let me go for the last option given there will be no air gap.

However, I've checked my specification which was passed and that says 50mm kingspan between rafters and 50mm kingspan after rafters. I can't see that achieving 0.2u given what I read elsewhere.

On checking, tlx do a new product called rafterfix which is designed for 100mm rafters and is designed to provide the between rafter insulation and you add 70mm PIR on top, which adds up to 170mm plus the plasterboard which is a saving of just 5mm if I do have to use 75mm in the rafter. I really don't see the point in multi-foil if it is just going to save 5mm!

jeds · 1 Sep 2009

With non breathable felt you will need a 50mm air gap. Using multi foil the overall thcikness will be 192 but it's a lot of mucking about. Upgrade the rafters to 150mm, fix 100mm pur between and 30mm beneath plus plasterboard is 194mm but a lot easier, and probably cheaper, to do.

neilw · 1 Sep 2009

Thanks again.

I've been doing more research into this and apparently if I do use multi-foil and put PIR/PUR before it (i.e. within the rafter) as some are recommending then there is the potential for condensation due to the differences in heat levels between the two.

I've also checked my plans (which are 3 years old) and the value required then (and is still valid as the plans were approved) is 0.25, hence why it was only specced at 100mm PIR.

I'm now seriously considering doing the following:

1. 50mm air gap
2. 50mm accoustic rockwool
3. actis super 10 (no gap required between wool and foil)
4. air gap
5. two layers of plasterboard.

This should give me adequate noise reduction, a decent thermal value (according to the blurb, which I know you have to take with a pinch of salt) and not a huge loss of roof height - about 170mm

I guess the thing to remember is the peak of the roof will be flat and so stuffed with standard insulation, as will a large proportion of the roof given it will have side walls where I can stick as much insulation as I want and an inch in height is a fair bit in reality. The gables will be fully insulated as normal.

Combover · 3 Sep 2009

I would be supprised if your BCO won't accept: felt / 50mm airgap / 50mm PUR / TLX (or Tri-Iso S10 etc) / counterbattens / 12.5mm p'board.

Is flanking sound a problem in your roof?

neilw · 3 Sep 2009

Hello,
I don't think so. Well, I've never noticed any before in the house. I have a detached house in a quiet area.

With regard to noise, my only concern is that coming through the roof given it's higher up and is a lot thinner than brick.

Would you recommend accoustic plasterboard, or two layers of plasterboard or maybe a layer of some other sound insulation on the roof or gable ends?

Looking at the accoustic rockwool I mentioned I wonder if this is ok fitted right next to the multi-foil without an air gap given it's twice as dense at least...

Combover · 4 Sep 2009

With the felt you have you will need an airgap between any insulation and said felt. No airgap is required between the multifoil and any other insulation used to back it up.

You could use sound block plasterboard but i'd be dubious over the benefits when compared to normal plasterboard.

I've not heard anyone claim that their loft convesion is too noisy and these are normally constructed in a similar way to the way I described above.

You could use mineral wool to back up the multi-foil but I doubt whther you'd be able to get the required amount in with the rafter depth you have. It's also difficult to maintain your required airgap but would be much better the PUR in terms of sound.

neilw · 4 Sep 2009

ah bums. Just realised from the structural drawings that the 100mm rafters are being replaced/supported by 150mm ones. So what I need to do now is find out the best way of insulating without going over the rafters at all (just plasterboard), i.e. I have a straight run of 100mm inside the rafter to get me 0.25u and the best noise reduction.

hotrod · 4 Sep 2009

neilw said:
ah bums. Just realised from the structural drawings that the 100mm rafters are being replaced/supported by 150mm ones. So what I need to do now is find out the best way of insulating without going over the rafters at all (just plasterboard), i.e. I have a straight run of 100mm inside the rafter to get me 0.25u and the best noise reduction.

As you know current Regs are 0.2 W/m2K, but if you want to achieve min 0.25 W/m2k you are still going to have to use more than 100mm PUR between the rafters with a 50mm air cavity. Why can't you just use 100mm PUR between the rafters, then underdraw with say 25mm PUR, then overboard. That will get you to 0.25 W/m2K I would have thought.

neilw · 4 Sep 2009

Hello,
My previous quote from celotex for 50mm in the rafters and 45 outside the rafters gave a u rating of 0.25. I'm hoping 100mm celotex just inside the rafters (with insulated plasterboard outside) will be the same. My regulations go back 3 years which had a rating of 0.25u.

If not I may well have to scrap the conversion as I think the room height will just not be big enough and I'd have to spend another 5-10k on full length dormers.

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rafter insulation query regarding multi-foil

hotrod

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