Loft conversion insulation

[pilsbury]

Christ it’s hard going to get started! Won’t go into the architect/SE design saga as I’ll just work myself up. Must be a boom on as I can’t give my money away!

It’s going to be self build so I’m using my energy to contemplate insulation. Getting mixed messages as to whether U values of the roof need to be 0.18 or 0.16. Which is correct?

Also type of insulation and method - typically PIR between rafters then a layer over the top before plasterboard. What thicknesses and where are the air gaps? My roof is sound but old and has no felt. Just plain tiles. 25mm between tile and insulation? Any gap between the full layer over the rafters and plasterboard?

what about this super quilt foil stuff? Looks easier. Again, mixed messages as to whether insulated plasterboard needs to go over the 2 layers you install.

advice on method, ease of installation and cheapest appreciated. (Cheapest is my language)

 

[JP_]

If you have room, put in more than current BS - more insulation, cheaper bills, less sweating in summer too.

 

[^woody^]

Typically 100 Celotex between rafters and 50 across the underside. If you have no felt you wont need to worry about a vented void, but you should seriously consider re-tiling and fitting a membrane as part of the work.

Don't waste your energy even thinking about multifoil.

 

[pilsbury]

If you have room, put in more than current BS - more insulation, cheaper bills, less sweating in summer too.

Understand this. Hot in summer, cold in winter seem to be known issues

 

[pilsbury]

Typically 100 Celotex between rafters and 50 across the underside. If you have no felt you wont need to worry about a vented void, but you should seriously consider re-tiling and fitting a membrane as part of the work.

Don't waste your energy even thinking about multifoil.

Stripping and retiling the roof is not really on the agenda. It’s a monster task that I don’t have the appetite for. Especially as the roof is in good order and currently leak free. Understand this would be the ideal gold standard, but not sure I have the motivation. It’s been solid for 100 years.

Plus you’ve almost convinced me not to by stating I need no air gap without membrane. I can just bung the 100mm in between rafters without battens. Saves me an inch and some faff.

so the order of the day looks like a total of 150mm PIR with plasterboard on top with no air gap. Correct?

 

[RandomGrinch]

Hi,

If you haven't seen any of the videos on this YouTube channel, they may be worth a look.
The videos cover the build of loft storage rooms and full conversions.

This one, in particular shows some of the extra work required battening out for the multifoil:

Good luck

 

[Ian H]

The Celotex stuff has Zero effect on noise, that’s the worst bit of ours.

 

[catlad]

Don't bother with the multi foil stuff, don't let your insulation touch your batons.
If it was my house I would put a layer of 11mm osb under the rafters, it will strengthen the roof and also allow to screw in the ceiling anywhere and you will hit a fixing point.

 

[pilsbury]

That chap in the video recons that space foil blanket is equivalent to 100mm PIR. Find that hard to believe. But I like the space ship look!

 

[pilsbury]

The Celotex stuff has Zero effect on noise, that’s the worst bit of ours.

Really? I thought it would being quite dense. You got any ideas on how you’d do it different?

 

[pilsbury]

Don't bother with the multi foil stuff, don't let your insulation touch your batons.
If it was my house I would put a layer of 11mm osb under the rafters, it will strengthen the roof and also allow to screw in the ceiling anywhere and you will hit a fixing point.

I think I’ve been shifted away from the foil stuff.

as for the osb under the rafters…. What are you suggesting? 100mm pir between joists…. Then what? 50mm pir then osb? Sounds like a lot of weight…. But might cure Ian’s noise issue.

 

[mikeey84]

That chap in the video recons that space foil blanket is equivalent to 100mm PIR. Find that hard to believe. But I like the space ship look!

Pretty sure the BBA certificate doesn't say its worth 100mm of PIR!

 

[catlad]

Yes 100mm between rafters then 11mm osb then 50mm insulation then foil backed plasterboard.

 

[Ian H]

Really? I thought it would being quite dense. You got any ideas on how you’d do it different?

I’m not sure but I’d have to do something. Maybe overbatten under the 50mm and use the SoundSlab stuff between the battens.

 

[JP_]

That chap in the video recons that space foil blanket is equivalent to 100mm PIR. Find that hard to believe. But I like the space ship look!

I found that I had the foil in the cavity of my 2002 Barratt home - only discovered when I had a loft conversion done.
Looked pathetic really, the builders at the time must have had a good deal on it, and believed it was better than rockwool.

Oh, bit of research shows that this was standard at the time:

"A 2002 house will already have some form of cavity insulation because the Buildings Regulations at the time required this. These requirements were always being upgraded. If it was a Barratt home for example, this could be as simple as as an air bubble silver foil wrap around the inner leaf. This was being done circa 1997 onwards. Others might have, say 35 mm Celotex. I recall this standard having been reached by 2003."
https://forums.moneysavingexpert.com/discussion/5750359/cavity-wall-insulation​