Insulating a pitched roof

[vanilla]

Sorry if this has been done to death but I did a search and couldn't find quite what I wanted.

I've recently been looking into insulation for my sister who's doing out her attic room. The house is a terrace built c. 1900 and aside from a window it's all original, just lath and plaster between the room and the roof. Rafters are 100mm x 50mm as far as I can tell.

After a bit of looking around we came across TLX rafterfit multi-layer foil stuff as an alternative to batts/foam board. Basic plan is foil between rafters then maybe 70mm PIR over the top of them and duplex plasterboard over the lot (is duplex overkill here due to there being foil on the two layers behind it?).

There are a couple of questions/problems I have with this situation:

1) The rafters are supported by several 12"x4" beams. If we go ahead with the above plan, the beams are still going to be well exposed, will there be a problem with thermal bridging and if so how much so? They are after all going to pass right through the middle of the insulation.

I had considered building some kind of framework within the beams and making the room smaller, but it's not something I really want to do.

2) Are these multi-layer foil products actually worth getting, particularly the more expensive ones? When reading around I found an article about Actis Tri-Iso and how it's claims of ~5m²K/W were found to be misleading, the real value more like 1.7m²K/W. Actis countered by saying the product wasn't installed correctly as it wasn't airtight on the living side.

If that's the case, does it really make a great difference what product you use if 2/3 of the claimed benefit comes from the installation preventing convection?
e.g. If Airtec Double has an r-value of 1.45 and Rafterfit is 1.69, is there any sense in paying hundreds of pounds extra for the latter when I could fit the Airtec in the same way and get 10mm thicker foam to compensate? Is there something I've missed?

Mmm, that ended up being longer than I anticipated, sorry if it's a bit waffly.

 

[freddiemercurystwin]

What’s the point/advantage of faffing around with a foil layer insulation when you're using some PIR anyway? You may as well just use thicker PIR and forget about the foil layer stuff. Phone your supplier eg Kingspan, have the beam sizes, rafter sizes/spacing’s available and they'll be able to claculate how much cold bridging will occure and they'll tell you exactly what thickness PIR you require. Will probably be around 130mm in total. 100mm between the rafters and a 30mm layer below.

 

[Chukka63]

as above but with no pics hard to comment fully.i personally would leave the beams showing,if in deed they do now.insulate the rest as freddy said .at the end of the day youre still upgrading what your sister has and keeping some features too.

 

[vanilla]

What’s the point/advantage of faffing around with a foil layer insulation when you're using some PIR anyway? You may as well just use thicker PIR and forget about the foil layer stuff.

TLX claim you can achieve a U-value of 0.2 with Rafterfit and 60mm PIR. The price difference between that and all PIR isn't much plus the foil is easier and quicker to install, though if we could use the cheaper foils it would be a relatively big saving.

Ignoring the fairly small insulation values of the internal/external air films, plasterboard, etc, the R-values work out at:

Celotex GA3060 60mm PIR below rafters R 2.6
TLX Rafterfit between rafters R 1.7
25mm unventilated air gap R 1

Total R = 5.3, U = 0.188, Cost of insulation = £700

Celotex GA3060 60mm PIR below rafters R 2.6
Celotex GA3060 60mm PIR between rafters R 2.6

Total R = 5.2, U = 0.192, Cost of insulation = £650

Celotex GA3060 60mm PIR below rafters R 2.6
Airtec Double between rafters R 1.45 (fitted in same way as Rafterfit)
25mm unventilated air gap R 1

Total R = 5.05, U = 0.2, Cost of insulation = £460

Is this a viable way of doing it? There's a lot of work needs doing on the house so a £200 saving is a big deal to us.

Phone your supplier eg Kingspan, have the beam sizes, rafter sizes/spacing’s available and they'll be able to claculate how much cold bridging will occure and they'll tell you exactly what thickness PIR you require.

I'll give that a go.

Will probably be around 130mm in total. 100mm between the rafters and a 30mm layer below.

Aren't we supposed to leave a ventilation gap under the inner surface of the roof? The rafters are only 100mm deep.

as above but with no pics hard to comment fully.

I'm more interested in the wellbeing of the occupants than having a pretty piece of wood across the ceiling

 

[freddiemercurystwin]

Aren't we supposed to leave a ventilation gap under the inner surface of the roof? The rafters are only 100mm deep.

If you have felt then yes you should, you asked a specific question about insulation not ventilation. I did not mention ventilation (or the multitude of other items needed to be considered when converting lofts) as you seem to be on the ball. All you do is have a thinner layer (50mm) between the rafters and a thicker layer beneath the rafters.
If you can achieve the U values you require using your foil layer insul and PIR combined and it works out cheaper for you and moneys is that tight then go right ahead. Despite most building control departments now accepting the foil layer insul for loft conversions there is still suspicion in the trade as to the merits of this type of insulation as your research has revealed. I do not ever recall anyone posting on here with a suggestion to use two different types of insulation in a roof above a loft conversion hence my suggestion to keep things simple.

 

[Chukka63]

tri iso was banned i believe a few years back and TLX looks the same.anyone know the current regs on this???

 

[freddiemercurystwin]

Tri iso was indeed banned as you put it however it is now fairly widely accepted by most Building Control departments. It would be prudent to check with your LA before purchase.

 

[vanilla]

I spoke to a building control surveyor at my local council and I was a little surprised by what he said after reading the responses here. He said he would use the foil rather than filling the rafters with PIR, so long as the gap between the foil and the inner surface was sufficiently airtight.

I'll inspect the inside of the roof, particularly at the bottom where air is more likely to find it's way up the inside of any insulation. If it looks like it will be tricky to seal it well and make an unventilated airspace using foil then I'll go with the PIR between and under the rafters.

Thanks for the replies, I've read around the subject a lot but because I've never actually done it before I felt a little nervous and wanted to make sure it was right from the start.