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.
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.