Mon, 25 May 2026 06:11:27 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301439#Comment_301439 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301439#Comment_301439 Thu, 04 May 2023 11:10:54 +0100 alm
I'm finally ready to order the boards and get insulating our loft bedrooms. It's a room in the roof on a hip, so I've got three pitches and a central horizontal section. I'm only doing under the rafters as the rafters are only just 50mm. I'll be trying my very best, but inevitably I think I'm going to struggle to make all the different angles meet flush on the insulation boards. I'm sure this is a basic question for a lot of you, but are there any recommendations for filling any gaps between the boards, just to keep the thermal qualities as much as possible. I know foam is an option, I'm not a big fan, but I will use it if its the only way. I was also worried about it expanding to touch the roofing underlay.

Any tips greatly appreciated, I've already got lots of others from reading through the forum but can't find anything on this in particular.

Thanks]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301441#Comment_301441 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301441#Comment_301441 Thu, 04 May 2023 12:23:38 +0100 neelpeel
I would PU foam fill, cut off the excess (I found a bread knife quite useful!) then aluminium foil tape.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301442#Comment_301442 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301442#Comment_301442 Thu, 04 May 2023 13:07:36 +0100 alm https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301444#Comment_301444 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301444#Comment_301444 Thu, 04 May 2023 13:37:15 +0100 WillInAberdeen
Now I have given up and cut them deliberately 3mm too small all round, they fit first time and I can get a good squirt of foam into the gaps.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301445#Comment_301445 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301445#Comment_301445 Thu, 04 May 2023 14:41:54 +0100 Peter_in_Hungary Has there ever been a study or other experience that shows that the temp under tiles can have a long term detrimental effect to EPS used between rafters?

trawled from the internet.

What temperature does EPS melt?
EPS will soften at a temperature of only 73 C (165 F), which will reduce its thermal performance capability. At 100 C (212 F), EPS begins to melt and drip, which can result in the complete loss of the insulation's thermal efficiency.

And

Thermal degradation of foamed polystyrene

S. Mehta, S. Biederman & S. Shivkumar

Journal of Materials Science volume 30, pages 2944–2949 (1995)
............... The results indicate that, when exposed to elevated temperatures, the polymer beads collapse at about 110–120 °C. The collapsed beads melt at 160 °C...........]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301446#Comment_301446 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301446#Comment_301446 Thu, 04 May 2023 19:25:23 +0100 Jeff B Posted By: WillInAberdeenAs a DIYer, I spent a long time trying to cut boards accurately to fit tightly, shaving a bit off here and there.

Now I have given up and cut them deliberately 3mm too small all round, they fit first time and I can get a good squirt of foam into the gaps.

100% agree - I too found out the hard way! Buy a professional foam gun with low expansion foam. The flow is easily controllable and even if you overdo it a bit, there is not much waste with the low expansion variety. When it's hardened, cut off the excess with a sharp, non-serrated, bread knife.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301447#Comment_301447 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301447#Comment_301447 Thu, 04 May 2023 19:27:02 +0100 Jeff B Posted By: Peter_in_HungaryUnder tiles can get v. hot especially if the roof faces S.
Has there ever been a study or other experience that shows that the temp under tiles can have a long term detrimental effect to EPS used between rafters?

Peter - the OP intends to use PIR boards though?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301453#Comment_301453 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301453#Comment_301453 Fri, 05 May 2023 09:59:38 +0100 Peter_in_Hungary Posted By: Jeff BPeter - the OP intends to use PIR boards though?
Oops - However
another study
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwiQvOGT293-AhVPhf0HHSExDq8QFnoECD0QAQ&url=https%3A%2F%2Fwww.mdpi.com%2F2073-4360%2F14%2F22%2F4926%2Fpdf&usg=AOvVaw1aC9x6tHTykQ9Fetn0NkNN

Abstract: Foam insulation materials are widely used in the construction industry due to their low
thermal conductivity attributable to their microstructures and their low-conductivity blowing agents
and affordability. In this study, we evaluate how the thermal performance of foam insulation materials
used for the exterior walls of buildings, viz., extruded polystyrene (XPS), polyisocyanurate (PIR),
and phenolic foam (PF), age over the life cycle of a building. To compare the aging of thermal
performance during the life cycle of a building, each material was tested at 70 and 110 â—¦C and with
slicing acceleration according to EN and ISO standards. The thermal conductivity of each foam
insulation material was measured using a heat flow meter at an operating temperature of 23 â—¦C and
converted into thermal resistance values. Different foam insulation materials have different aging
procedures according to material-specific EN standards, while ISO 11561 applies the same procedure
to all material classifications. Upon comparing the aged values according to ISO and EN standards
to the initial values, the analysis showed a change rate of 23 to 26% in PIR and 18 to 20% in PF. In
XPS, a rate of change of 10 to 23.8% was calculated

End Quote

So it would appear that solid board insulation will degrade over time with heat. I suppose the next question is - will the temps of the insulation under tiles get to the temps used in the test? Probably at the top surface but not all through and what degradation over time will be seen (if any) in real life application.

Glass or mineral wool is good to about 1000 degC is cheaper than board insulation but greater thickness is needed for the same U value and whilst easer to instal (IOM) they can suffer from slump over time and compression of the wool can result from careless installation.

Any advantage in using PIR on the north and east side and glass/mineral wool on the south and west side ?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301457#Comment_301457 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301457#Comment_301457 Sat, 06 May 2023 14:49:16 +0100 CWatters
https://www.screwfix.com/p/diall-aluminium-foil-tape-silver-45m-x-75mm/4415V?kpid=4415V&ds_rl=1249416&gclid=Cj0KCQjw9deiBhC1ARIsAHLjR2DG6fNuYwUvAv4c3UxJttSgD5dl4xLczTwN80lOSeiAhHR4pCGP8KsaAhaBEALw_wcB&gclsrc=aw.ds

Mobile phone reception may well be affected.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301465#Comment_301465 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301465#Comment_301465 Sun, 07 May 2023 08:23:53 +0100 Jonti https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301466#Comment_301466 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301466#Comment_301466 Sun, 07 May 2023 09:18:59 +0100 WillInAberdeen
https://spsenvirowall.co.uk/assets/technical-resources/technical-data-sheets/2625_SPSenvirowall_Limited_Expanding_Foam_TDS.pdf]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301468#Comment_301468 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301468#Comment_301468 Sun, 07 May 2023 16:28:55 +0100 revor When it came for us to do the floors I ordered 100mm and 50 mm boards to cross lay but when they arrived the 100 mm boards were concave all around the edges they had shrunk in the same way as the builder had described. They were rejected and replacements obtained. The merchant accused me of being too fussy and any one else would have used them. One of the reasons I reckon why houses do not perform.
Coming back to the original posting I recall some years ago seeing at an exhibition a foam tape product you wrapped around the slightly undercut board and push fitted it between rafters and the tape took up the gaps. But if the board shrinks then not much use I guess. Recall it being expensive I not recall the name of it.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301469#Comment_301469 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301469#Comment_301469 Sun, 07 May 2023 20:26:10 +0100 djh Posted By: revorTo his horror he discovered that there were big gaps between the rafters and the PIR. They had shrunk with edges forming a concave. When we did our build we put 170 mm rockwool between truss rafters and overlaid 100mm PIR boards and taping joints and fixings with aluminium tape so as to avoid such an event.Does the aluminium tape stop the boards shrinking then?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301470#Comment_301470 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301470#Comment_301470 Mon, 08 May 2023 08:53:23 +0100 revor Posted By: djhDoes the aluminium tape stop the boards shrinking then?
I can see the ambiguity in my post. It is the rockwool that prevents the shrinkage not the taping.
I would not have thought tape would be enough to stop shrinkage it would probably pull off or split but In my situation as there is the 170 mm rockwool between the slate roof and membrane so guess would not get hot enough. The joints between the boards rest on the truss timbers this also avoids the "cold bridge" through the timber the tape helps make the joints more air tight. The boards are secured with long screws and plastic wall tie retaining clips for insulation into the truss timbers. The screw heads and securing rings are also taped over. Having had the report from the builder and my experience with my delivery I wonder if PIR between rafters is a good idea. I also wonder how SIPS roofs behave.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301471#Comment_301471 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301471#Comment_301471 Mon, 08 May 2023 10:55:52 +0100 djh https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301963#Comment_301963 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301963#Comment_301963 Mon, 12 Jun 2023 14:39:32 +0100 kebl2825 Posted By: revorComing back to the original posting I recall some years ago seeing at an exhibition a foam tape product you wrapped around the slightly undercut board and push fitted it between rafters and the tape took up the gaps. But if the board shrinks then not much use I guess. Recall it being expensive I not recall the name of it.

For anyone else looking, I think the product you're referring to is Gapotape.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301965#Comment_301965 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301965#Comment_301965 Mon, 12 Jun 2023 18:23:01 +0100 cjard
I'll get some pics if I can at some point..

Now I'd probably say wool between rafters and board over. I might even be tempted to put two layers of half thickness boards in with staggered joints


I'd also use a decent foam, not a Soudal can. The two part foam I used to fill in between my PIR in the roof of my current house, was amazing..
-

Alu tape will not stop board shrink]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301970#Comment_301970 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17829&Focus=301970#Comment_301970 Mon, 12 Jun 2023 20:29:46 +0100 Nick Parsons