Tue, 19 Dec 2023 08:38:13 +0000 http://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300368#Comment_300368 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300368#Comment_300368 Mon, 20 Feb 2023 20:09:15 +0000 philedge
Question is how to insulate. The interior has been decorated in the last year or so and with a second baby on the way they really dont want any/much disruption inside. There's enough eaves overhang to get a bit of EWI fitted so thoughts are to work from the outside, remove the external cladding, fit 50/75mm of PIR between the studs and a further 75 over the outside of the studs. Vertical battens fixed through the PIR into the studs with timber cladding fixed to the batten.

With no VCL are we going to hit problems even though weve got an equivalent or more insulation on the outside than within the studs? Second question is, if we can use foil faced insulation, do we need a membrane over the outside of the insulation?]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300373#Comment_300373 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300373#Comment_300373 Tue, 21 Feb 2023 08:26:15 +0000 Nick Parsons
This should also give you a chance to improve air-tightness, even though the decorated interior denies you access to the key 'interface'. I would suggest a condensation risk assessment. I don't know of a free version but I think others on here have used free or 'trial' versions. Just finding a risk does not mean you will get an interstitial condensation issue - there are so many variables - but it may help you to find what the model says the risk is and where it predicts it will be. Improving the air-tightness will/may reduce the volume of water vapour 'transported' in any case. If we use the '66% outside;33% inside' rule of thumb for 'hybrid' insulation then it perhaps points to 50 between studs and phenolic, not EPS externally.

You asked ''With no VCL are we going to hit problems even though weve got an equivalent or more insulation on the outside than within the studs?''
As above, not necessarily, and in any case more outside is 'prescribed', not more inside.


''Second question is, if we can use foil faced insulation, do we need a membrane over the outside of the insulation?''

I would not use foil-faced outside, but if you watch Grand Designs you'll see lots who do. We don't get the 'after story' though. I suspect foil-faced is only used externally because that's what the merchants most readily sell.]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300375#Comment_300375 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300375#Comment_300375 Tue, 21 Feb 2023 09:24:38 +0000 djh
IMHO it sounds like now is the wrong time to try to insulate it. Doing the job with no (or little) disruption will probably result in a less than ideal result that will annoy everybody for evermore. Far better to wait until they are in a calmer position and can face doing the job right. JMHO, YMMV. ]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300376#Comment_300376 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300376#Comment_300376 Tue, 21 Feb 2023 09:42:04 +0000 fostertom http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300377#Comment_300377 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300377#Comment_300377 Tue, 21 Feb 2023 10:55:22 +0000 djh http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300382#Comment_300382 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300382#Comment_300382 Tue, 21 Feb 2023 14:54:48 +0000 fostertom http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300385#Comment_300385 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300385#Comment_300385 Tue, 21 Feb 2023 20:23:04 +0000 WillInAberdeen
Could you not bother with the between-studs insulation - just cover the outside of the studs with something airtight, then 100mm PIR/phenolic, then battens, cladding?

Probably work out cheaper, similar insulation value, and no doubts about condensation. Option to use 150mm EPS wool etc

But you mentioned this is for the upstairs side walls of a semi, so I guess there will be junctions with the downstairs, the roof, the cavity gables, and the neighbours?

It will take quite some thought how to do those without thermal bridges and without returning the insulation inside the decorated envelope. In our case, those thermal bridges made it meaningless to go for sub U=0.2 EWI because the bridges lost more heat than the EWI.]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300390#Comment_300390 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300390#Comment_300390 Wed, 22 Feb 2023 12:02:20 +0000 philedge
Still a bit uncertain over the way forward though!

I had a play with the demo calculator on u-wert.net and ended up with diagrams below that include a statement that "this component is free of condensate under given climatic conditions". Is that the answer Im looking for??]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300391#Comment_300391 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300391#Comment_300391 Wed, 22 Feb 2023 12:39:03 +0000 WillInAberdeen
I dislike the humidity method in that software which looks fancy and detailed but is actually an (over) simplified method from an EN standard. EG it shows a big humidity gradient in the air space (would actually get evened out by convection) and no humidity effect of the foil facings on the PIR (would actually trap more humidity on one side than the other, that's what they're for).

If this calculation is going to change your mind about the design one way or the other then Wufi would be more trustworthy IMO. The PIR/phenolic manufacturers might already have an analysis they could give you for free?]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300422#Comment_300422 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300422#Comment_300422 Fri, 24 Feb 2023 10:49:47 +0000 philedge Plasterboard
4x2 timber studs without insulation
VCL over outside of studs
100mm PIR
Breather membrane
Vertical battens fixed through to studs and hopefully strapped to ends of rafters for support
Horizontal cladding

Longer term when interior needs decorating we'll have the option to remove outer VCL, add a bit more insulation and VCL from the inside.

Thanks for everyone's input]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300429#Comment_300429 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300429#Comment_300429 Fri, 24 Feb 2023 18:06:23 +0000 djh http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300433#Comment_300433 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300433#Comment_300433 Fri, 24 Feb 2023 19:00:32 +0000 philedge Posted By: djhJust curious but why PIR rather than phenolic?

Availability locally at a good price and Ive already got some large offcuts. I've never really looked at phenolic but it seems slightly better performing but at a disproportionately larger price. Am I missing something??]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300434#Comment_300434 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300434#Comment_300434 Fri, 24 Feb 2023 20:32:59 +0000 djh http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300455#Comment_300455 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300455#Comment_300455 Sat, 25 Feb 2023 20:50:24 +0000 philedge Posted By: djhPerformance in a fire?
For the limited area we're doing, the benefits of phenolic seem to be trumped by the cost saving and availability of PIR]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300498#Comment_300498 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17748&Focus=300498#Comment_300498 Tue, 28 Feb 2023 11:25:46 +0000 fostertom Posted By: philedgeJust spoken with Celotex tech and insulation between the studs without a VCL is a non starterSuch firms' reps put out that kind of blanket ruling that just shows poor understanding and analysis. They may think it's fail-safe but actually VCLs in many cases make matters more risky, when re-drying ability becomes dominant in the everyday behaviour of a particular configuration of the 'sandwich' plus local climate.]]>