PIR thickness

[Babybirddog]

Hi,

Our architect has specified 95mm between our frame and 100mm on top in the cottage. 95mm impossible to get hold of, also 45mm not common so not sure if same problem if I use it with 50mm. Also worried about getting perfect fit with 95x45 stud frame. What problems would I have if I ask him to change to 90mm, easier to source.Any other alternatives.

Ta

[Crofter]

Fitting PIR into a timber frame is a right pain, because as you have found the sizes don't match! Timber will be nominally 4" but actually be regularised to 95mm, and then your 100mm PIR cannot fit. The other thing that makes it a pain is the tedious gap filling with expanding foam around the edge of every board. You only need to leave a few gaps to cause significant drop off in performance.

If you have a contractor local to you, consider blown in cellulose insulation instead. The on-paper performance isn't as good but it will totally fill the space, and may actually perform better in practice.

Or switch to a glass/rock wool for the frame, which is what I did- but I had to overlay this with a layer of PIR to get good enough performance. This continuous layer had another advantage though because it cuts off the thermal bridging.

[AliG]

Architects do some strange things.

 

I found 95mm PIR for roofs when I googled which you presumably could use, but surely he knows that 90 or 100mm would be much more normal.

 

95mm is the standard size for a stud wall. Historically these were insulated with rockwool, but the U-Value would be too low now. 90mm would be fine, I see a lot of walls with cut PIR in them and the builders seem to do a decent job of fitting it in place. As @Crofter says you could use rockwool and then insulated plasterboard on the inside. This may well work out cheaper, but will lose you some interior space. 90mm of PIR is basically at the minimum acceptable level of insulation for a wall today.

 

100mm in the roof is too thin. There are various standards depending on whether your building is a conversion, or an extension or new build. I am not sure from your first post. But 100mm of PIR will give around a 0.22 U-value for the roof. The roof is normally the easiest place so get better insulation unless you have a height restriction. Unless you are planning to use heated roofspace 400mm of rockwool gives around a 0.11 U-value and would also be much cheaper.

 

 

 

 

[jfb]

I presume you meant 100mm PIR over the timber frame as well as the 95mm between the frame. I'd use rock wool or something similar for between.

[Babybirddog]

Yes 100mm over the frame, thanks for suggestions, time to give him a call.

[ProDave]

I recommend Knauf Earthwool frametherm 35 for between the frame, a lot less nasty to work with than other types of glasswool insulation. This was a top tip from the builders that built and erected my frame.

 

[Triassic]

3 hours ago, Babybirddog said:

Also worried about getting perfect fit with 95x45 stud frame.

Our timber frame supplier has a large table saw especially for cutting PIR. They cut and foam it in place when making up the frames. Whatever you do don't fit it yourself, as I did, possible the most horrible job ever, the dust when cutting the stuff gets everywhere.

 

what level of insulation do you have in the foundations?

[Babybirddog]

The slab has 150mm PIR on top of hardcore etc with concrete 125mm on top,going in at the moment.

[AliG]

I thought you meant the roof when you said 100mm on top!

 

100mm PIR plus rockwool in the frame would be the best value solution as suggested. that will give around a 0.15 U-Value. 2 layers of PIR would be lower again, but the benefit would not offset the considerably higher cost.

[PeterW]

Downside of 100mm insulation on the frame is very long fixings needed and they are not cheap. 

 

Is there a batten on the inside too ...? Or are you just using the plasterboard to hold back the insulation ..?

[ProDave]

You are addressing this from the wrong angle.

 

I would be asking why are you even considering a 100mm frame today?  Our previous house, built in 2003, used a 150mm frame. That was about the time up here that 150mm became the normal, because you simply could not get enough insulation in a 100mm frame. Now "ordinary" houses up here use a 150mm frame with added PIR on the inside.  Our new house has a 190mm frame in our case with added insulation on the outside.

[CC45]

Our frame is 140mm. I put pir in myself, get a good mask and use a saw. Take your time & you wont need to foam much. The biggest pain is the inconsistency in the pir.  I would strongly advise you fit pir thats 10mm thinner than the frame ie 140 frame - use 120 or 130 pir. It wont alter the u value really.  We've got horizontal battens on the inside and then more pir between then.  It takes time so if i was to do this method again I would see if the frame company would stick the pir in to be honest.

[Babybirddog]

Thanks for all the suggestions. I have managed to find 25mm+70mm  and also 100mm to go over the frame at sensible prices as they are more of a stock item.Using the frametherm was something I would have considered but the cottage internally is narrow and can't afford to loose any more space to pack enough in to get an acceptable u-value. I am now checking my schedule to see if any other materials are going to cause a problem, if I can spot them. Already picked up the post on here about low expansion foam, very happy I found that one.