Disadvantages of only using PIR in timberframe?

[AliG]

Hi,

 

We were planning a frame with 140mm Frametherm 32 between the studs and 40mm PIR over the studs to get a quoted U-Value of 0.14.

 

Apparently there is a shortage of Frametherm 32 and so they have changed the spec to 70mm PIR between the studs and 50mm over. Using Kingspan K12 with a 0.02 thermal conductivity I do not get 0.14 U-Value. which I have not queried yet.

 

However, when offered the option of PIR fill I suggested changing to the 0.11 U-Value option which I had previously discounted due to the thicker wall. This would be 140mm full fill PIR and 50mm over, which does give a 0.11 U-Value.

 

TBH the difference between 0.14 and 0.11 does not pay for itself, around one third if the outside walls are windows, but I feel that thicker full fill insulation should be better for a modest extra cost.

 

However, I am wondering if using only PIR could cause any issues. I liked the idea of a mix of PIR and Frametherm as the Frametherm will help with noise insulation and decrement delay.

 

Should I be concerned re noise or decrement delay with only PIR. The wall build up will be render on render board, 50mm cavity, 9mm OSB, 140mm stud with 140mm PIR, 50mm PIR over stud, 35mm service cavity and then Fermacell. Maybe the extra weight of the Fermacell means that I do not have to worry. The builder was somewhat incredulous about using Fermacell instead of plasterboard.

 

I did wonder if an alternative to get some of the benefit of a different kind of insulation in the wall is to put 25mm acoustic insulation in the service cavity.

 

I am also wondering if I take the walls down to 0.11 am I getting to the point where at least we do no need heating upstairs. We will have roughly 0.1 floor, 0.11 walls, 0.7 windows and 013 roof(maybe a bit better if we can get more insulation in). The only issue is that of 225 square metres of outside wall, 70 is windows and doors which increases the heat loss, although most of the windows are downstairs. Ex the hall, which is double height, only around 15% of upstairs wall is window.

[tonyshouse]

Go for the best you can now, I would aim lower on the roof for sure, good workmanship is crucial, I like fitting joist or rafter, then insulation then the next one etc

 

what is your air tightness target? 

[AliG]

The roof will be a flat roof with insulation over. In Scotland you pretty much have to use a warm flat roof construction like this. BC do not like insulation between the joists and a ventilated space above, they like a fully sealed roof here. This causes an issue as the height of the roof parapets is already set and so there is a limit to how thick the insulation can be.

 

The architect specified 175mm, I think we can take it up to 190mm which is 0.12, although again this makes hardly any difference. The windows account for over half of the fabric heat loss.

 

It is an MBC frame, as we cannot use their standard cellulose roof insulation we have not yet specified air tightness, but I would expect we can get a figure below 1.

Edited August 30, 2020 by AliG

[Mr Punter]

I have used Fermacell in the past and I would never use it again.  It is a nightmare to work with.

[AliG]

There have been quite a few threads where people reckon it is a better product installed. The builder said his guys will install it if I want it. Is it just working with it you found a problem or something else?

[DamonHD]

My builders didn't like Fermacell much, espacially in combination with my favourite aerogel:

 

http://www.earth.org.uk/note-on-Spacetherm-aerogel-thermal-insulation.html#Spacetherm-F

 

Rgds

 

Damon

[Onoff]

If you want quiet inside, then look at @PeterStarck's wall make up. Icynene between the wall timbers and from memory Rockwool batts outside. 

[AliG]

Thanks guys.

 

I was literally about to sign the contract when I noticed the change from Frametherm and PIR to PIR only. I am sure some people have PIR only frames I was wondering if they found any issues with it.

 

I then thought we could just put acoustic insulation in the service void if I wanted a combination of different insulation with different properties. I don't know if anyone has done this?

 

9 minutes ago, DamonHD said:

My builders didn't like Fermacell much, espacially in combination with my favourite aerogel:

 

 

The proposal is just to use standard Fermacell with the FST. It won't be going on until next spring so can be changed, but it seemed from various threads that people like the finished product even if it is hard to work with. A lot of the issues are weight and if you have a team doing a whole house this should be less of an issue.

[Thorfun]

1 hour ago, AliG said:

Thanks guys.

 

I was literally about to sign the contract when I noticed the change from Frametherm and PIR to PIR only. I am sure some people have PIR only frames I was wondering if they found any issues with it.

 

I then thought we could just put acoustic insulation in the service void if I wanted a combination of different insulation with different properties. I don't know if anyone has done this?

 

 

The proposal is just to use standard Fermacell with the FST. It won't be going on until next spring so can be changed, but it seemed from various threads that people like the finished product even if it is hard to work with. A lot of the issues are weight and if you have a team doing a whole house this should be less of an issue.

if you haven't signed the contract with MBC yet have you thought about approaching another company? or changing to their blown cellulose twin-stud wall? are all other manufacturers struggling to get Frametherm 32? it's what I want to install in my open-panel timber frame so maybe I need to start sourcing that now!

[AliG]

We had a building width restriction so can’t use the 300mm wall.
 

I tried to use sone frametherm 32 in my current house and couldn’t get it at the time.

 

PIR is more expensive so I’d get more insulation for the same price. I’m just wondering if there will be any issues. 

[Thorfun]

Fair enough. Can’t help with the question about potential issues but I do thank you for the heads up about lack of frametherm 32!

[Andrew]

We’ve just finished having our frame insulated and they used this between the studs - https://www.isover.co.uk/products/timber-frame-batt-range

 

Didn’t seem to be any shortage as far as I could tell and seems to have the same performance as Frametherm 32.

[Miek]

The main issue will be fitting. PIR is far more time consuming to to fit tightly, which is important so as to avoid thermal bypass via the gaps around the edges. Mineral batts are far easier to fit. So how confident are you that it will be fitted well? Badly fitted PIR can have most of its better thermal performance wasted if it's done badly enough.

[Temp]

20 hours ago, AliG said:

The wall build up will be render on render board, 50mm cavity, 9mm OSB, 140mm stud with 140mm PIR, 50mm PIR over stud, 35mm service cavity and then Fermacell.

 

I think what will make this work is the 9mm OSB. It should stop air in the 50mm ventilated cavity getting around the gaps in the PIR. I wouldn't go for PIR without it or some other gap sealer.

[the_r_sole]

I'd say the uvalue they are quoting is on the low side so you need to ask them for a spec... A lot of this comes down to what you want the wall to achieve, 0.14 is decent for walls and the PIR insulation is diminishing returns on the investment...

[AliG]

I am happy that we can get the U-Values we want to achieve depending on the thickness of the PIR. Previously I would have been concerned re PIR not fitting well but factory fitted PIR should fit tightly and not cause any issues.

 

I am more considering other things that are lost in the focus on U-Values. PIR is lighter and stiffer than Frametherm so transmits sound better and has lower decrement delay. Really wondering if in the real world anyone has found this an issue or by the time you consider all the elements of the wall this concern goes away.

[the_r_sole]

if both the constructions have the PIR on the inside of the kit is that not the thing which would have the biggest impact on the decrement delay? 

If you are worried about sound transmission you could look at resilient bars internally - I'm guessing that the different in the insulation won't be a big factor in sound transmission in your build up as you have a direct connection between the outside face and inside face through the render board/timber kit/internals so the insulation won't be doing much... I'm no expert in this stuff btw, there's some on here who are much more knowledgeable in the details

[CC45]

We have a 140mm timber frame, I filled it with 140mm celotex - dont.  Use 130mm because it isn't accuate enough so I had to trim some of them which is a real pain.  I tried my best to get them tight - most you couldn't get a credit card down but if I did it again I would get them roughly there, pin them in place and then spary foam them in tight.  Air tight barrier then with some 25mm battons as service runs.  Most of these battons were infilled with 25mm celotex - so we have got 165mm in total.  This summer the house has been cooler than the outside with the windows left adjar over night.  Now temps have dropped - it is warmer inside.  Far more noise comes in through the windows than the walls.  We have got a 50mm cavity with a brick outer skin. Celotex takes a long time to install - not sure I'd bother again - I'd rather pay a few hundred more in heating bills tbh.  If I was building panels myself then I would prob use it but still use foam and then some ply to seal it all.

[AliG]

Thanks, that is useful. Luckily the difficulty in installing it will be MBC's problem

[Oz07]

Digressing but as @CC45 says above I think it's better to fit loose and foam. It's what I always do now. A lot quicker because they can be cut to a common size and the foam reduces any of the wind washing effect bypassing the insulation. To me it seems a much better method but I wouldn't profess to be a boffin with regards to the science of insulation.

[Mr Punter]

On 30/08/2020 at 19:28, AliG said:

Is it just working with it you found a problem or something else?

 

It is heavy, really dusty to cut, needs special tools to cut and special fixings, sparky will curse you, the FST stuff is not easy to apply, although it will hold a screw you need to pilot first.

 

Positives - if it is kept dry it is very durable, impact and noise resistant.  They use it in double layers for high security institutions.

 

We recently just used Knauf Soundshield Plus plasterboard as it is good for impact, fire and acoustic rating, loads cheaper than Fermacell and simple to fit and skim.  We did double layer 15mm on walls and ceilings, which will give all the decrement delay you need.

[AliG]

Thanks @Mr Punter. I am getting a price for Fermacell and plasterboard and once I know the difference I  have the option of using plasterboard. I did look at the Soundshield Plus you mentioned.