Timber frame exterior wall insulation

[sansserif]

Hi all

 

Wondering if anyone has any thoughts re: exterior wall insulation for my timber frame build. I'm shooting for an energy efficient build, but not chasing perfection by trying to hit passiv standards. I'd like to hit a sweet spot between price and performance.

 

It looks like the "standard" exterior wall build-up from TF firms is 140mm studs + PIR to hit a decent U-value. But I'm wondering wondering if I'm missing a trick by not considering other insulation options? In particular, it would be great to have something that's a more organic material, lower VOC etc...

 

Anyone have any advice? What's worked well for you all?

[saveasteading]

24 minutes ago, sansserif said:

any advice

before any further thoughts could you advise on the roof verge detail?

 

ie if you built 150mm or so out from the walls, does that fly past gutters and roof covering?

similarly at the bottom, are there any obstructions such as manholes?

And what distance is the nearest wall to a boundary. roughly if it over 2m, otherwise exactly in case it affects fire regs or any access you have.

 

[sansserif]

The designs have an allowance for a ~300mm exterior wall thickness, excluding rain screen and plasterboard etc. Would be great to not need the full 300mm though. Design has a gable roof with a  that overhangs about 50mm over the gable end.

 

Distance to nearest boundary is 0.8m, and around 1.8m to the nearest building. No (known) nearby obstructions!

[Mr Punter]

1 hour ago, sansserif said:

In particular, it would be great to have something that's a more organic material, lower VOC etc...

 

Insulation tends to be mineral wool or PIR.  What is the proposed "rain screen"?

[sansserif]

Rain screen would be brick (ground floor) and render (above)

[Mr Punter]

You could fully fill with 140mm pir between studs and line internally with 50mm pir, held on with 38 x 50 battens to create a service cavity.  You could put the 50mm on the outside but you will need different wall ties and another layer of breather membrane etc.  You will need about 30mm cavity to the brickwork.  If the timber frame co quote for the insulation it saves a lot off faff and mess on site.

[Roundtuit]

Typically I think 140mm studs are filled with 120mm PIR due manufacturing tolerances, then we lined with another 40mm.  I'm sure there are other rockwool/cellulose options you could look at with 300mm to play with. A 50mm cavity between frame and outer skin is required if you have masonry I believe.  Mixing insulation will probably need some sort of condensation risk doing to avoid a dew-point in the wrong place.

[G and J]

Ummm, the title confused me.   At first I thought you were talking about insulation outboard of the frame/OSB. 

 

But I think you are looking at insulation between the studs of the frame and inboard of that.

 

I think the 120mm PIR in a 140mm stud is about an air gap next to a reflective coating.   So better insulation overall than full fill PIR.  
 

I am biased.   I hate PIR with a passion.   Used it under our floor a bit and regretted it.  Should’ve made sure I had enough space for polystyrene only.   
 

We used mineral wool in our frame and another layer inside.  I may be kidding myself but I now believe it’s a nicer job to do and nicer to live in.  The only place PIR looks good is on a spreadsheet. 

[Iceverge]

300mm external wall is really really tight to meet insulation regs. 

 

Brick

Cavity

Osb

140mm studs

Plasterboard 

Skim 

 

 

Comes to about 290-325mm by my calculations. 

 

Can you stretch the wall thickness to 400mm?

 

 

 

[Roundtuit]

1 hour ago, G and J said:

I think the 120mm PIR in a 140mm stud is about an air gap next to a reflective coating.   So better insulation overall than full fill PIR.  

There's a +5mm tolerance on the PIR boards, so I wouldn't attempt full fill.

[G and J]

10 hours ago, Roundtuit said:

There's a +5mm tolerance on the PIR boards, so I wouldn't attempt full fill.

That’s true.   When I laid the 120mm layer of polystyrene for our floor it snuck together beautifully tight and flat. The polystyrene has some spring in it so it could be cut to an interference fit and shoe horned in, lovely.  
 

The PIR in the other hand, was a bitch.  Really nasty dust in the cutting, and however hard I tried still tiny gaps between the boards and to add insult to injury it sat there showing off how different thickness each board was.  
 

PIR.  Just say NO! lol

[sansserif]

14 hours ago, G and J said:

We used mineral wool in our frame and another layer inside.  I may be kidding myself but I now believe it’s a nicer job to do and nicer to live in.  The only place PIR looks good is on a spreadsheet. 

Why did you end up going with mineral wool out of curiosity? What makes you think it's nicer?

 

I'm trying to understand how to make a decision on this. The TF companies steer heavily towards PIR, but my impression is that it's because it's easiest for *them*.

 

 

13 hours ago, Iceverge said:

300mm external wall is really really tight to meet insulation regs. 

 

Brick

Cavity

Osb

140mm studs

Plasterboard 

Skim 

 

 

Comes to about 290-325mm by my calculations. 

 

Can you stretch the wall thickness to 400mm?

 

Sorry, I wasn't clear. 300mm is the thickness allowance for just the osb+studs+service batten.

 

Complete allowances:

- Brick walls: 500mm for everything (brick + cavity + OSB + studs + plasterboard + skim)

- Rendered walls: 350mm for everything (render + OSB + studs + plasterboard + skim)

[ProDave]

Go got 195 studs then you can get 2 goof layers of Frametherm 35 type mineral wool between studs.  Less nasty to work with than other types of mineral wool and is stiff enough to just push in and stay put when cut a little over size.  

 

If that does not get the U value you need over board the inside with PIR, no need to cut sheets to fit, butt the edges together and tape them then 25mm battens for service void.

[JohnMo]

PIR is great insulation value in theory, but theory and practice are not generally the same thing. Cutting and then foaming to get a good fit between studs is a pain to do well, if you don't do it well, the practical U value suffers. Mineral wool is forgiving you can cut oversize and it can be squished into place, cut it undersize you can add more. Mineral wool is easy to do well. Use some thing like Frametherm 32 or Rockwool Flexi. Add PIR to inside face in full sheets if you need to after. 

[-rick-]

Mineral wall has better sound insulation too so leads to a quieter house compared to PIR.

[G and J]

1 hour ago, sansserif said:

Why did you end up going with mineral wool out of curiosity? What makes you think it's nicer?

 

 

1 hour ago, -rick- said:

Mineral wall has better sound insulation too so leads to a quieter house compared to PIR.

 

 

Absolutely this drove us, (we had a previous room in roof (done by others) and the result was not great) and then we found it really easy to work with. Used the frame therm batts as others have described.

[Alan Ambrose]

Posi-joists with blown in cellulose or wood fibre? This is easy to do - insulation installed in a couple of days. The problem is finding a reliable & professional installer. Unfortunately, we didn't .

[Nickfromwales]

13 minutes ago, Alan Ambrose said:

Posi-joists with blown in cellulose or wood fibre? This is easy to do - insulation installed in a couple of days. The problem is finding a reliable & professional installer. Unfortunately, we didn't .

Gordon Lewis is tha’ man. If anyone wants his number, ping me a PM. 

[G and J]

10 hours ago, sansserif said:

Why did you end up going with mineral wool out of curiosity?

I (G) did hours and hours of working through options for wall build ups.  Drove J, the architect and a lot of buildhub dudes mad with my endless ‘what ifs’.  
 

In the end the conclusion surprised me.  Getting the most wall insulation is not paramount.  Adding insulation underfloor and in the loft is easy.  Improving airtightness (don’t get me started) is massively more important than absolute wall u value.  Glazing further undermines the benefit of excessive wall insulation. 
 

So then decrement delay (ask a nearby grown up to explain that one) and sound attenuation become the driving factor.    Had I known what a bugger PIR is to work with I’d have factored that in too, but I didn’t back then.  
 

Frame batts (not roll as it falls out) is nicer to work with too.  Just don’t push too much in.  And don’t ask me why unless you’re wearing ear defenders lol

[sansserif]

The points around sound insulation seem really valid, that's something that is valuable to me.

 

In general, I've been surprised how adamant the TF firms are around using PIR. Has anyone worked with a manufacturer that's actually open to something else? Feels like in twisting their arm to even discuss something different.

 

Architect is in favour of cellulose, anyone have thoughts there? He's mainly thinking from an eco perspective, which I appreciate but it's not my only consideration.

 

How are you guys thinking about structural warranty / STA assured wall build-ups?

[Iceverge]

7 minutes ago, sansserif said:

Architect is in favour of cellulose, anyone have thoughts there? He's mainly thinking from an eco perspective, which I appreciate but it's not my only consideration.

 

 

It's a great product if installed correctly. 

 

Are you stick framing on site or ordering a kit? 

 

 

 

 

[G and J]

17 minutes ago, sansserif said:

In general, I've been surprised how adamant the TF firms are around using PIR.

Is that because they are specifying it but not installing it?

 

In a calculation, PIR makes a wall build up seem great from a thickness/u value perspective.   So at the point of buying a timber frame it helps the sale.  
 

We took the view that a few inches off room sizes was an ok price to pay for a nicer ‘sounding’ house with still reasonable wall insulation. 
 

22 minutes ago, sansserif said:

Architect is in favour of cellulose, anyone have thoughts there? He's mainly thinking from an eco perspective, which I appreciate but it's not my only consideration.

Your architect may be different, but most (it appears to me) never build stuff.  Shredded paper looks good on websites, but one lesson from our build is that doing non standard stuff is like pushing a pebble uphill with your nose.  Fighting that one through for airtightness was worth it.  Given the properties of mineral wool which is widely available and understood I would struggle to build a case for cellulose.  But it’s a good talking point over canapés I guess.  
 

I really must find out what a canapé is lol

[Roundtuit]

7 minutes ago, G and J said:

Is that because they are specifying it but not installing it?

Maybe the opposite.  Our frame came with PIR factory-installed; very tight and accurately fitted, and foamed in for good measure.  I'm pretty sure they weren't geared up for a rockwool or cellulose solution, so the job would maybe become less appealing for them if that was the choice.

[G and J]

I didn’t realise that was an option.  Rolly and I manually erected our frame so that would have stressed the PIR fill so wouldn’t have worked for us anyway but for normal builds where a crane is used I can see that addressing all the in-frame PIR fitting issues.  

 

[-rick-]

2 hours ago, sansserif said:

Architect is in favour of cellulose, anyone have thoughts there? He's mainly thinking from an eco perspective, which I appreciate but it's not my only consideration.

 

I think that tends to be seen as the gold standard for a timber frame house. Easier to build and install so long as you have the right installer. @Nickfromwaleshas someone he recommends every now and then. But companies selling a 'package' will want to do things they way they usually do them. Anything else will cost more because it will be more complicated and costly for them to accomodate.