Timber frame exterior wall insulation

[Nickfromwales]

13 minutes ago, -rick- said:

 

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 accommodate.

@sansserif can simply ask them to remove it from their quote, if it is costed remotely and is cheaper elsewhere or better done by a trusted 3rd party; where the preference is to remove ambiguity over the quality / robustness of the work.

 

Other examples are UFH, design and install / manifold / pressure test, groundworks for pipes / ducts etc, and so on; things that some folk prefer to focus on specifically / more intensively, and often that steps beyond the comfort zone of a turnkey contractor or supplier.

 

This is one of my favourite things about MBC and Pro ICF, the ability for me to ‘stick my oar in’ and finesse things on the clients behalf way in advance of the construction phase.

[saveasteading]

12 hours ago, G and J said:

what a bugger PIR is to work with 

All designers should spend a day putting it into wall or roof framing.

I wonder if the TF companies that fit it are simply much better at doing it, I'd think they could readily have found the method that we one-timers haven't.

 

The reality is gaps and expanding foam, then shiny tape over the gaps, and it won't be questioned.

 

Bottom line though is the theoretical insulating property and skinnying down of the studs.

[-rick-]

34 minutes ago, -rick- said:

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.

 

I should add, that I think you tend to need a thicker wall section to make blown in cellulose work. Like Mineral Wool cellulose is not as insulating on paper as PIR. It tends to be used in higher performing homes anyway where the thicker walls allow serious insulation values and easy installation (also very very good sound insulation).

[Nickfromwales]

28 minutes ago, -rick- said:

 

I should add, that I think you tend to need a thicker wall section to make blown in cellulose work. Like Mineral Wool cellulose is not as insulating on paper as PIR. It tends to be used in higher performing homes anyway where the thicker walls allow serious insulation values and easy installation (also very very good sound insulation).

A very good point. Many heads always better than one  

 

Minimum 250mm, MBC is 300mm, but their 140mm mineral wool + PIR IWI option is better on paper; (imho poorer overall in real life though, as it’s quite acoustically transparent).

[Square Feet]

Has anyone used this stuff or considered it? It's natural fibre hemp insulation batts.

https://www.indinature.co

I'm at a very early stage in my project so it's far too early for this conversation really, but I saw this product and I liked the idea because the factory is about 2 miles from my plot plus it's not made from nasty chemicals. Just wondering....

[Nickfromwales]

8 minutes ago, Square Feet said:

I'm at a very early stage in my project so it's far too early for this conversation really

Unless that’s at least 12+ months or more, as usually I’m on these types of projects as early as the client will entertain. 11 months before concrete was poured on one of my current projects and still discovering opportunities / ‘tweaking’ on the fly!

 

Keep asking lots of questions, and use this time well to design M&E / interior and fabric choices etc, as when the build kicks off you’ll have far less capacity / brain-space than you think you will,  .

[Square Feet]

1 minute ago, Nickfromwales said:

Unless that’s at least 12+ months or more, as usually I’m on these types of projects as early as the client will entertain. 11 months before concrete was poured on one of my current projects and still discovering opportunities / ‘tweaking’ on the fly!

 

Keep asking lots of questions, and use this time well to design M&E / interior and fabric choices etc, as when the build kicks off you’ll have far less capacity / brain-space than you think you will,  .

It may well be 12 months away. I've just had a 'subject to planning' offer accepted on the plot so very early stages. I get your point though - hence the question.  

[JohnMo]

31 minutes ago, Square Feet said:

Has anyone used this stuff or considered it? It's natural fibre hemp insulation batts.

https://www.indinature.co

I'm at a very early stage in my project so it's far too early for this conversation really, but I saw this product and I liked the idea because the factory is about 2 miles from my plot plus it's not made from nasty chemicals. Just wondering....

Never too early to start making plans, they will take a few iterations to get what is correct for you - not the architect. We ended up changing the full build method from timber frame to ICF, so all the Warrant plans needed to be redone. Lots of details that maybe standard for the architect but end up being a thermal bridge or key details just missed altogether that have you scratching your head or the builder doing what he normally does and mess thing up for you.

[sansserif]

What's the thinking around STA assured wall build-ups?

 

I'm thinking if you were to piecemeal the wall -- TF co supplies uninsulated frame, insulation is fitted on site separately...

 

Would you run into issues with mortgage-ability or structural warranty going down that route?

[JohnMo]

24 minutes ago, sansserif said:

TF co supplies uninsulated frame, insulation is fitted on site separately...

Isn't that the norm - i.e. what they always do. Buying pre-insulated cassettes is not the norm.

[sansserif]

I've found that some of the TF vendors factory fit the (PIR) insulation. The argument being that factory installation is going to have better tolerances and quality control.

[G and J]

43 minutes ago, sansserif said:

I've found that some of the TF vendors factory fit the (PIR) insulation. The argument being that factory installation is going to have better tolerances and quality control.

I imagine it would be quite difficult to get a reliable cost comparison between factory fit and on site fit. I get the convenience, erection speed and possible quality improvement over on site PIR fit.  
 

But until such things become the norm and competitive pressures push down prices I’d worry about the extra cost.  But then, I’m a tight wotsit.  

[G and J]

1 hour ago, sansserif said:

What's the thinking around STA assured wall build-ups?

Also just to mention, our timber frame supplier (no insulation provided) is a member of the STA and our warranty provider (and B regs) just asked for their calculations etc.

[stu w]

I have used this method many times on builds and extensions, as have many.

- Reflective thermo breather membrane (external)

- 11mm OSB for racking strength and airtight substrate

- 140mm timber stud

- Full-fill Rockwool NyRock Frame Slab insulation

- 25mm or 50mm PIR 

reflective thermo VCL

- Service batten

- Internal plasterboard 

 

Everything taped to help with air tightness.

My opinion, you will always install insulation and take more care carry out the insulation and taping works yourself   although it is time consuming 

[sansserif]

I've done a bit more research and so of course have even more questions.

 

MBC and a few others have a twin wall cellulose offering. I feel confident that this approach would be a quality option, but there are a few issues:
- It requires really thick walls and roofs, approaching 400mm before cladding.

- It's not exactly cheap

- It feels like overkill for what I want to achieve thermally. I'd prefer thinner walls and a bit more space.

 

...but what are my other options?

 

I'm finding that timber frame manufacturers standardise on 140mm studs, which doesn't seem enough for anything but PIR.

- 225mm mineral wool would achieve what I need thermally (if my calcs are correct), but how would I make that work with a 140mm timber frame kit?

- If I went with 140mm mineral wool + PIR board, what would the acoustic performance be like?

 

Now I'm also wondering about the relative importance of wall insulation relative to glazing, especially with large spans of glazing? Money better spent on the insulation or glazing?

[sansserif]

On 19/07/2026 at 11:09, stu w said:

My opinion, you will always install insulation and take more care carry out the insulation and taping works yourself   although it is time consuming 

 

Also a fair point. I'm pretty good at grunt work

[Nickfromwales]

5 hours ago, sansserif said:

If I went with 140mm mineral wool + PIR board, what would the acoustic performance be like?

Very poor vs 300mm blown frame. Been in both offerings and it's quite a significant downturn. 

 

Do you really think you'll miss 6" of GIA?

 

Home for life?

 

Factor in all the hidden benefits with MBC, turnkey, SE included, slab & frame fully coordinated and project managed, with guarantees, guaranteed airtightness, fully service-battened interior, cheaper downstream 1st and 2nd fixes, list goes on.

 

I had this very conversation with a clint who had almost instantly dismissed MBC as "too pricey", and they were the most expensive at a glance. After I broke down all the other quoted they'd had from the other big hitters, and realised the opportunities and hidden cost of others / reduced spec / poorer airtightness unless you want to buy some more bells & whistles, and so on, and on, and on.

 

Needles to say, MBC ended up with the job and it's going very well, very VERY quickly, and is likely only a slight bit cheaper than the best of the others (who then didn't make grade).

 

Concrete poured end of March '26, and I am pushing the project forward with some very good people so they can have xmas dinner there this year (less landscaping and 2 of the 4 bathrooms tiled etc).

 

Needs an impartial perspective to show you who is cheaper, and for what (sometimes hidden or 'not immediately obvious) reason(s).