Is insulation *outside* studs considered unconventional?

[lineweight]

This is a kind of general question - but most likely to apply where there's a timber frame external wall with some kind of cladding such as tile hanging. It might be newbuild or it might be the face of a dormer in a loft conversion.

 

There's a basic choice between putting your "continuous" insulation layer inside or outside of the structural studs. Of course, this can be supplemented with some level of insulation between the studs as well.

 

In principle it usually seems to me that insulating outside the studs is preferable - keeping the structure warm removes much of the worry about condensation, and co-ordinates with a general strategy of keeping structure on the inside of the insulation layer, avoiding various thermal bridging situations.

 

However - it still feels a bit like it's a non standard approach. For example - if you look in the TRADA "timber frame construction" book (5th edition, published 2011) nearly all of the example details assume insulation in between studs and on the inside. The option of external insulation is mentioned - but only under a section called "alternative wall constructions".

 

I'm interested in any opinions on whether this approach is generally considered an unusual one ... and/or whether it's becoming more common over time?

[Pocster]

I’m no expert ( as others will confirm ) . I put insulation outside my timber frame and also between the internal timber frame .

[joth]

I'm also no expert, but we're using wood fiber insulation (retrofit + extension with 50% external walls are now timber frame) and this has a continuous layer of pavatherm EWI wrapping all the B+B and TF walls

https://www.pavatex.com/en/application/wall/external-wall-insulation-in-timber-construction/

https://www.pavatex.com/en/application/application/

 

[lineweight]

32 minutes ago, pocster said:

I’m no expert ( as others will confirm ) . I put insulation outside my timber frame and also between the internal timber frame .

 

Is that using conventional PIR type insulation?

[Pocster]

Just now, lineweight said:

 

Is that using conventional PIR type insulation?

Yes . Foiled backed stuff . All around the timber frame externally ( with either cladding or a thermolite wall beyond that ) . Then pir in the timber studs . Also in between ceiling joists .Then 50mm pir over all of that and foil taped all joints .

You can only insulate once ......

[redtop]

140mm woodfibre outside of timber frame, 150mm rockwool between the studs and then racking osb on inside. The propassive stuff for the airtight layer. Service void and plasterboard. Oh an vapour wrap on external woodfibre, then will have battons and ceder cladding

[lineweight]

I think it might in fact be the 'normal' way to do it when you're using woodfibre insulation?

 

My interest is largely related to how a regular builder (ie one who'd probably generally be used to mainstream products like PIR and Rockwool) would see it ... because that tends to influence cost and their willingness to take on the project.

[joth]

15 hours ago, lineweight said:

I think it might in fact be the 'normal' way to do it when you're using woodfibre insulation?

 

ah yes, sorry I wasn't very clear but the links I linked upthread were exactly to make this point. Thanks

 

[lineweight]

On 25/11/2020 at 18:01, pocster said:

Yes . Foiled backed stuff . All around the timber frame externally ( with either cladding or a thermolite wall beyond that ) . Then pir in the timber studs . Also in between ceiling joists .Then 50mm pir over all of that and foil taped all joints .

You can only insulate once ......

 

What was your outer cladding in this instance?

[Pocster]

2 minutes ago, lineweight said:

 

What was your outer cladding in this instance?

Depends ! Architect made it different in different sides .

Side are just rendered Thermalite with a cavity tied through the pir to the timber frame . Front and rear are vertical studs with pir in between. Then horizontal stud with pir in between. Then vertical studs with no pir to allow air flow . Render board type cladding fixed to that .

[Pocster]

Oh and pir internally in all stud walls 

[lineweight]

1 hour ago, pocster said:

Front and rear are vertical studs with pir in between. Then horizontal stud with pir in between. Then vertical studs with no pir to allow air flow . Render board type cladding fixed to that .

 

Cheers. So in that case, there's not actually a continuous layer of insulation outside of the timber frame (it's interrupted by the horizontal studs). But in the bits with the blockwork outer skin, the insulation is continuous across the timber frame (only the wall ties crossing it).

 

The reason I ask... when I look carefully at, say, Celotex's guidance for the use of their insulation boards, it's fairly clear that it's ok to run them on the outside of the timber frame when the timber frame forms the inner skin of a cavity wall.

 

It's a little bit less clear, when you want to to the same, but then have some form of cladding (say, vertically hung tiles, or render board as per your case) on the outside, instead of a self supporting masonry skin.

 

When you go and look at the BBA certificate for the celotex PIR, it only really mentions build-ups with a masonry outer layer. I'm curious why this is. Is there something about the combination of insulation-outside-timber-frame and then a cladding layer that they are wary about recommending?

[Pocster]

1 minute ago, lineweight said:

Cheers. So in that case, there's not actually a continuous layer of insulation outside of the timber frame (it's interrupted by the horizontal studs). But in the bits with the blockwork outer skin, the insulation is continuous across the timber frame (only the wall ties crossing it).

You got me ! Lol !

There’s a continuous layer outside left and right sides ? It does ‘continue’ front and back but due to the cladding had to have studs . If we didn’t have the cladding it would of fully wrapped around all 4 external walls . I forget the make up of my own build until someone prods me ! ?

 

 

[Mr Punter]

10 minutes ago, lineweight said:

 

Cheers. So in that case, there's not actually a continuous layer of insulation outside of the timber frame (it's interrupted by the horizontal studs). But in the bits with the blockwork outer skin, the insulation is continuous across the timber frame (only the wall ties crossing it).

 

The reason I ask... when I look carefully at, say, Celotex's guidance for the use of their insulation boards, it's fairly clear that it's ok to run them on the outside of the timber frame when the timber frame forms the inner skin of a cavity wall.

 

It's a little bit less clear, when you want to to the same, but then have some form of cladding (say, vertically hung tiles, or render board as per your case) on the outside, instead of a self supporting masonry skin.

 

When you go and look at the BBA certificate for the celotex PIR, it only really mentions build-ups with a masonry outer layer. I'm curious why this is. Is there something about the combination of insulation-outside-timber-frame and then a cladding layer that they are wary about recommending?

 

I doubt that Celotex will propose any solution with their insulation externally plus rainscreen cladding.  Grenfell put a stop to that.

 

There may be an increased chance of interstitial condensation if you us a foil backed board externally, but not enough to lose sleep over.

 

If the cladding is heavy and the insulation is thick, fixings can be an issue.  With the insulation internally you only have the plasterboards and the floor can take the weight of these (I prefer on packers in case of leaks).

[lineweight]

6 minutes ago, Mr Punter said:

 

I doubt that Celotex will propose any solution with their insulation externally plus rainscreen cladding.  Grenfell put a stop to that.

 

There may be an increased chance of interstitial condensation if you us a foil backed board externally, but not enough to lose sleep over.

 

If the cladding is heavy and the insulation is thick, fixings can be an issue.  With the insulation internally you only have the plasterboards and the floor can take the weight of these (I prefer on packers in case of leaks).

 

yes, I wondered if Celotex have quietly changed their recommendations post Grenfell, even for situations under the 18m limit.

 

Indeed, the issue of fixings is one to check out and I can see how this probably becomes what limits the thickness.

 

Not quite sure what you mean about the interstitial condensation risk... the idea of keeping the insulation outside of the timber frame is to eliminate that entirely, as the inner face of the insulation will be warm, and then then it's continuous through to the outside.

[Mr Punter]

I think the risk may be where you have some insulation in the frame without a vapour barrier and the dew point moves within the frame and the condensed water cannot escape.  Don't ask me a follow up on this as I am struggling now!

[lineweight]

The general principle is that it should be ok if the insulation outside of the frame is equal to, or more than the insulation inside the frame.

[Ian]

With regard to condensation risk here's a couple of extracts from the current version of BS 5250: 'Code of practice for control of condensation in buildings'

 

Some of the key points are:

Page 55: A VCL with a vapour resistance of at least double that of the [external] sheathing should be provided on the warm side of the insulation.

Page 92: If different types of insulation are used in successive layers, the layer with the greater vapour resistance should always be placed closer to the heated interior.

[lineweight]

27 minutes ago, Ian said:

With regard to condensation risk here's a couple of extracts from the current version of BS 5250: 'Code of practice for control of condensation in buildings'

 

Some of the key points are:

Page 55: A VCL with a vapour resistance of at least double that of the [external] sheathing should be provided on the warm side of the insulation.

 

 

 

 

Note though, that comment is made specifically in relation to situations where the insulation is internal to the timber frame.

 

This is why putting the insulation on the outside of the frame is much preferable from the point of view of reducing condensation risk.

[lineweight]

I just asked about this on a TRADA timber frame webinar.

 

Asked why insulation external to the timber structure seems relatively conventional if you are talking about a roof, but not a wall.

 

The answer was basically to do with ease of construction, how you fit the external layer and then provide support for whatever cladding type you are using.

 

However, they also said, that things might need to start changing, now that building regs are more concerned about thermal bridging (and achieving better U values in general). So, we might start to see details with external insulation become a bit more comonplace.