Getting old oak framed walls insulated

[andymet87]

Hi all,

Currently renovating a cottage that dates from 1730 (not listed).

Upstairs at the front of the house we have tile hung timber framed walls. The structure of the wall is (from inside to outside):

• 100mm oak frame (with lathe/plaster inset between the wood, the rest of the space between the wood is a void)
• battens on the back of the oak frame
• clay tile hung

So... as you can see it's not that warm over winter atm!

We are looking to resolve this and are considering our options with the hope of achieving a U-Value of under 0.3 (current building regs would require 1.8 I believe). We want to achieve the same aesthetics internally and externally with the finished wall. Unfortunately the lathe/plaster is not in a good state and will have to be replaced regardless...

The current plan is to do the following:

1. remove tiles & battens
2. install TLX Gold to the outside of the oak frame
3. re-batten and tile the wall
4. from the inside fit 50mm Wood Fibre Batts between the oak frame
5. from the inside fit a breathable board and lime plaster between the oak frame

I think this will achieve a u-value of about 0.44 for the wall which isn't great but much better then the current situation.

Has anyone got any suggestions of a better way to do it?

Thanks in advance!

 

[bennno2005]

Have you got scope to form another timber frame on the outside that you could insulate, then counter-batten to fix the tiles, or does the wall need to remain the same thickness?

 

[andymet87]

Have you got scope to form another timber frame on the outside that you could insulate, then counter-batten to fix the tiles, or does the wall need to remain the same thickness?

Afraid it needs to stay the same thickness really, the upstairs frame sits on top of a brick wall. We could add an extra few cm on the outside if we have to but no more then that.

I'm also reviewing my calculations and am getting thoroughly confused by R-values and u-values!

 

[bennno2005]

Have you got scope to form another timber frame on the outside that you could insulate, then counter-batten to fix the tiles, or does the wall need to remain the same thickness?

Afraid it needs to stay the same thickness really, the upstairs frame sits on top of a brick wall. We could add an extra few cm on the outside if we have to but no more then that.

I'm also reviewing my calculations and am getting thoroughly confused by R-values and u-values!

0.44 does seem very low using a TLX system.

Have you got scope to form another timber frame on the outside that you could insulate, then counter-batten to fix the tiles, or does the wall need to remain the same thickness?

Afraid it needs to stay the same thickness really, the upstairs frame sits on top of a brick wall. We could add an extra few cm on the outside if we have to but no more then that.

I'm also reviewing my calculations and am getting thoroughly confused by R-values and u-values!

I see.

0.44 seems a very low-u-value with the use of TLX, so maybe check the calcs again.

Don't forget when you install TLX it needs to have an air gap either side to work (i.e you can't compact it), so your best bet is using battens on the Oak frame, then TLX then more battens fixed directly on top of the other ones.

 

[andymet87]

thanks - I've just phoned TLX and they're running the calcs for me so will see what they come up with!

Any suggestions for better ways to do it?

 

[bennno2005]

thanks - I've just phoned TLX and they're running the calcs for me so will see what they come up with!

Any suggestions for better ways to do it?

Can't think of anything other than using PIR board instead of the wood fibre batts inbetween the Oak frames.

 

[andymet87]

So the very kind lady at TLX has just provided me with the below U-Value calculation for the wall and I think it looks very good - better then I had thought it would be but I'm not sure if she has allowed for plasterboard between or in front of the timber frame... I will go back and confirm.

Calculation below...

any thoughts on this?

 

[bennno2005]

So the very kind lady at TLX has just provided me with the below U-Value calculation for the wall and I think it looks very good - better then I had thought it would be but I'm not sure if she has allowed for plasterboard between or in front of the timber frame... I will go back and confirm.

Calculation below...

View attachment 103553

any thoughts on this?

I'd go for it. Like I said previously the only way to improve it further would be to substitute the steico flex for a PIR board (which has twice the thermal resistance - a 50mm Celotex CG5000 PIR board has 2.35 vs your 1.55 for a 60mm thick wood fibre batt.

 

[andymet87]

I'd go for it. Like I said previously the only way to improve it further would be to substitute the steico flex for a PIR board (which has twice the thermal resistance - a 50mm Celotex CG5000 PIR board has 2.35 vs your 1.55 for a 60mm thick wood fibre batt.

Many thanks - out of interest how would this affect the calculation if that change were to be made? I will look into the likely impact of making the wall not "breathable"!

 

[bennno2005]

I'd go for it. Like I said previously the only way to improve it further would be to substitute the steico flex for a PIR board (which has twice the thermal resistance - a 50mm Celotex CG5000 PIR board has 2.35 vs your 1.55 for a 60mm thick wood fibre batt.

Many thanks - out of interest how would this affect the calculation if that change were to be made? I will look into the likely impact of making the wall not "breathable"!

I'll see if I can work it out for you. I wouldn't think you'd need to worry about it not breathing in a 300 year old house.

 

[andymet87]

I've just asked regarding the use of PIR in the wall and the response was:

Although on the face of it PIR will give better insulating value this would depend on the edges being very well sealed – not always possible with an irregular frame – and this moisture-carrying air is what would normally cause condensation. However TLX Gold prevents this by altering the temperature profile and dewpoint location according to the standard Glaser calculation.

However, getting more sophisticated, WuFi condensation analysis indicates that with old breathable walls like this the obvious strategy of using an internal vapour barrier can actually be harmful –(the Scottish Historic Buildings have done research on this). So that’s why I think your proposed buildup is good.

This tallies with my understanding of it all so think that the setup proposed above is probably the best I'll get given the frame it's going into and the need for it to breath...

Thanks for your help bennno2205