First floor timber extension wall insulation

[tanneja]

Not for me, for a friend.

 

Have a first floor extension in timber, 100mm timber frame with 90mm celotex cut and offered into the frame space between noggins. Internal detail planned was just plasterboard, externally cement board, membrane and render.

 

Surely the areas of the wood frame (maybe 10-20% of the wall) will just be 100mm of timber U value equivalent? I've heard them say they will put 20mm of PIR internally, but by my math that gets to around U=0.53 over any wood element.  To reach 0.28 for extensions the online calculator tells me 65mm of PIR needed.  Whats the sensible approach? Is a room calculation % based given some areas are frame but others the 90mm PIR?  Is the tennant better to insist on thicker insulation to have no spots breaching U0.28? 

 

I should add I am aware that space in the room is a premium.

 

Thanks in advance for any wisdom.

[the_r_sole]

the uvalue calcs take account of the amount of timber in the wall construction - with 100mm kit you'll definitely need some insulation on the inside too rather than just between studs

[Russell griffiths]

That spec looks like a poor loft conversion in the 90,s, it will be cold. 

Unless some serious internal insulation is done as well as good air sealing. 

[tanneja]

@Russell griffiths I was afraid someone would say that, I will recommend they look at non-trivial internal PIR at the sacrifice of the room area.

[the_r_sole]

Usually we'd go for 120mm between (140mm studs) and 35-50 internally

[tanneja]

@the_r_sole I was just corrected that the studs are 150, so 50mm internally on top of that seems to take U = 0.28ish consistent with regs, obviously would be far more for the areas between the joists where it doubles on the insulation.  I have suggested they try and find 50mm on the walls and 100mm on the ceiling (ceiling is also 150mm timber and they planned to put 50mm internally)

[the_r_sole]

what calculator are using for those values?

Kingspan calc gives a lot better on those thicknesses https://www.uvalue-calculator.co.uk/calculator/walls/timber frame/insulation between timber studs with (insulated) plasterboard/140mm/timber cladding/37.5/80/

 

Edited October 21, 2020 by the_r_sole

[tanneja]

@the_r_sole This was my attempt.  I am aware that this only applies to the areas where the studs are, those lengths will have localised poorer U value than the areas between them (which are packed with PIR within the studwork voids).  Still it feels like that comprises a non-trivial area of the wall (10-20%) so presumably you want to bring that amount of area at least up to the extension Regs min?

 

I may be incorrectly using the U value Calculatiors

[the_r_sole]

the kingspan/celotex/any other manufacturers calculations all account for the amount of timber in the wall construction, the uvalues are area weighted, you don't do different calcs for each bit of the wall

[ADLIan]

Default is to use 15% bridging of the insulation in timber frame - manufacturers will take this into account in their U-values.

 

90mm Celotex in the above wall will achieve approx 0.35 W/m²K. adding a further layer of say 25mm internally would drop this to approx 0.24 W/m²K

[tanneja]

Ok, so it is weighted based on % of wall area, thats very reassuring to hear, thank you @the_r_sole. @ADLIan that is precisely what they want to do (90mm celotex intra studwork, 20mm over the top), ok, their plan is sensible for the extension work.  I sincerely appreciate the education.

[tanneja]

@ADLIan my apologies, but just to ensure I use these calculators correctly going forward, I see the below for your 90mm celotex and 25mm internally, getting materially different overall U value, can you see what I am managing to mess up?

 

(fyi it says the 37.5mm insulated plaster board is made up of 12.5mm plasterboard and then the remianed (25mm) is PIR))

Edited October 21, 2020 by tanneja

[A_L]

2 hours ago, tanneja said:

getting materially different overall U value, can you see what I am managing to mess up?

 

here is my version of your wall, U=0.23, if I remove the 15% timber fractions the U value falls to 0.18

 

 

[tanneja]

@A_L You have taught me, this is outstanding, thank you