Spray foam u value

tails_naf · 18-02-2019 11:04pm #1

Hi All,
I am considering insulating between rafters on a new build with a warm attic and was looking a spray foam as an option. I had assumed the u value would be the same as rigid board insulation, but from what I've tracked down it seems to be closer to 0.36w/m2/k vs the rigid board value of nearly 0.2w/m2/k. Any advice on this? Are there different foams, where others achieve better values? A value of .36 means pretty thivk insulated slab is required, so wondering what people do typically.

Calahonda52 · 19-02-2019 10:16am

why not go outside the rafters with the insulation?
what have you planned as the roof build up?

listermint · 19-02-2019 10:18am

I presume its already built ?

tails_naf · 20-02-2019 9:39pm

Wouldn't going outside the rafters take space from the interior of the attic? Plan is to slab the attic and use it as stogage/livable space, ideally achieving .17 u value.

Funnily I've been talking to a few foam suppliers and the are slow to give out numbers like restivity or u value.

The only advantage I see right now is it potentially gives better insulation coverage as it will expand everywhere and fill all gap with an airtight finish.

Calahonda52 · 20-02-2019 10:04pm

For in roof attic insulation, you need to consider, as well as U value,

specific heat capacity
decrement delay
density

dathi · 21-02-2019 11:29am

tails_naf wrote: »

Wouldn't going outside the rafters take space from the interior of the attic? Plan is to slab the attic and use it as stogage/livable space, ideally achieving .17 u value.

Funnily I've been talking to a few foam suppliers and the are slow to give out numbers like restivity or u value.

The only advantage I see right now is it potentially gives better insulation coverage as it will expand everywhere and fill all gap with an airtight finish.

https://www.nsai.ie/certification/agrement-certification/search-agrements-certificates/

you can find the values you are looking for here if you search by product name

tails_naf · 22-02-2019 12:14am

dathi wrote: »

https://www.nsai.ie/certification/agrement-certification/search-agrements-certificates/

you can find the values you are looking for here if you search by product name

That's a great link, thanks. Also debunks what i was told by one supplier that their open celled foam would get a u value of 0.22 with 125mm depth. That would suggest a lambda of 0.027 - which seems optimistic after looking at a few certs there

tails_naf · 22-02-2019 12:16am

Calahonda52 wrote: »

For in roof attic insulation, you need to consider, as well as U value,
specific heat capacity

decrement delay

density

I'm familiar with the first item, but as for the second, what is that in regard to? Also how does density affect it - I mean is the lambda/u value not quoted based on the typical application of the product at it's normal density?

Lumen · 22-02-2019 7:32am

tails_naf wrote: »

I'm familiar with the first item, but as for the second, what is that in regard to? Also how does density affect it - I mean is the lambda/u value not quoted based on the typical application of the product at it's normal density?

Explanations and stuff here, albeit on a page pimping wood fibre.

http://www.acaraconcepts.ie/wood-fibre-insulation/summer-heat-protection/

Calahonda52 · 22-02-2019 12:25pm

Lumen wrote: »

Explanations and stuff here, albeit on a page pimping wood fibre.

http://www.acaraconcepts.ie/wood-fibre-insulation/summer-heat-protection/

Thanks for this Lumen.

OP, the issue is that tiles/felt in summer can reach 45/50 degrees which creates a much higher delta T for heat transfer through insulation.

Spray foam u value

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