140mm Frametherm vs 140mm PIR

[BMcN]

I am still costing up my build and I am looking for advice on insulation.  

 

House will be timber frame with a block + render skin.

 

Working off Scotframe spec as a guide to follow their 4* or 3* build.  I am slightly unsure on the external walls though, they show 140mm Frametherm 32 on the 3* open and 140mm PU on the 3* closed .  Both show with a U' Value W/m2K of 0.15.

 

If I try this on an online U calc, I get 0.15 for PU,   0.13 for PIR and 0.23 for glasswool.   

 

Any ideas?

 

 

[MikeSharp01]

4 minutes ago, BMcN said:

If I try this on an online U calc, I get 0.15 for PU,   0.13 for PIR and 0.23 for glasswool.   

The manufacturers numbers Sound odd given a common thickness of insulation the two / three have very different insulating characteristics. Your numbers look more in line with what might be expected.

Edited October 16, 2017 by MikeSharp01
Improve readability

[BMcN]

34 minutes ago, MikeSharp01 said:

The manufacturers numbers Sound odd given a common thickness of insulation the two / three have very different insulating characteristics. Your numbers look more in line with what might be expected.

I have just noticed the 3* open has 140mm Frametherm + 25mm PU.  This would still be less than 140mm PU though I'd have thought?

[ProDave]

Are you buying a package of frame with insulation fitted, or buying the frame and fitting the insulation yourself?

 

 

[MikeSharp01]

37 minutes ago, BMcN said:

I have just noticed the 3* open has 140mm Frametherm + 25mm PU.  This would still be less than 140mm PU though I'd have thought?

Ah - that makes more sense -  the thickness is now 25mm greater ad the total U value of the build up (all the bits) is worked out using the process:

 

First find the Resistivity (R1) of the frametherm portion = material thickness (d ) / Thermal conductivity coefficient (k) [Known as Lambda]

then for R2 (the 25mm of PU) = material thickness (d ) / Thermal conductivity coefficient (k)

 

add R1 + R2 to get a total R

 

And the U value = 1/R

 

So plucking some values for frame therm and PU the double build up is

 

U = 1/(.025 / .022) + (.140/.032) = 0.18

 

while for just PU

 

U = 1/(.140/.22) = 0.15

 

Much more as quoted but they will also include the sheathing board, internal plasterboard, air gaps and everything else in the build up, all of which have Lambda values and thickness.

 

 

[BMcN]

45 minutes ago, ProDave said:

Are you buying a package of frame with insulation fitted, or buying the frame and fitting the insulation yourself?

 

 

It will be built on site and insulated myself.  Just using Scotframe as an initial guide for pricing.

 

Thanks Mike, by putting in everything from render to plaster it does indeed come down to 0.15-0.16 with 30-40mm PIR.  

[Jeremy Harris]

Worth looking at decrement delay, not just the U value.  PIR can given a good U value in a thin frame, but has a very short decrement delay, so may be less comfortable to live in.  Decrement delay is important to comfort, in my view perhaps more important that just the U value of the structure.

 

It's worth reading this article to get an idea as to how decrement delay impacts on comfort:  http://www.greenspec.co.uk/building-design/decrement-delay/

[MikeSharp01]

+1 on the @JSHarris offered link above its a great article and has several very good references.

Edited October 16, 2017 by MikeSharp01
Typo

[ProDave]

35 minutes ago, BMcN said:

It will be built on site and insulated myself.  Just using Scotframe as an initial guide for pricing.

 

Thanks Mike, by putting in everything from render to plaster it does indeed come down to 0.15-0.16 with 30-40mm PIR.  

The reason I asked is to do with ease of fitting.  If they made the frame with the PIR insulation it would likely be machine cut and a good fit. If you are fitting it, then it is a lot harder to cut by hand and get a perfect fit. So for DIY fit, the frametherm has many advantages. It is flexible enough to compress a little, so cut a few mm over size and push it in. It is then stiff enough to stay put.

 

As already noted the frametherm will have a better decrement delay.

 

My own build has 190mm of frametherm 35 and 100mm of wood fibre board over the outside of the frame. Adding it all up, render, wood fibre, frametherm filled frame, OSB, service void, plasterboard gives a U value of 0.14.

[BMcN]

JSHarris, thanks. I will have a good read through tomorrow.

 

Thanks for the points on the frametherm.  Certainly is a lot of work installing the PIR if cutting.  

 

Just had a quick google of the wood fibre and it seems pretty expensive, in the region of 10p/m.  Does that sound about right?  Also I presume that would need to be decided early on to allow the frame to be spaced in a bit from the blockwork?