Foil insulation worth paying for better performance?

[Happy Valley]

We will have a warm roof with 100mm of PIR. Below the roof we plan to staple multifoil insulation across the whole of the roof area and tape it up.

 

We have the choice of Superfoil SF40 with an R value for a roof of 3.47 costing £1120 + VAT for materials or:

 

YBS superquilt with an R value for a roof of 2.5 costing £595 + VAT for the materials.

 

Obviously the Superfoil provides better thermal performance but is it worth another £535 + VAT to get there?

[SteamyTea]

What thickness are they?

[Happy Valley]

Superfoil is 60mm

YBS quilt is 40mm

[SteamyTea]

This translate into a U of:

Superfoil 0.29 W.m^-2.K^-1

YBS 0.4 W.m^-2.K^-1

 

So effectively 0.1 W.m^-2.K^-1 different.

 

Check the details carefully as in the past multifoils only met the manufacturers claims if they were installed with other types of insulation.  

 

[Thorfun]

11 minutes ago, SteamyTea said:

Check the details carefully as in the past multifoils only met the manufacturers claims if they were installed with other types of insulation.  

and they need to have an air gap to get the claimed performance, right? at least that's what I remember although I have been known to be wrong before.

[SteamyTea]

1 minute ago, Thorfun said:

and they need to have an air gap to get the claimed performance, right? at least that's what I remember although I have been known to be wrong before.

I seem to remember that as well.

Also seem to remember that under the normal thermal conductivity testing regime, they performed worse than traditional insulation..

Bit of light reading here:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=125&page=1#Item_0

[Thorfun]

5 minutes ago, SteamyTea said:

I seem to remember that as well.

Also seem to remember that under the normal thermal conductivity testing regime, they performed worse than traditional insulation..

Bit of light reading here:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=125&page=1#Item_0

yeah, I did a lot of reading on this stuff when I started out as ScandiaHaus use it. I decided it wasn't for me (and ScandiaHaus were stupidly expensive!) but I know that some on here have had very good results with it. I'm a sceptic though and decided to stick with 'normal' insulation.

[keith65]

If this is a warm roof adding insulation under the roof timbers might cause condensation in the future. Best to keep all insulation on the outside on a warm roof.

[Happy Valley]

15 hours ago, SteamyTea said:

I seem to remember that as well.

Also seem to remember that under the normal thermal conductivity testing regime, they performed worse than traditional insulation..

Bit of light reading here:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=125&page=1#Item_0

 

Thanks for that.

 

If the Superfoil performs within 85% of it's claims then it will be within the expected design performance. 

Edited December 3, 2021 by Happy Valley

[Olf]

21 hours ago, SteamyTea said:

I seem to remember that as well.

Also seem to remember that under the normal thermal conductivity testing regime, they performed worse than traditional insulation..

Bit of light reading here:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=125&page=1#Item_0

 

My opinion (no evidence to support it though) is that the manufacturers' marketing departments came with 'equivalent U value' where they calculate (or make up) gains due to low emissivity of the foil face (hence the gap required) and improved airtightness and turn reduction of the heat loss in their model into what U value of conducive losses would be.

 

In already airtight house with foil facing rigid boards (or other low-e faced material used) the benefit of using them will be close to 0.

In droughty, wool filled house, they would do something good, but still not sure if offer value for money

[Gordo]

On 02/12/2021 at 13:12, Happy Valley said:

We will have a warm roof with 100mm of PIR. Below the roof we plan to staple multifoil insulation across the whole of the roof area and tape it up.

 

We have the choice of Superfoil SF40 with an R value for a roof of 3.47 costing £1120 + VAT for materials or:

 

YBS superquilt with an R value for a roof of 2.5 costing £595 + VAT for the materials.

 

Obviously the Superfoil provides better thermal performance but is it worth another £535 + VAT to get there?

The sf40 R value of 3.47 equates to equivalent to 75mm PUR insulation or 115mm mineral wool 

 

the YBS equivalent to 55mm PUR insulation.

 

the sealed air gap, reflective surface and additional surface resistances will also contribute a little more to overall U value

 

you don’t quote the cost per m2 so don’t know if it’s good value

 

these foil insulation’s were the subject of protracted legal cases over false advertising. They were unable to prove that they performed as advertised (using the recognised BS hot box test) therefore they should quote R values for comparisons. But this is often hidden in small print. And although they may show BBA logo on advertising you will find it difficult to get hands on actual BBA certificate for their independent information.

 

although I’d usually advise against a hybrid roof make up I doubt the lowish performance of the foil will be a concern with interstitial condensation but best do a condensation risk analysis 

Edited December 3, 2021 by Gordo

[Happy Valley]

Thanks for all the responses.

 

The roofing contractor says there will be no issue with interstitial condensation but this is beginning to concern me slightly now.

 

So other than foil insulation already mentioned or the expensive and time consuming option of 70mm PIR insulation between the rafters/spars what other options do I have to improve the efficiency of the roof? The spars are 47mm x 200mm spaced with 450mm centres.

[JohnMo]

Have been on to the two companies websites and downloaded the installation instructions, they include all the approved installation schemes and the expected u values.  It tells you where you need air gaps and what depth they need to be etc.

 

 

[Thorfun]

29 minutes ago, Happy Valley said:

Thanks for all the responses.

 

The roofing contractor says there will be no issue with interstitial condensation but this is beginning to concern me slightly now.

 

So other than foil insulation already mentioned or the expensive and time consuming option of 70mm PIR insulation between the rafters/spars what other options do I have to improve the efficiency of the roof? The spars are 47mm x 200mm spaced with 450mm centres.

we used Proctor Roofshield (https://www.proctorgroup.com/products/roofshield) as our roofing membrane. This is allowing us to fully fill our 197mm rafters (actually putting in 190mm due to availability of thicknesses) of Isover Frametherm 32 glass mineral wool batts with 90mm PIR below the rafters.

 

 

you could try this route. speak to your preferred insulation manufacturer and ask them to do a condensation risk analysis on your chosen make up and they'll do the calculations for you.

[Happy Valley]

Superfoil have come back with the following with regards to condensation:

 

"The condensation risk analysis does show a miniscule risk between the OSB and EPDM layer, this is unlikely a cause for much concern and down to assuming a worst case for the EPDM layers vapour resistance."

[Gordo]

1 hour ago, Happy Valley said:

Superfoil have come back with the following with regards to condensation:

 

"The condensation risk analysis does show a miniscule risk between the OSB and EPDM layer, this is unlikely a cause for much concern and down to assuming a worst case for the EPDM layers vapour resistance."

Bump up thickness of PUR should sort this if just marginal a risk.