Warm roof specification

[Becca]

We signed with a timber frame supplier last week ?.  I asked the timber frame company about changing to a warm roof as their spec had no insulation in the small attic space above the horizontal ceiling of our "rooms in roof" attic.  They have come back with this as an optional extra:

 

"To replace supply only 280mm quilt between bottom chord of top hat truss with 150mm loft roll insulation and supply only of 140mm PIR to slope of truss. No additional VC foil/service battens or 
plasterboard included."

 

The attic area is a very small crawl space, but will be the likely site of the MVHR unit and otherwise is just for storage.  Cost of this optional extra is nearly £2k extra, supply only....

 

I always said we wouldn't compromise on insulation and airtightness, but does this spec seem okay?  Cost-wise I wondered whether to leave the 280mm quilt, and buy what we need separately, but would any heat actually reach the attic area with that level of insulation on the horizontal? ?

[ProDave]

That does not sound like a true warm roof.  A true warm roof needs insulation above the rafters before the tiles and then you can full fill the rafter space.  Clarify with them what they mean by 140mm PIR to slope of truss"  To me that sounds like they mean under the rafters but I could be wrong.

[Becca]

Thanks, will check, but I think you're right - it's PIR between the rafters.  The question is what to do if this is the only upgrade on offer.  Perhaps best left with the original spec and insulate the MVHR, or move it into the store on the first floor.  

[Iceverge]

I’d keep the MVHR out of the attic If I could. 

 

Easier to get a good airtight envelope, avoid possible noise issues, no difficulties accessing for servicing, no summer overheating issues with the MVHR drawing in very hot air from just above the slates if you have roof terminals. 

 

If you only use PIR between the rafters your insulated space in the roof will get bonkers hot in the summer. It has a low decrement delay meaning heat will soak through very fast from the sun heated slates.  A natural insulation like woodfiber will help with this but is not cheap. 

 

What we did was blow 400mm cellulose at ceiling level in our attic. Not the cheapest but less than PIR and, if you have the space, better in every way. 

 

 

 

 

Edited April 7, 2021 by Iceverge

[Becca]

On 07/04/2021 at 23:21, Iceverge said:

I’d keep the MVHR out of the attic If I could. 

 

 

Thanks, will keep the MVHR out of the attic if we possibly can.  Still much to learn, but glad to have this forum!