Loft conversion - roof insulation

[Engineer1]

Hello,

I hope someone can help. I had a loft conversion built, by a company with a good reputation, who took me to see other work they had done. The work looked great, and their clients were very happy. So, I was happy to proceed.

All went well, until toward the end of the job(*). It's been 'finished' now, bar the snagging. Or so I thought. They used a private building control company, who have informed me that the pitched roof insulation is inadequate. There is Celotex between the rafters, but the plasterboard is fixed directly to the rafters. It seems that continuous insulation is required between the rafters and the plasterboard.

The most pragmatic solution (although not ideal) is to have PIR put over the existing, plastered, ceiling. Or, celotex and then plasterboard.

I'm struggling to find out what thickness of insulation will be adequate to satisfy BC. Does anyone have any recommendations for this?

(* It turned out that the company owner, who, until the toward the end of my build, had been exceptional, developed a serious cocaine habit, and destroyed the company, and lots of other things. He's now done a runner to Canada, leaving me, and lots of other people, in a significant mess.)

 

[^woody^]

It depends what thickness and type is between the rafters (and rafter spacing). It may be 50 or 60mm or could be more.

If using the existing rafters, then having two layers of plasterboard may create a [over] loading issue

 

[tony1851]

For roof insulation, you're looking at a total 100mm min. thickness if using PIR.
So if you had - say - 50 between the rafters, you'd need 50 + plasterboard below.
Howcome the inspector didn't advise the builder before applying the plasterboard?

 

[freddiemercurystwin]

Cos he was private probably.

 

[frutbunn]

Howcome the inspector didn't advise the builder before applying the plasterboard?

Had he even been on site?

 

[Engineer1]

For roof insulation, you're looking at a total 100mm min. thickness if using PIR.
So if you had - say - 50 between the rafters, you'd need 50 + plasterboard below.
Howcome the inspector didn't advise the builder before applying the plasterboard?

There is 100mm between the rafters. The issue is that the plasterboard can't be fixed directly to the rafters, because of potential condensation problems, as the rafters can get very cold. There needs to be a continuous insulation layer between the plasterboard and the rafters.

It's the thickness of this 'extra' layer that I need to determine.

 

[Engineer1]

Had he even been on site?

No!

 

[freddiemercurystwin]

Just login to the celotex U Value calculator, pop in the details and voila.

 

[frutbunn]

No

Sounds about right for a private inspector!
I have a friend who mainly did lofts and used to use a private inspector for that very reason, unfortunately he exhausted most of the local ones and eventually had to return back to the LA as, unfortunately, we can't refuse his work unlike a private inspector.
Cheeky sod even advised a builder I was dealing with on a job on the opposite side of the road from one of his lofts to use a PI as "they hardly ever come out".