airgap - insulating old slate roof with celotex kingspan pir

[sharknose]

Would it be ok to just have a 30mm air gap (rather than the standard 50mm) between the kingspan pir and the under side of the roof battons plus the extra bit of space (0mm - 10mm ) between the under side of the battons and the lime mortar underneath the slate. I dont have any felt. just the lime mortar which fills in the gaps between the slates. I am only insulating 3 meters of the bottom part of the roof as the top part is attic space. There is no air vents at eves so i will probably drill a few holes at the eves to allow some air flow to get through inbetween each rafter. Any input anyone? Does that sound ok to you?

 

[geraldthehamster]

Would it be ok to just have a 30mm air gap (rather than the standard 50mm) between the kingspan pir and the under side of the roof battons plus the extra bit of space (0mm - 10mm ) between the under side of the battons and the lime mortar underneath the slate. I dont have any felt. just the lime mortar which fills in the gaps between the slates. I am only insulating 3 meters of the bottom part of the roof as the top part is attic space. There is no air vents at eves so i will probably drill a few holes at the eves to allow some air flow to get through inbetween each rafter. Any input anyone? Does that sound ok to you?

If your slate is unfelted, then in theory I don't think an air gap is required at all.

Cheers
Richard

 

[sharknose]

What ive read up about on the internet is that if it is unfelted you could use a 25-30mm gap because the air could circulate easily through the gaps of the slate. Problem is with my roof there is a sort of lime mortar under the slates, arching from batton to batton, presumably to seal the drafts from entering the gaps in the slate, which would i turn stop that circulation I need. I could take of all the lime mortar I guess but dont fancy it if i dont have to.

 

[geraldthehamster]

What ive read up about on the internet is that if it is unfelted you could use a 25-30mm gap because the air could circulate easily through the gaps of the slate. Problem is with my roof there is a sort of lime mortar under the slates, arching from batton to batton, presumably to seal the drafts from entering the gaps in the slate, which would i turn stop that circulation I need. I could take of all the lime mortar I guess but dont fancy it if i dont have to.

Does the mortar cover the whole of the joints between slates? I've lived in a couple of Victorian/Edwardian houses that had cement fillets under the slates, on the top side of the battens, but this wasn't continuous. Are you saying that your tiles are, effectively, rendered underneath?

I had a loft conversion done in one of those houses, and the BCO was happy for the gaps between the rafters to be packed, without an air gap. Although I have to say that I *can* see the virtue of an air gap, and it certainly couldn't hurt to have one. I don't think anyone would be asking for the 50mm that would be needed for an imperviously felted roof (as opposed to one with a breathable membrane).

Bear in mind that lime mortar is also pervious to moisture.

Cheers
Richard

 

[sharknose]

Yes, it is more or less rendered. I can't see any slate in any case. It would be a batten thickness (about one inch) at the batten and then arches to about 5mm or so just under the slate half way between each batten. I think I will go with a 25mm airgap from bottom of the batten and with the extra 10mm or so in the arch (effectively 35mm), I think this would be sufficient.

The info of the CBO coupled with the breathability of the lime mortar has definetly settled my mind on the smaller airgap issue

Thanks again.
Justin