Cold roof insulation - general tips and materials recommendations?

11 Sep 2021
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United Kingdom

I’m hoping to improve the loft insulation of a cold roof of a large wood clad, timber frame bungalow that is currently poorly insulated and struggles with heat retention. It is currently largely single glazed (I’m working on secondary glazing with Magnetglaze Extreme Pro and 4-6mm Perspex). It’s been extended multiple times over several decades. Much of the construction appears to be wood clad single skin brick or breeze block with no insulation. Roof is a lightweight wooden structure with a thin metal roof tile (Decra Classic).

Given the poor thermal performance of the building, I’m inclined towards over-insulationg it / being overly cautious - unless of course that makes no sense.

(A) Loft over bedrooms – currently partially insulated with loose fibre.
  • Planning to:
  • Add downlight covers over LED lights*
  • Add more loose fill mineral wool where there is none between the joists (so c. 200mm).
  • Then add another layer (I assume 100mm – but perhaps 200mm worthwhile?) everywhere over the joists/existing insulation, save for the area where the solar inverters/batteries, etc are. There I plan to create a “walkway” (well, crawling patch) from the loft hatch to there for occasional access with some PIR boards (75mm thick? E.g. Celotex GA4075) (instead of topping over joists with loose fibre). Though given size of loft hatch and limited space in the loft, it might be more sensible to order smaller sizes of PIR board!




(B) Skylight
There is a large skylight.
  • Pictured from inside the loft:

  • The other side is plasterboard into the room:

The (presumably plywood) wooden frame around it is partly insulated. Planning to:

Downlight covers over LED lights*
- Replace (and uses elsewhere) existing insulation between the (c. 45-47mm) studs. I’m tempted by 40mm Rockwool RW3 (60kg/m3) between the studs (for its acoustic performance, as well as thermal). But perhaps PIR should be considered instead?
Then clad over it all, studs included, with PIR boards (appropriate thickness). How best to secure it? (Part of this will need to be inside that secondary 100mm frame)
- So a bit like this – but with rockwool between the studs:

(C) Pipe lagging - recommendations welcome.

* I gather fire retardant downlight covers will be required for the LEDs (e.g. Loft Lids).

Comments on how best to approach A, B and C, tips, and recommendations for suitable materials (sizes, brands, etc), suppliers, etc would be most welcome.

I’ve assumed that because I’m essentially topping up existing insulation that there’s little sense in changing horses on material, e.g. sheepwool or aerogel.
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