Ceiling insulation and voids

[cwr]

Original plan for part out build was to have cold roof with fink trusses, 300mm mineral wool between and over at ceiling, 25mm PIR below, airtight membrane below that then service void (22mm battens and counter battens) then plasterboard.

 

Wondering about no PIR and mineral wool stuffed in service void after 1st fix.

Is the PIR layer worth it?

[JohnMo]

If you are adding an airtight membrane and battons, look at something like PHS Hi Therma, which with a service void gives an increase R value of 0.79. Then skip the PIR.

[Iceverge]

Hi @cwr

 

I've run your choices through Ubakus. For me I would do 400mm earthwool and an unventilated service cavity. Cheapest and easiest to build.

 

If you want better cellulose is best for heat protection, soundproofing , airtightness,  embedded carbon, ease of installation, fire protection, real world continuity of insulation and thermal bypass. It's what I did on my own roof. Also rolling out loft roll is awful. 

 

@JohnMo

Reflective layers do help in keeping heat in but their contribution is very small.  Most of the insulation in your case comes from the stationary air. Here's a study if you like but I've highlighted the last line anyway.  https://www.insulationstop.com/resources/TR_2011-01_Reflective_Insulation_in_Cold_Climates.pdf

 

Original Proposal

 

 

[JohnMo]

The main reason for using was really airtightness and vapour control, so was applied per instructions and used their tapes and jointing materials.  If it adds anything to thermal efficiency that's an added benefit.

[cwr]

Thanks for the replies and for running the figures through Ubakus. So more insulation above the simplest way to go.

 

Do you think a layer of PIR below the joists helps when considering the other members of the truss may create thermal bridging through the insulation, or is this not a significant effect?

[cwr]

Moving forwards with this. Next question is pitch of battens, is 400mm required or 600mm ok for suspending plasterboard off?

[markc]

20 minutes ago, cwr said:

Moving forwards with this. Next question is pitch of battens, is 400mm required or 600mm ok for suspending plasterboard off?

600mm is fine for ceilings, or walls that won’t take any knocks.

Edited November 22, 2021 by markc