Roof insulation and cavity size

[Walshie]

Hi,

I’m in the latter stage of finishing a new roof.

 

I could use some advice on insulation and cavity size.

 

I am able to get 150mm of insulation between the rafters and then plaster board, leaving a 25mm air cavity.

 

My questions are:

 

Will this be ok from a building regs point of view?

 

Will it be sufficiently thermally effective?

 

Any thoughts please?

Thanks v m,

 

 

[Redbeard]

UK Gov't guidance (https://assets.publishing.service.gov.uk/media/61d727d18fa8f50594b59305/retrofit-room-in-roof-insulation-best-practice.pdf) now suggests 50mm ventilation. The resultant  125mm of PIR between rafters is not, I think, going to meet the Building Regs target of 0.16W/m2K. Rough values: 125mm PIR (to leave that 50mm vent gap): R value 5.68m2K/W approx. 'Base case' R value of uninsulated roof (if indeed you allow it any): 0.5m2K/W. Total 6.18m2K/W. 1/R = U so U value (v. approx) = 0.1618W/m2K, but this leaves no allowance for the timbers which interrupt the insyulation every (?) 400mm, so it's not 'real'. When I was doing attic room re-fits on a regular basis I  used to ignore the 'base case' value and look for a depth of PIR which would give me 0.16 on its own. Of course you could simply add 25mm PIR below the rafters and you have not only cloaked your thermal bridge but, with 125mm between the rafters, you'll achieve a 'gross' U value (of course assuming 150 throughout; it's not area-weighted) of around 0.147W/m2K. A lot depends on headroom as battens and plasterboard & skim will lose you another 40mm.

[Iceverge]

53 minutes ago, Walshie said:

Hi,

I’m in the latter stage of finishing a new roof.

 

I could use some advice on insulation and cavity size.

 

I am able to get 150mm of insulation between the rafters and then plaster board, leaving a 25mm air cavity.

 

My questions are:

 

Will this be ok from a building regs point of view?

 

Will it be sufficiently thermally effective?

 

Any thoughts please?

Thanks v m,

 

 

 

I assume that you are thinking about PIR between the rafters.

 

Don't! 

 

To hard to fit, too wasteful, shrinks and falls out, bad in fire, bad for noise etc etc etc....

 

I assume you have 175mm rafters at 600cc? 

 

Full fill, use 75mm insulated service cavity with mineral wool. 

 

 

 

[Nickfromwales]

2 minutes ago, Iceverge said:

 

I assume that you are thinking about PIR between the rafters.

 

Don't! 

 

To hard to fit, too wasteful, shrinks and falls out, bad in fire, bad for noise etc etc etc....

 

I assume you have 175mm rafters at 600cc? 

 

Full fill, use 75mm insulated service cavity with mineral wool. 

 

 

 

Typo (75mm should read 175mm?) 

[Nickfromwales]

Defo should have an additional layer of PIR going over the rafters to the room interior, to stop the quite adverse repeat cold bridging; (25mm to 40mm thickness for this layer, depending on how much head height you can sacrifice?).

[Iceverge]

If you must use PIR put it outside the roof and diligently tape the OSB as an airtight layer. 

 

 

Far better to have all timbers inside the insulation for long term durability. etc. This is also a very easy way to make a room in roof airtight if you return a membrane to the inner wall surface through a double sandwiched wall plate. 

[Iceverge]

2 minutes ago, Nickfromwales said:

Typo (75mm should read 175mm?) 

 

----furiously rereads post.----------

 

 

Nope, 75m service cavity to get below 0.16W/m2K. I would prefer 220mm and 50mm service cavity as it is nail gun friendly but as we're starting with 175mm rafters we're stuck.

[Nickfromwales]

1 minute ago, Iceverge said:

 

----furiously rereads post.----------

 

 

Nope, 75m service cavity to get below 0.16W/m2K. I would prefer 220mm and 50mm service cavity as it is nail gun friendly but as we're starting with 175mm rafters we're stuck.

You didn't have to reread it furiously lol . Just wanted to check.

[Nickfromwales]

@Walshie

 

Room in roof or warm attic?