Talk to me about Phenolic Insulation - Kingspan Kooltherm / Xtratherm Safe-R etc

[DC5]

Hi Folks,

 

Some input appreciated on the following!

 

In our loft conversion our architect has most recently specified in agreement with BC -

 

150mm insulation between the 160mm deep rafters (externally sarking, high performance breathable membrane and slate) 

25mm on top of the rafters to stop cold bridging etc (then plasterboard to finish).

 

Originally we had planned 100mm between the rafters with PIR but Building control wanted a roof vent top and bottom on every!!! rafter bay for the 60mm void (which externally would have looked ridiculous) but it was because we have no eaves space for top to bottom and bay to bay air flow (very old house the rafters terminate in the wall head). 

 

Because of this we have gone for the route at the start, this also means we only need the 25mm insulation on top of the rafters mentioned which increases ceiling height internally.

 

So what I'm not sure about is the architect and BC officer have agreed on the 10mm void behind the insulation with a highly breathable membrane over the sarking externally (under slate).

 

What are peoples thoughts on this my feeling is that there should be no void in this application compared to say a 50mm void behind insulation board where vents are present (I have spoken to the kingspan technical team and they said it was fine but for some reason I wasn't totally convinced! Is there benefit to the 10mm gap?

 

I was also surprised to see on a Kingspan Kooltherm that it has small pin holes compared to none on normal PIR does this mean that vapour can pass through it?

 

Finally how does xtratherm safe-r phenolic compare to kingspan kooltherm?

 

In general thoughts or musings appreciated!

Edited January 16, 2019 by DC5

[MarkyP]

I noticed you didn't get a response so thought I'd share some thoughts..

 

what is the sarking made of? If it's lengths of timber with small gaps between each strip then there ought not to be a need to ventilate as it will be vapour open (with your breather and slates above) so the condensation risk on the underside of the sarking would be low, or any that did form could evaporate away readily when conditions suited. If the sarking is sheet material, like ply or OSB, then this is less breathable and the standard approach would be a ventilated void to mitigate perceived risk of condensation forming on the underside of the sarking. However, there are multiple examples of builds which have eschewed this common thinking and fully filled under OSB sarking ( using spray foam, blown cellulose or mineral wool) with vapour control to the warm side.

 

In your case, I don't understand what the 10mm void is doing. Perhaps PIR boards are easier to source in 150mm rather than 160mm? Or perhaps this is just a fitting allowance - you mention an old house, are the rafter voids all universally 160mm deep? Again, if the sarking is strips of wood with lots of nice breather gaps, then I don't think the void is an issue. You could use a warm side VCL as an additional measure for peace of mind (just taping the foil face PIR under rafter would do, I think)

 

I can't comment on the questions about the differences between the sheet in insulation types.

 

 

 

 

 

 

[Ed Davies]

If the sarking is traditional 150 mm treated boards then maybe the idea is that the 10 mm gap allows any moisture under the middle of a board access to the gap above or below.

 

My house has 22x150 sarking boards which will have mineral wool underneath. The BCO was happy with the idea that the mineral wool is vapour open enough to allow this access but I doubt he would have been if it'd been planning PIR between the rafters.

[LeeVanCleef]

I did a similar thing a few years ago and left a 50mm gap. I also noticed an abundance of nails poking through the sarking which were holding the slates on the other side in place. The 10mm might be so you don't interfere with those? Presumably the insulation would just get skewered if you didn't have a gap (or a nail might get pushed out?!)

[scottishjohn]

 my thoughts would be to use counter battens

this would be the make up 

membrane on top of sarking boards 

vertical battons attached to sarking 

then membrane that very slightly sags with counter battons on top of it 

that gives you your 50mm void and nice draft up roof to dry things out and just enough sag to stop water gathering up behind the cross battons

then fix slates to counter battons.

fit bug screens at lower edge of roof  ay gutter level and some ventilated ridges at top 

this should mean you can fill under sarking  with insualtion  as it is now a cold roof with ventilation 

Edited January 25, 2019 by scottishjohn