Roof insulation

[JeremyM]

This cold weather has just made it too cold in my unheated garage to work, yes its been cold even in Swanage! So it has concentrated my mind on sorting out some insulation for the roof.

I have a pitched roof with breathable membrane above the rafters which are 130mm deep. Is it best to insulate between rafters, and if so do I need an air gap between membrane and insulation or should I put insulation under or even both! I'm not looking for high temps and am generally happy working if 50f or above.

It seems simpler to fix below the rafters will 25mm Kooltherm do the job? I'm undecided whether to leave this bare or skin with ply. I would welcome
any advice.

Jeremy

 

[MikeG.]

You should leave a min 50mm clear airgap below the sarking (felt). That leaves you 80mm of the rafter depth available, so I would suggest 75mm mineral wo0l, with 25mm of Kingspan underneath the rafters....50mm if you are feeling rich. I would then screw on a "ceiling" of OSB or ply and paint it white.

Mike

 

[Tenko]

I have conventional rafters and old asbestos tiles which are mainly intact. I was intending to seal the odd crack or two with a bitchumen sealer/filler and then buy 8x4 ft 3inch polistyrene sheets and saw them to fit between the joists. Is that not wise ?

 

[JeremyM]

Mike many thanks for your prompt reply, I will get on and cost this.

Jeremy

 

[MikeG.]

I have conventional rafters and old asbestos tiles which are mainly intact. I was intending to seal the odd crack or two with a bitchumen sealer/filler and then buy 8x4 ft 3inch polistyrene sheets and saw them to fit between the joists. Is that not wise ?

Tenko,

I would strongly advise against having polystyrene over your head. This stuff is a serious fire hazard, dripping globules of hot sticky stuff like nepalm, and producing awful, deadly smoke. The only time I would ever condone its use is if it is fully encased, both sides, such as under screed or entirely within a timber frame faced back and front with fire-resisting material. Nasty stuff.

Mike

 

[miles_hot]

I have conventional rafters and old asbestos tiles which are mainly intact. I was intending to seal the odd crack or two with a bitchumen sealer/filler and then buy 8x4 ft 3inch polistyrene sheets and saw them to fit between the joists. Is that not wise ?

Tenko,

I would strongly advise against having polystyrene over your head. This stuff is a serious fire hazard, dripping globules of hot sticky stuff like nepalm, and producing awful, deadly smoke. The only time I would ever condone its use is if it is fully encased, both sides, such as under screed or entirely within a timber frame faced back and front with fire-resisting material. Nasty stuff.

Mike

bummer - I think that I something like polystyrene in my garage roof space (covered by hardboard), I will have to have a look I think.

Miles

 

[chippy1970]

If you go the Kingspan route try your local roofing merchant when I built my timber frame workshop years ago I used 50mm all over roof and walls but instead of Celotex or Kingspan I used non branded boards at nearly £10 a sheet cheaper. There is no real difference in quality and I can tell you that my workshop takes about 5 minutes to heat up in the winter and in the summer its very cool inside on a hot day so it works well.

 

[Jake]

Watch out for PUR rather than PIR though, the former is very much more flammable (not far from polystyrene). They look pretty much identical but PUR is cheaper. Some (but not all) of the smaller brands are PUR.

 

[MikeG.]

Good shout Jake.........

Kingspan, Celotex, or seconds of either, are my recommendation.

Mike

 

[JeremyM]

Mike sorry to ask a naive question how do you secure the mineral wool in place prior to applying the under-the-rafter kingspan?

Jeremy