Getting .2 from a pitched roof

KingPuck

I am considering my options for insulation in the roof of my dormer bungalow. I am thinking of using wool between the rafters and an insulated board under the rafter.

The two options are 1) 140mm Knauf Frametherm with 38mm insulated board underneath or 2) 150mm Isover Metec with the same 38mm board.

I have been told by a hardware rep that I would need 500mm board with the Frametherm on account of it only being 140mm thick. I have 7" rafters so the thicker 180mm Frametherm is not an option.

No6

Why dont you use a 100mm PIR board between the rafters, with the 38mm board. If you are using a ventilated roof construction then you ned a 50mm or 2" air gap over the insulation therefore you can only use 125mm of either insulation with 7" of 175mm rafters.

d'Oracle

I'm gonna say that the best/most logical approach to this is to use the polyiso insulation between the rafters with a polyiso backed plasterboard layed across the rafters. And make sure its properly taped up.

Its more expensive than the fibre, but it allows better performance from thinner material.

KingPuck

No6 do I still need a 2 inch gap between felt and insulation even if felt is breathable? It's not Tyvek but is breathable felt, about €70 a roll.

dunie001

do I still need a 2 inch gap between felt and insulation even if felt is breathable? It's not Tyvek but is breathable felt, about €70 a roll.

you need the gap for ventilation purposes. Since you are building a dormer house then the top part of the roof (above ceiling in dormer) will need ventilation.

Seeing as the only ventilation getting into the roof is from the eaves then you need to allow air to flow along the rafters to the top of the roof. A 2" (50mm) air gap is the standard.

Breathable felt just stops condensation building up on the inside of the felt and doesn't ventilate the roof.

The best method of insulating the rafters is with solid insulation boards like kingspan or Xtratherm insulating boards. Xtratherm and kingspan boards have twice the thermal resistance as wool or rockwool for half the thickness. Also it doesn't sag or settle like wool.

Wool could settle on itself to the bottom of the rafters over time unless it is attached to the rafters. Insulation boards just need to be cut correctly and pushed between the rafters.

dunie001

You can then use a breathable membrane and tape to seal any air gaps (5mm or less) and improve the air tightness of the roof.

beyondpassive

Just note that the PIR boards will have lots of gaps and the wind pressure in the roof will draw air through these. If you used softboard outside and cellulose or mineral wool pumped in, you would have no dew point at the felt. Put OSB as a liner and you have a very robust windtight and vapour/air tight roof, without cuttin bits of PIR board which will never provide a continuous insulation barrier.

Casati

You can fill the gaps around the PIR board with a can of foam and if you use one of the 'accordian' style boards then can help get a tighter fit.

I am doing the same thing but am told I need to get min .18 to comply with B3 as a minimum. To get .18 I am looking at a 125 PIR board between the rafter with a 37.5mm board below

Southern Comfort

Casati wrote: »

You can fill the gaps around the PIR board with a can of foam and if you use one of the 'accordian' style boards then can help get a tighter fit.

I am doing the same thing but am told I need to get min .18 to comply with B3 as a minimum. To get .18 I am looking at a 125 PIR board between the rafter with a 37.5mm board below

Yes, that "accordian" style insulation would be ideal. The most widely used one is Xtratherm Rafterloc. http://www.xtratherm.com/products/literature/thinrrafterlocirl.pdf

If you use 125mm of it and plasterboard with 30mm of insulation, you'll achieve 0.18 and if the plasterboard has 40mm of insulation, you'll achieve 0.16. Pretty good!!!!

Rafterloc is made for rafters with a 400mm span. It'll squeeze in and hold itself in place. Aeroboard make a similar product. It's easier to fit but not as dense and won't give a U Value as good as the Rafterloc. http://www.aeroboard.ie/images/brochures/Aerodorm.pdf

L driver

Hello,
the grooves/cuts in the concertino insulation are surely full of cold air? I don't think it is a good insulation material just a gimmick and should not be used

Casati

Having just priced it I dont think I will be using it - I was looking at Xtratherm and the Rafterloc is almost twice the price and I dont think the labour gain will pay for it. I am not sure re the air gaps though it makes sense if its full of air then it cant be as good as a solid board?

Am amazed how some companies boards are much wider for the same U value, how can they compete?

Southern Comfort

Casati wrote: »

Having just priced it I dont think I will be using it - I was looking at Xtratherm and the Rafterloc is almost twice the price and I dont think the labour gain will pay for it. I am not sure re the air gaps though it makes sense if its full of air then it cant be as good as a solid board?

Am amazed how some companies boards are much wider for the same U value, how can they compete?

The width of an insulation product doesn't have anything to do with U Value. It's the thickness that's important. The reason that Xtratherm has a better (lower) U Value is that it's a much denser product. Aerodorm is an aeroboard type product, is lighter and not as dense as Xtratherm.

L Driver is probably right in what he says but, if the Xtratherm is fitted exactly, (very tightly) the gaps will be squeezed out of existence. It's much more difficult to do this with Xtratherm than Aerodorm.

paddy147

What about Quinn thermal boards???

Are they cheaper than Xtratherm boards??

d'Oracle

L driver wrote: »

Hello,
the grooves/cuts in the concertino insulation are surely full of cold air?

This is irrelevant.
They will close up mostly, that being the point in them, to allow the board to be packed easily between the rafters.
Also rafter insulation should be taped up which will prevent the cold air getting in.
Also the issue isn't cold air in the insulation, its prevention of conductive heat transfer across the insulation. Many effective insulation materials utilise trapped air in the material layer.

L driver

d'oracle,
how do you propose to close the gaps? Best of luck to you.
The board has grooves both front and back so maybe 10-20mm between warm and cold:eek:
What does air infiltration do to insulation, plenty on boards about this.
How does insulation differ with it's densities

Poor Uncle Tom

The insulation is compressed, like a spring, before it is fitted. So when it is fitted the gaps are closed and the insulation is under a bit of pressure on the rafters either side helping it stay put.