Best Insulate Dormer Roof?

D

dj01

Registered User
Messages
142
Hi.

I'm in the process on a self build and we are just starting the roof. It's a half dormer. I'm wondering what the best way to insulate the roof area is to minimize heat loss. The rafters will be 7 x 2, which means if I'm leaving a 2 inch air gap on the under side of the roof, I've only got a depth of 5 inches(125mm) for insulation, which I'm concerened is just not enough.

Would I be better increasing the rafter sizes to accomodate more insulation? I had tought of combining a polyiso (xtratherm etc) insulation board with rockwool in the rafters, but to do this I'd need a depth of 50mmm + 100mm + 140mm (air gap + ins. board + rockwool), which is 12 inches. This doesn't seem practical as it eats into the head room of the upper floor.

Has anyone any advice on what might be the best solution to provide high level of insulation without eating into head height space? Any advice would be great.

thanks
dj01
 

depends what level of insulation you want.

you can construct the roof so that the air gap is actually over the rafters, thus you can the incorporate 150mm of insulation between the rafters.
this is done by battening and counter battening over the rafters.
 
Thanks Sydthebeat. Wanted to go a little beyond regs. I might increase the rafter size to 9 x 2's, which would give me a little extra space. What did you use in your house in rafters and intermediate floor?
 
djo1

I am currently insulating the pitched roof of my dormer. What I decided on in the end was Rockwool between joists, multifoil insulation stapled to joists, counterbatton an then insulated (37.5mm) plasterboard. You hear a lot of arguements about multifoil but if it does even half of what it claims then I'll be happy. I bought a pallet of the stuff from the UK for half the price it costs here...
 
multi foil is fine if its used as a vapour check layer. It cannot be included though, when calculating the u values of the roof, therefore all the other insulations need to comply with the regs.
Boyler, in your example above you would need to have roughly 220mm of rockwool to comply with minimum regs, with that spec..... i assume you dont have this....
 
Syd...what is the rationale for excluding the foil product from u-value calculations? Presumably, it hasn't satisfied some test or other pertaining to heat loss? Appreciate any insight as there's a huge gain in terms of reduced labour cost & speed of fitting. Thanks, apple1
 

thats exactly it...

it hasnt been able to show by testing that it has the insulative properties its salespeople claim. Testing for u values calculates the heat resistance/transmittance of particular materials, whereas foils main insulation property is reflective rather than resistive.
Therefore it cannot be included in any calculations to rate u values.

The only BBA cert for a foil is as a vapour check and not a stand alone insulator.
 
Not sure if I am allowed to put up details about products but as Rockwook, Kingspan, etc have all been mentioned I assume I can put up the link to the company who supply the multifoil in th uk. I have no links and was just a customer. The specs are there for the multifoil along with the price...

[broken link removed]

I agree that you cannot rely on it as the sole form of insulation but it does have good thermal properties. Hence the reason I installed the rockwool and the insulated slabs with it.

Regs aside, it is a form of insulation and not a Vapour Control layer. All you have to do is put your hand on it and you immediately feel your heat reflected.

In the UK they do recognise it as a form of insulation and leave it to the local Building Control Officers to allow or not. I my view (only from what I have read) this was the UK Govs way of passing the buck so they did not upset the vested interests by approving it.
 
Apologies to Brendan for posting on the same message on tow seperate threads:

Looking for the elemental U-vaue for the combination of the following between the rafters of my dormer dwelling

2 layers of 50mm Xthratherm between the rafters with
12.5mm plasterboard/25mm insulation fixed directly to the rafters.
 

i do appreciate this boyler, but the problem comes when the actual u values of the construction are to be calculated.
can i ask you what the u value of your roof construction is??? and how did you calculate it??

with multifoil insulation the majority of insulative properties are from a reflective trait. When calculating u values you measure how solw or fast heat transfers through the material and herein lies the proble. when measured this way multi foil only has the same thermal resistance of about 28mm of quilted insulation,,... thats a far cry from the 200mm they claim.

personally i would have no problem accepting this product once it has been properly tested and a genuine thermal conductivity or resistanc efactor has been achieved... until then it is an unproven product and i for one certainly wouldnt stand over it.
 
Here is how I calculated it. Let me know if I have made a mistake as I do not do this for a profession...

I assumed a thermal resistance of 1.7m2K/W for the multifoil. This is the value that the NPL in the UK published from the "hot box" results. The manufacturers claim a much higher thermal resistance value(>3m2K/W) so in reality I am hoping for a much better value than 1.7m2K/W. I also remember seeing a set of experiments that examined the air gap below the multifoil which demonstrated an improved thermal resistance.

Even using a value of 1.7m2K/W the overall U-Value for the pitched roof should meet the regs. of 0.2W/m2K from the calc below.

Thermal Resistance (m2K/W)
External Surface 0.1
Slate 0
Batten 0
Felt 0
Cavity2'' 0.18
100mm Rockwool 2.27(= 0.100m/0.044W/mK)
MultiFoil 1.7(Per UK BRE Hot Box Test)
Insulated Plasterboard 1.15(37.5mm K18 Kingspan)
Plaster 0.1
Inside Surface 0.12

Total 5.62m2K/W

U Value 0.18W/m2K
 
You must log in or register to reply here.
Menu
Log in
Register