Insulating roof between rafters

[dougiem]

I have a 200 year old house with slate roof and no felt, I am about to reslate and put felt, but it has no insulation between joists, if I put EPS (polystyrene)insulation boards between joists, do I push it in contact with the sarking boards or do I need to leave an air space, and the same with inside, if I plasterboard fixing to the joists do I have to leave a space between the plasterboard and insulation or should it be touching

Thanks for any comments

Dougie

 

[dougiem]

when I say joists I meant rafters

 

[noseall]

an air gap is necessary on the cold side of the insulation, i.e. directly above the insulation.

have a look at this site.

 

[dougiem]

Hi Noseall

Thanks for the reply, so I dont need to leave a gap between the insulation and the plasterboard in the inside, only on the cold side towards the sarking boards


Thanks again

 

[PaulJR]

Sorry to be padantic, but don't use EPS in your roof, use Polyurethane (PUR) Kingspan/Celotex/Isotherm - it's much safer.

EPS is a real fire risk, a lot of it is foamed using air, and once it ignites it cannot be extinguished. In the mid nineties a couple of superstores burnt to the ground in a couple of hours because they used EPS foam filled cladding.

when you re-felt use a breathable type, and set your tiles up on battens (& counter battens) to let the ventilation take place above the felt and then you canfill upto the underside of the sarking boards with insulation.

 

[dougiem]

I was going to use a flame retardant EPS which I believe when it is behind plasterboard has the same firerating as most other materials, but I will take on board what you have said and look at the fire ratings of other materials

For the roof, the my roof is connected to my neighbours on a straight run, it is also in a conservation area, so I can not alter the height of the roof, which I think it would if I use wooden battens, but I am interested in the breathable felt, should I still use this or standard felt

Thanks

Dougie

 

[PaulJR]

Go breathable - it's not going to hurt, it will help the ventilation.

 

[freddiemercurystwin]

Personally I'd say the jurys still out despite their widespread acceptance by building control etc. Every now and then a thread pops up on here when some ones got a breathable membrane and still got condensation.

 

[JohnD]

but it seems to me these are usually the people drying their washing indoors, or with no bathroom extractor, or with hot steaming loft tanks.

 

[noseall]

as woody has pointed out, condensation within domestic dwellings, is a lifestyle issue.

you can have identical houses with the same number of people living within, but registering significantly different levels of condensation.

personally i use nothing but breathable felt/membrane, simply because it is a superior product. i can not remember the last time i used bituminous sarking felt.

 

[JohnD]

as woody has pointed out,

what?

 

[noseall]

as woody has pointed out,

what?

...and johnD.

it was woody's' choice of words, i.e. "lifestyle" issue that i was quoting.

having said that, you would be a fool to ignore any advice about condensation, given by the master of condensation, JD.