Dormer attic insulation.

mikehn

Hi All
I want to insulate a section of the roof of my dormer. The attic portion is well insulated as are the dormer walls. There is no insulation between the ceiling and felt of the sloping section. I am looking at slipping 4inch foam insulation boards up between the rafters but dont know what specs to look for. Budget is fairly tight so I was a bit gobsmacked when I was quoted €70 for a sheet of 8X4 4" insulation in my local builder providers. Is this about the going rate. Any suggestions about alternatives gratefully appreciated. Thanks in advance.

Lumen

Any insulation added between the rafters needs to leave at least 50mm air space below the felt for ventilation. There are some opinions that this can be reduced or eliminated if you have breathable felt.

Even if you can meet the ventilation requirements it is going to be incredibly difficult to get a tight fit between the insulation boards and the ceiling, and you will have cold/hot spots in the ceiling from thermal bridging through the joists which is a condensaton risk.

The best approach is probably to remove the ceiling and apply thinner PIR between the rafters and then continuous PIR over the rafters before refitting the plasterboard (with appropriate air tightness), but this is going to be expensive and you will lose head height.

mikehn

Thanks Lumen for your detailed reply.
If I had the budget I would certainly go down the route that you suggest but sadly cannot.
I am aware of the requirement for the air space and had intended to factor this into the thickness of the board if I went down that route. The house was built in1995 and I assume that breathable felt would not have been the norm at the time so I going on the basis that it is not.
I accept that I cannot do an ideal job due to budget but I was going on the basis that any insulation is better than none,so I reckoned that if my efforts were only 50 or
60% effective that it would be an improvement on what I currently have.
What I had not considered was your comment regarding condensation due to thermal bridging from the joists. I guess I will have to go back to the drawing board on this. Thanks

Calahonda52

As someone who lived in and converted a dormer bungalow, your project quite simply won't work with any degree of effectiveness.

You are assuming everything is all //, square, smooth, etc and that the PIR will slip in like a piston in a cylinder..
Won't happen and you may get to the point where a sheet will get stuck on you and break when you try remove it.
Likewise, the, if driving it up from below you risk catching and tearing the felt over laps....
It could turn out to be a complete cluster...k

The other issue here is summer over heating.
Is that an issue?

More later if you respond

mikehn

Thanks Calahonda52
Have not had an issue before with summer overheating, so hopefully the level of additional insulation i'm considering wont significantly cause a problem. I have been doing a little digging today on the various board options and was wondering if the Quinntherm 60mm foil backed board would be worth considering. Using these would give me a considerable safety margin in terms of avoiding contact with the felt.

MicktheMan

mikehn wrote: »

I accept that I cannot do an ideal job due to budget but I was going on the basis that any insulation is better than none,so I reckoned that if my efforts were only 50 or
60% effective that it would be an improvement on what I currently have.

Insulation boards not done properly are more than likely >90% ineffective (not done properly in this context is where a gap between the insulation board and the ceiling is more than 6mm). How will you ensure that the board is tight to the ceiling slab and tight to both rafters also when sliding or more realistically pushing up from underneath?
You have a better & more realistic chance of success with a more flexible product such as fibre on a roll which can be installed more easily with the aide of something like a long 'T' shaped stick.