Warm roof construction

[marsaday]

This house needs a reroof at some point and it also needs insulating. So rather than rip down the attics ceiling to insulate, i will add the insulation when i reroof.

is it straight forward to do a warm roof in this situation.

i will put insulation between the rafters and then over them. i would plan on using 25mm kingspan over them, would this be correct?

i understand i dont need to ventilate the roof because it is all warm, correct ? View media item 15890

 

[Deluks]

Yes, provided you ventilate the attic room itself. 25mm is a bit stingy, can you stretch to 50mm?

 

[marsaday]

can do, but i will use polystyrene between the rafters.

i have put in an air brick in the room (25x15cm) at the very highest point where i have been having condensation. Will this be enough ventilation ?

What else could i do if not enough

 

[Deluks]

Fresh air needs an inlet and an outlet if you want air to flow. You could put an extractor fan at the opposite end of the room to the airbrick.

Mineral wool (rockwool) would be an improvement on polystyrene, but still not as effective as foil backed rigid board such as kspan/celly.

 

[marsaday]

can you use rock wool in a roof space with no ventilation? i was lead to believe that it will sweat.

 

[Deluks]

The disadvantage is that it has no inbuilt vapour barrier, so yes it will let moist air pass through. What is the attic boarded out with, standard plasterboard or foil backed? (should be the latter)

 

[mointainwalker]

@Deluks

Why do you say that rockwool would be an improvement on polystyrene ?

As far as I know the U-values are the same.

If there is a cost-difference there is no point in buying foil-backed board as the board is impervious to moisture.

 

[Deluks]

I'm not up to speed on exact u-values, but I favour rockwool over polystyrene for it's fire and sound resistance.

If polystyrene or kingspan is used will the vapour barrier not be necessary.
If there is no current insulation in the roofspace and the attic room is being used, then a vapour barrier is necessary, hence my suggestion that there should be foil backed plasterboard.

 

[marsaday]

the ceiling is the original one, so lath and plaster.

Will i need a membrane in the roof space then? would it go under or over the kingspan which would be placed over the rafters?

 

[Deluks]

No, the foil acts as a vapour barrier itself.

 

[marsaday]

i have decided to insulate from inside, so ripping off the old ceiling.

i will use 50mm celotex between and 25 underneath rafters. will this work prevent the condensation from happening in the loft as it should all be nice and warm now.

 

[r896neo]

I should certainly hope so.

 

[Deluks]

Is the attic just an attic? with insulation on the floor? Or is it a proper room, used by the tenants?
If the latter then you are being a bit stingy with the depth, it won't keep the heat in for that long and will cost money in heating bills. Consider adding at least another 50mm. If you're worried about the extra expense then raise the rent by a small amount, pointing out that they will make the money back when the bills come in.

 

[m9aco]

use our multifoil insulation over or under the rafters with rigid board insulation between rafters. no need for foil back plasterboard either and the multifoil is a vapour barrier also.