Insulating a flat roof - is condensation likely?

[Andy23]

It's a chalet style semi-detached property with bedrooms on the first floor. Since the roof slope start at floor level of first floor, the bedrooms have not brick walls and are almost not insulated and very cold.

The bedroom ceiling is half in loft and half has flat roof with felt on the top. Loft has 4 inch insulation, but when I look from inside the loft at the void between bedroom ceiling and the flat roof, this has no insulation at all.

I was thinking of somehow filling this 4 inch gap with some insulation. This void is closed from all sides (I could see it after removing some loft insulation that was pushed into the mouth of the void) So I assume air is not intended to flow in this void.

Will filling this void with some insulating material pose any problems with condensation?
Please help
Andy

 

[stem]

Came across this recently whilst looking for something else.
Case Study Insulating Chalet Bungalow

 

[noseall]

We recently insulated a vaulted rafter situation and building control wanted 100mm p.u.r. foam board (celotex, kingspan etc) between the rafters, with 40mm across the rafters internally. This was coupled with a 50mm air gap above the insulation.

Unless you have 250mm of usable void between the inner and outer faces of the internal and external wall then fibreglass is of no use.

 

[Andy23]

Thanks but the question still remains unanswered!

I have just now filled this void space between joists that are between bedroom ceiling and flat felt roof. This space is 370mm wide between each joist and 100mm high. I have squeezed in 150mm glasswool into it so that there is absolutely no air gap in it (void is totally filled). There are 8 such voids between 9 joists. They are 6 feet in length.

Should this cause any condensation?

Help will be appreciated
Thanks
Andy

 

[noseall]

Glasswool is full of air.

My reference to void size was joist or rafter depth. You need to have 250mm of usable depth in which to accommodate adequate thickness of glasswool insulation. So, only having 100mm puts you at a high risk of condensation.

You would have been better off fully filling with pir foam board then taping and sealing all joints, then finishing off with a foil backed plasterboard.

 

[mointainwalker]

In respect of the last post by noseall, how do you then seal the joins between the pb sheets so you can skim over them ?

 

[Andy23]

Glasswool is full of air.

My reference to void size was joist or rafter depth. You need to have 250mm of usable depth in which to accommodate adequate thickness of glasswool insulation. So, only having 100mm puts you at a high risk of condensation.

I do not fully understand this..
Are you saying that the joist depth of 100mm in my case is not enough to stop condensation?

Is it the air in the glasswool that condenses?
If yes, surely if I fill a 250mm deeper and bigger void with glasswool it will have more air in it. why does this air not condense?

Please explain or let me know if I am missing something
Thanks
Andy

 

[noseall]

Agree regards air. it is everywhere!.

The point is you need an adequate vapour barrier and to reduce any air pockets to a minimum.

Snug fitting foil laminated pir board offers the best resistance to the problems of condensation.

Foil backed plasterboards can not be fully sealed but do provide vapour resistance.

Every little helps. 100mm of fibreglass is better than nowt but is no guarantee against cold spot.