Garage conversion roof insulation.

[AndrewR]

Hi,

I'm considering converting an existing garage/utility into living space and am unsure on the best way of insulating the existing cold flat roof. The joists are 8" deep and it's currently ventilated via the open cavity. Should I either:-
a) completely fill the void between the joists with insulation with rockwool or kingspan and seal the cavity?
b) partially fill the void with insulation leaving a ventilation gap, if so how thick should the rockwool or Kingspan be?
c) apply kingspan insulation to the underside of the existing garage ceiling, and how thick would it need to be to comply with the regs?

I'm reluctant to apply insulation to the underside of the ceiling as it would reduce overall room height (it's already 150mm lower than the adjacent rooms).

Your thought/guidance would be appreciated.

Andrew

 

[noseall]

How do you mean "already vented by the open cavity?"

The roof void will require through ventilation if you go for a cold deck.

The simplest way is usually to fix a warm deck above the roof joists thus avoiding awkward ventilation issues.

 

[Nickam]

Hello Andrew,
The easiest way which avoids any relaying of roofing is to construct a cold roof. This has the insulation between the rafters with a 50mm min. ventilation gap above. You then provide vents in the soffits on opposing sides to give equivalent of 10mm continuous gap. Have you not had Building Regulations approval as you will have to comply with min heat loss requirements as well as other criteria?
Cheers,
Nick

 

[Chukka63]

not as easy as that if its a flat roof with no soffit overhang.

 

[Nickam]

No, that's true. In that case it's either a warm roof construction (insulation over the joists and boards) or take off the flush fascia, extend the joists and put on a new projecting fascia. Not sure which would be most work. Probaly be most work to extend the fascia as this would also require refinishing the roof covering anyway.

 

[noseall]

Through ventilation is only possible on buildings with three accessible sides otherwise you would be looking at installing mushroom vents.

Yuck!

 

[Nickam]

Through ventilation is only possible on buildings with three accessible sides otherwise you would be looking at installing mushroom vents.

Yuck!

There is a flashing vent that can be used in that case. I've used it with a flat roof adjacent to parapet walls.

 

[Nickam]

Through ventilation is only possible on buildings with three accessible sides otherwise you would be looking at installing mushroom vents.

Yuck!

Surely you mean two accessible sides in the case of the roof being bounded by two higher walls?