I have a long, narrow detached garage used as a workshop and storage for a car. It currently has an insulated and well sealed garage door, double glazing (no drafts) and the walls are hung with fireproof kapok blankets that provide some wall insulation. The roof is flat, felted over plywood, and uninsulated however.
It takes forever to warm up in the winter, and I have to run an electric heater all day to keep up with the heat loss through the roof, so I'd planned to make it a cold roof with PIR boards between and/or under the joists, leaving the usual air gap, plus a vapour barrier and then plasterboard to avoid the chance of grinding or welding sparks setting the PIR foam on fire.
The problem I have is that the joists run longitudinally down the garage, supported on cross beams at two points. To get ventillation in to the air space, I'm going to need to make holes at both ends of the garage between each joist.
At one end of the garage, the joists terminate on top of the end wall with a soffit across the outside, so I can make ventilation holes no problem. At the door end however, the joists terminate inside the garage and are supported on a batten attached to the solid end wall. The wall extends up with a brick parapet. It looks like, to make ventillation holes, I would have to replace some bricks in the end wall with air bricks, but I'd need several, and that is the exposed side of the garage - subject to wind-driven rain - so it's likely rain would be blow in to the air space.
My other (easier) option seems to be to add mushroom vents in the roof itself, but I've no desire to refelt that roof (it's in good order), so would they give a weathertight seal to the existing felt?
Failing that, do I need to even bother with an ventillation air gap above the insulation as it's an unoccupied space?
Or are there other options to reduce heat loss through that roof without re-roofing?
It takes forever to warm up in the winter, and I have to run an electric heater all day to keep up with the heat loss through the roof, so I'd planned to make it a cold roof with PIR boards between and/or under the joists, leaving the usual air gap, plus a vapour barrier and then plasterboard to avoid the chance of grinding or welding sparks setting the PIR foam on fire.
The problem I have is that the joists run longitudinally down the garage, supported on cross beams at two points. To get ventillation in to the air space, I'm going to need to make holes at both ends of the garage between each joist.
At one end of the garage, the joists terminate on top of the end wall with a soffit across the outside, so I can make ventilation holes no problem. At the door end however, the joists terminate inside the garage and are supported on a batten attached to the solid end wall. The wall extends up with a brick parapet. It looks like, to make ventillation holes, I would have to replace some bricks in the end wall with air bricks, but I'd need several, and that is the exposed side of the garage - subject to wind-driven rain - so it's likely rain would be blow in to the air space.
My other (easier) option seems to be to add mushroom vents in the roof itself, but I've no desire to refelt that roof (it's in good order), so would they give a weathertight seal to the existing felt?
Failing that, do I need to even bother with an ventillation air gap above the insulation as it's an unoccupied space?
Or are there other options to reduce heat loss through that roof without re-roofing?
