Cold roof insulation and ventilation

[RobertPW]

I’ve recently built a lean to shed abutted to my garage with a GRP roof. I underestimated the ventilation requirements and have ended up with mould on the underside of the roof deck despite soffit vents and air bricks. There is no insulation in the roof, so I am considering the following options and invite comments/advice…

1. Spray foam insulation to the underside of the deck.

2. Use rigid insulation between joists with a 50mm air gap. This will need to be vented into the abutted garage using a core drill as I did not use an abutment fascia vent.

3. As option 2 with the addition of mushroom vents.

Any other options appreciated. Thanks in advance.

 

[noseall]

I’ve recently built a lean to shed abutted to my garage with a GRP roof. I underestimated the ventilation requirements and have ended up with mould on the underside of the roof deck despite soffit vents and air bricks. There is no insulation in the roof, so I am considering the following options and invite comments/advice…

1. Spray foam insulation to the underside of the deck.

2. Use rigid insulation between joists with a 50mm air gap. This will need to be vented into the abutted garage using a core drill as I did not use an abutment fascia vent.

3. As option 2 with the addition of mushroom vents.

Any other options appreciated. Thanks in advance.View attachment 373725

Why are you fussing with insulation over a shed?

Glue some 25mm or so, insulation to the underside of the deck to prevent localised condensation.

 

[RobertPW]

It’s not really a matter of fussing, just trying to avoid the ridiculous amount of condensation I’m getting. My research suggests that insulation stuck to the bottom of the deck could make it worse.

 

[noseall]

IMy research suggests that insulation stuck to the bottom of the deck could make it worse.

You had better stick with your research then. Good luck.

 

[Alastairreid]

My research suggests that insulation stuck to the bottom of the deck could make it worse.

As Noseall suggested getting rid of the cold spot with rigid tight fitting insulation
Will work.

 

[noseall]

As Noseall suggested getting rid of the cold spot with rigid tight fitting insulation
Will work.

It will work. The thicker the better, but not for deck cold spot reasons (25mm would probably deal with that). Adding thicker insulation means you reduce the risk of the joists wicking the cold, due to the superior insulation depth etc.