Insulating pitched garage roof?

[Mangokid]

I have a rendered, single skin block garage with a pitched slate roof. There is a membrane between the slated and the rafters which I don't believe is permeable...

I am looking at insulating the garage, as cheaply as possible, just to make it a bit more comfortable and so I can heat it to protect tools.

I have done a search of the forum, but it can be a confusing issue.

In relation to the roof, can I just fit polystryrene between the rafters, leaving a 50mm air gap above and leave it at that? Just to get me started.

Not sealing the joints with tape or anything just yet? I realise this is not the greatest insulation but I can get the polystyrene for free. I may upgrade this down the line by covering that with an insulated plasterboard etc and so I wasn't going to tape or add a vapour barrier at this stage unless it's definitely needed to protect from any condensation issues?


Thanks.

 

[RossJarvis]

From my limited experience the principle seems fine. The main issue I've found is getting the gaps round the insulation filled. with polystyrene and celotex it can be practically impossible to get a snug fit to all the twists and bows in the rafters. This is why people often use expanding foam to seal the gaps. I think the movement of air around the edges can significantly reduce the effectiveness of the insulation. I think if you cover the whole thing a great improvement will be noticed. I would think the single skin wall would also keep things cold. Additionally one problem with sealing places up can tend to increase condensation as natural ventilation can be blocked up.

 

[Yetty]

Done something similar on a attic conversion, leaving a 50mm gap between insulation and tile membrane.

However, there was a definite requirement to install both Sofit Vents and Tile Vents (positioned high up near ridge level) to get the air moving / control condensation as RossJarvis mentions. Otherwise I reckon you'll be fine.

 

[RogerS]

Is it advisable to have polystyrene in the roof space? I'm thinking about a fire and burning polystyrene dripping down. Seem to remember something about polystyrene ceiling tiles being a no-no these days.

 

[Mangokid]

Thanks all for the input. I will be doing the walls also, but I'm happy about the approach to take there.

Done something similar on a attic conversion, leaving a 50mm gap between insulation and tile membrane.

However, there was a definite requirement to install both Sofit Vents and Tile Vents (positioned high up near ridge level) to get the air moving / control condensation as RossJarvis mentions. Otherwise I reckon you'll be fine.

Vents are a concern I had, and something I know little about.

No vents in the wood/pvc soffit at the minute, and I could only fit them to one side as the garage is attached to the neighbours garage on the other side via a cavity wall, so I'm assuming I would need a roofer to look at any options for vents there, if they are needed at both sides, or is the cavity good enough for ventilation?

I'm also assuming I would need to install a vent under each rafter but I'm not sure. The rafters are 400mm apart. I had hoped to do that myself without getting a roofer in, but if tile vents are also needed? It's never simple!

 

[Hornbeam]

I totally agree with comments regarding fire hazards of polystyrene. Much better to use a PIR foam insulation board which is a form of PUR insulation but with much better fire performance. PIR is now the industry standard for foam insulated panels

 

[skipdiver]

You could use penny vents in the soffit but i reckon you would need quite a few, especially if there is only one soffit. If you don't want to pay a roofer and you are fairly competent with building work, you could apply a dry ridge system that doesn't require cementing into place.

I also wouldn't use Polystyrene personally, it's a fire hazard and it sweats. I used 50mm Knauf Earthwool in mine and left a 50mm air gap but i have breathable membrane. I also vented the soffits just to add more airflow. Wool is cheap and there is a lot of info on the Knauf site for different applications.