How to safely insulate this? With pictures

[bosyuk]

How can I safely insulate the back of this dormer?





It slops back about 90cm.

I was thinking 50mm of celotex in between the sloped joists leaving a 50 mm gap behind. Then filling the gap between the floor joists which is the living room below with loft insulation.

Is this ok from a moisture point of view?

Looking forward to your replies so I can ordering!

 

[Deluks]

You don't need to board the rafters, You should insulate the ceiling joists though, and between the studwork, followed by some insulated plasterboard over the studs.

 

[bosyuk]

Thanks that makes sense. So insulation between the ceiling joists to stop the heat from downstairs going straight out that 90cm section of ceiling. Then 50mm celotex insulation in between the joists in the bedroom side leaving the sloped roof joists as they are now.

Then more celotex over the bedroom joists followed by a vapour barrier then plasterboard.

That sounds sensible to me.

 

[Deluks]

Yup. But thicker in between studs if you can, then you'll get away with say 25mm over the studs, preserving room space.

 

[freddiemercurystwin]

This complies with the regs in terms of thickness of insulation etc.

Untitled

• freddiemercurystwin
• 25 Apr 2010
• 24

 

[bosyuk]

I'm finally doing this part of the room!

I'm a bit concerned the 50mm air gap behind the celotex could be a condensation risk because unlike the diagram of a normal roof from freddymercurystwin my dormer isn't ventilated at all. I assume this will make a big difference.

I was thinking of maybe fully filling the space between the studs. Is this a bad idea?

Any thoughts would be great as the whole vapour thing is trying me a bit crazy!

 

[alittlerespect]

Hi

The 50mm air gap is there to avoid a condensation trap and ensures that the roof void is ventilated this also reduces the risk of timber decay.

You may also need to consider introducing some eaves vents.

Regards

 

[bosyuk]

Thanks alittlerespect.

I think I'm low on options here. Behind this slope you can see in the pictures is a single storey flat roof extension. So there are no eaves to ventilate.

I actually don't see how I can ventilate the space. Leaving the 50mm gap without ventilation sounds like a really bad idea

Really not sure what to do. Full fill with rockwool then celotex over the studs and paint two coats of vapour barrier on the living room ceiling below? It used to be full fill but didnt have the celotex over the studs before so will colder behind there now. Maybe get some vent tiles?

There must be loads of people that have been in the same situation. I'd hate to have to redo this in a few years because I get it wrong now! All this insulation wasn't cheap.

Thoughts would be great.

 

[bosyuk]

I think I'm going to fit three vent tiles http://www.kloeber-home.de/uk/products.php?products_id=25 so the vents will come though the slopped felt you can see in the picture.

I believe this is my only option to safely reduce the chance of condensation without changing to breathable felt. If anyone has any thoughts do let me know.

Cheers.