Looking for some advice on suitable insulation for a retro-fit garden office build

[Pete251]

Hi

I am looking for some advice on insulation for a retrofit build project.

I have a existing pre clad timber framed outbuilding that I am looking to convert into a garden office and I am looking for advice on how to install insulation in the roof and walls but in a suitable way so as to prevent any troublesome condensation from building up.


The structure is mainly timber 6m x 3m with a pitched apex roof that is constructed with 18mm OSB boards and covered with 1.5mm EDPM.

The whole structure rests on a concrete base and there is a single layer of bricks running the perimeter. Sitting on top of the single layer of bricks are the walls, which are wooden and consist of 5mm plywood sheeting with 10mm cladding with 95mm x 40mm timber stud supports.


I believe insulating such a structure is what’s referred to as a ‘Cold Roof’ as the insulation will be going on the inside and my understanding is that I need to leave an air gap in the roof of approx 50mm, which needs to be vented to allow a through flow of air in order to avoid condensation.

My question - How do I do this and do the walls also need venting to allow a through flow of air? Note: There is No breathable membrane installed on the outside walls, I do intend to install a vapour barrier on the inside.


I have searched the Internet both high and low but as yet not managed to find any suitable solutions to this retrofit insulation problem, any advice and suggestions would be very much appreciated.

Many thanks

Peter

 

[motorbiking]

You can just cut celotex or similar and insert it. Use nails in the joists to stop it falling out of the roof. As long as it's properly air tight there is no risk of condensation. Either foam filler and/or foil tape. Alternatively you could go with loft roll.

I'd probably go with something around 70mm unless you have building regs to meet. That will mean the space can easily be heated by a 500w fan heater.

 

[Pete251]

Hi
Thank you for your reply,
Celotex, Kingspan etc Pir is the insulation route that I have been considering but I have read that blocking in all the space especially in the roof without leaving atleast a 50mm air gap with most likely create condensation where the cold and warm surfaces meet,
From my understanding with it being a pitched roof, the point at the top is where condensation is most likely to form, hence my concerns.

 

[^woody^]

Full-fill with any insulation tight to the outer sheathing and roof boards, PIR type is more efficient for any given thickness, fill all gaps, fill in between and across timbers if you can, and you don't need any air gaps or any vapour barrier.