Rafter Insulation

[Nicksun]

Hi all, hope you can offer some advice. i have a 1959 detached dormer bungalow. I want to insulate the short rafters below the dormers. If i fit insulation between the rafters will i need any ventilation to the space? The ceiling joists below have already been fitted with 100mm jablite but because the wall between the bedrooms and bathroom is only 3" blocks, there is effectively no insulation to the rooms from the void/outside.

Thanks.

Nicksun.

 

[Nickam]

Putting insulation between the rafters does mean leaving 50mm min. of ventilation over the insulation. The only way to avoid this is with new construction which uses a breathable type of sarking felt which your older building doesn't have.

 

[Nicksun]

Cheers Nickam, So 100mm celotex an 100mm rafters with breathable felt will be ok?

 

[Nickam]

Yes it would but to fit the breathable felt you will have to strip the roof of tiles so don't think you would want to do that. And obviously with a 50mm gap and then 100mm insulation you will need to batten out as that comes to 150mm.

 

[Nicksun]

It's no problem removing and replacing the tiles and battens. My query was with the insulation/condensation/ventilation of the space. I'm a carpenter and do a little roofing from time to time. Thanks for your help.

Nick.

 

[Chukka63]

you can put less insulation between the rafters and more under the rafters if thats possible in your case.saves the depth of under battening needed.

you will need the 50mm air gap no matter what.

you dont need to change your existing felt if there is adequate ventilation.adding soffitt venting,ridge /tile vents may be cheaper than reroofing.

 

[Nicksun]

Thanks Chukka63,
Getting sheets of insulation into the void could be difficult but not impossible. The roof will be stripped back and refelted whatever option i take. I will be replacing the fascias and soffits, so over fascia vents or breathable felt are both an option.

Nick

 

[Nickam]

Thanks Chukka63,
Getting sheets of insulation into the void could be difficult but not impossible. The roof will be stripped back and refelted whatever option i take. I will be replacing the fascias and soffits, so over fascia vents or breathable felt are both an option.

Nick

Hi Nick, If you are stripping the roof it may be worth replacing the whole construction with a warm roof construction. It's more efficient and saves all that messing around cutting insulation between rafters. It does, of course, add to the height of the roof slightly and would probably require fascia replacing for higher ones.
Cheers,
Nick

 

[ajrobb]

You'd get better insulation putting 100 mm of Celotex under the rafters held in place with wood strips. The avoids thermal bridging by the rafters. Tape the joints to complete the VPL. The devil is in the detail - sealing the roof void.

Alternatively, put Celotex between the wall studs up against the plasterboard and fill between the joists (150 mm) with mineral wool and cover with more mineral wool (100 mm) to a total depth of at least 250 mm. You avoid the problem of sealing the roof void.

 

[Nicksun]

Thanks ajrobb and Nickam, I can't add to the roof height as it runs quite tightly under the upstairs window cills. The wall between the rooms and the void is blockwork, so not easily insulated. Putting the insulation under the rafters is the ideal scenario, but not easily acheived, although probably worth the aggro as I'm never planning on moving from here.

Cheers again.

Nick.

 

[ajrobb]

The wall between the rooms and the void is blockwork, so not easily insulated.

I would consider pinning Celotex against the outside of the block work using wood strips and maybe hammer fixings - I don't imagine it would take much to keep them together. Again, no worries about sealing the roof void.

 

[Chukka63]

ive just completed a loft conversion and the BCO told me some insulation is required between the rafters no matter thickness goes under the rafter.cant remember the reason for this at the moment.