Insulating knee wall in loft

[wwwebber]

Hi, been asking a few questions regarding various things this week and you've all been very helpful - thank you.

Now, onto my next question :-

My loft has been converted into a semi habitable space but it is very very drafty and cold as it has not been insulated properly. I've identified that I need to insulate between and under the rafters but will be keeping a cold space behind the knee walls and into the eaves for storage, ventilation etc. The knee walls currently consist of thick plywood only which is inadequate. What should I do here to insulate properly ?

Also, the knee walls only go as far as the floor boards of course and thus air is whilstling under the floor boards between the floor joists into the loft room. I intend to insulate between the joists with rockwool etc for sound and heat insulation but this wont stop the wind blowing through - so what do I need to do here to seal the flooring off from the eaves ?

Thank you.

 

[Ian H]

Is it plywood into a 4x2 frame?

With mine I put 100mm celotex between the 4x2 them 50mm over the lot. You can just about see here:

 

[Doggit]

If you don't insulate behind the knee wall, then you'll end up getting draughts through the storage doors. Best to take the wall down, lift the floor boards and insulate underneath, then insulate between the rafters, rebuild the knee wall and insulate behind that, then insulate under the rafters, and on the front of the knee wall as well. You want to make sure that there's an airflow up and over room, and under the tiles; you very likely need to batten out the rafters before fitting the insulation between them. You should be aiming for a total of 125mm of Cellotex, 100mm between the joists, and 25mm under them to stop cold bridging, then you fit a vapour control layer, and then plasterboard.

 

[wwwebber]

Ian H - no, it's very much non standard as it's a diy job by previous owner and lots of mistakes have been made. Thank you anyway.

Doggit - Yes, I intend to take the knee wall down as we need to fit beams to support dormer windows etc. The roof doesn't currently have a membrane and I'm told that this means that I don't need an air gap when fitting celotex ?. The tiles provide a lot of air circulation. You say insulate under the floor boards - would that be rockwool or similar ? how do I stop draughts going under the floor / knee wall ?

Thank you for you response.

 

[Doggit]

Is this being done with planning permission and building control, as the dormer window will need to be done properly to keep the roof structurally sound. I'd still leave an inch between the cellotex and the tiles even though there's no membrane or felt, and yes, 170mm of Rockwool under the floorboards; that'll stop moisture getting up there, and keep some of the rising heat downstairs. If the cellotex goes down toward the soffits, then that'll stop a lot of the draughts coming through the wall and the floorboards.

 

[wwwebber]

Thanks Doggit.

The dormer window already exists and now that I've done some further investigation they will not now need any additional support etc. But I still want to insulate properly. The rafters are 3x2 with no membrane as already mentioned. I'm not aiming for regs but simply to improve things as there currently no insulation between the rafters with plasterboard backed with 25mm insulation screwed underneath them. So I'll fit 50mm celotex leaving an inch gap and then re-fit the insulated plasterboard under the rafters. That should dramatically improve things.

You mentioned a vapour control layer - doesn't the celotex have this built in ?. Also, I need to install insulation stops to ensure everything is level - what do you suggest for this ?

Thanks again.

 

[Doggit]

You just need a 2" piece of wood to set your distance, and then nail in a length of 1x1 behind it. I didn't add a VCL when I did my loft, but have since fund on here that I shouul have. It'll stop any mousture getting into the rafters, as well as the air space behind the cellotex, as you'll never get it completely tight. Use alli tape on any joints, and then look at the job at the end to decide on VCL.

You could think about adding another 25mm on top of the rafters, doing the VCL (or not) and then plasterboarding. 120mm is the required amount by building control.

 

[wwwebber]

You just need a 2" piece of wood to set your distance, and then nail in a length of 1x1 behind it. I didn't add a VCL when I did my loft, but have since fund on here that I shouul have. It'll stop any mousture getting into the rafters, as well as the air space behind the cellotex, as you'll never get it completely tight. Use alli tape on any joints, and then look at the job at the end to decide on VCL.

You could think about adding another 25mm on top of the rafters, doing the VCL (or not) and then plasterboarding. 120mm is the required amount by building control.

Thanks again Doggit - I'll do just that with the 1x1 before slotting in the 50mm celotex. I'll then follow it up with a VCL and then plasterboard backed with 25mm of insulation.

What would you recommend as a VCL ?

 

[Doggit]

Sorry, I didn't use it, but now know that I should have, so you need to look for a Vapour control layer such as this

 

[Ian H]

I think you can also foil tape the joints on the Celotex for your VCL or use the foil backed plasterboards.