Insulation for habitable room, sloped roof with 70mm rafters

[Hugh1983]

Hello,

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• Hugh1983
• 28 Aug 2011
• 1

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• Hugh1983
• 28 Aug 2011
• 2

I have uploaded a couple of pictures to assist with your responses. I am just considering my options and the pictures show the initial thoughts on how I would insulate the whole of the top floor.

My house is Victorian with 4 floors. The top floor is shown in the two pictures that I’ve uploaded. The room opens up directly from the staircase.

I don't have a problem with how to insulate the flat ceiling level. I'm going to use the 100mm loft roll in between the rafters (as shown) and then roll 170mm loft roll at a right angle to this across the top of the rafters giving 270mm insulation. As far as I'm aware this is the correct thing to do. For the ceiling roof level I'm thinking of screwing in 9.5mm plasterboard to the rafters and then getting someone to skim over it.

My problem is with the sloped roof part as I do not believe that I should be insulating it as I have shown in the pictures. The rafters are 70mm thick. As you can see, I've cut loft roll to fit this space between the rafters. This is 100mm loft roll and I'm guessing it is less effective as it is slightly compressed in this space. However, from the research I have tried to do, it sounds as if this is the wrong way to insulate this space anyway. It seems that I should be using sheet material to insulate, leaving a gap between the roof and the sheet material for insulation?

When I initially gutted this floor I found that it had been insulated with loft roll on the sloping roof parts and across the ceiling level. This is why I initially planned to re-insulate it in this way but I'm now thinking that this is the wrong way to do it. However, I'm not aware that the way it had been insulated had caused any problems and makes me think why not re-insulate using the same method. I am very tempted to carry on and fully insulate the room as I have started. With loft rolls from Builder centre at £3 a roll, it wont end up costing that much. It seems the sheet material option will be a lot more expensive and I don't know if I can even do it effectively with 70mm to play with, I gather that I would need to leave a 50mm gap for ventilation which would only allow me 20mm for insulation. Can I reduce the gap to 20mm and insulate with 50mm sheet board if I don’t go ahead with the loft roll option?

I am on a budget as I have a four story project on my hands so please bear this in mind when giving advice as your suggestion may be too expensive to consider.

Your responses and discussions are very much appreciated.

 

[marsaday]

i am a diyer and i think you will have problems with the soft rolled insulation - no breather gap and so sweating will occur.

You need to use kingspan or such like and leave a breather gap as you say of 50mm. To solve your problem you need to extend your rafter. You just screw some more wood to extend it downwards. this is fine if head height is not a problem.

Then you fit the insulation inside the space plus you have room for the air gap.

 

[Hugh1983]

i am a diyer and i think you will have problems with the soft rolled insulation - no breather gap and so sweating will occur.

You need to use kingspan or such like and leave a breather gap as you say of 50mm. To solve your problem you need to extend your rafter. You just screw some more wood to extend it downwards. this is fine if head height is not a problem.

Then you fit the insulation inside the space plus you have room for the air gap.

Thanks for the response, if i were to extend the rafter, do you think 100mm is good enough so that i have 50mm gap/50mm insulation. obviously the more the better but i am on a budget and if 50mm will be ok, i will probably go with that. I imagine it will be better than the way it was previously being stuffed with loft roll.

 

[marsaday]

i think 50mm = 100mm of soft insulation. i may be wrong, but i think that is about right.

 

[mointainwalker]

PIR (Kingspan/Celotex) initially provides 60% more insulation than fibre-wool, so 50 mm = 80 mm wool.

 

[freddiemercurystwin]

Normally you would fit Rigid insulation between the rafters leaving a 50mm air gap. As mentioned you would add a spacer batten beneath the existing rafters of say 50mm. then you would fit say 70mm Kingspan between the rafters and then a continuous layer of around 50mm and then your foil backed plasterboard. Though some on here have managed to fix very long screws all the way through a big chunk of insulation and the plasterboard and avoided the additional battens.

A bit like this:

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• freddiemercurystwin
• 25 Apr 2010
• 24

What nobody seems to have mentioned is that the air gap needs to be open to the air at the eaves and the ridge (or close to it with several roof vent tiles). this is vital to ensure adequate ventilation is maintained. If the air gap is not vented to the open air you may as well not bother having it.