90mm Rafters Insulation Questions

[shamusdb]

I feel a bit guilty about clicking on the 'New Topic' button as there so much information on here about the topic but if you could spare the time to advise me that would be great.

I have purchased a house with 2 rooms in the pitched roof space. The rafters are 90mm deep 35mm wide with spacing (between) of 400mm. The insulation coverage is approx 45m square meters. At the moment there is some kind of rockwool between the rafters with 3mm plyboard nailed/screwed under the rafters. I'd like to replace this with Celotex or Kingspan type insulation. I don't want to encroach on the room too much. I'm thinking I should increase the depth of the rafters slightly and insulate between them leaving a 50mm gap (due to non breathable felt) and then adding a layer of insulated plasterboard under the rafters. My questions are: -

1. Is it better (or not) to insulate between the rafters, reducing the insulation layer under the rafters? i.e better U value, easier to find screws to fix thinner layer to rafters.
2. About 1000mm down from the peak is my ceiling height effectively leaving a large void behind the ceiling for the length of the room. Does this help the ventilation or affect the requirement for 50mm gap down the sides?
3. When the insulation is installed between and under the rafters, does it just butt up against the floor or does it need to be sealed along this edge?

Any suggestions for a way to go would be great

Thanks for your time
Shaun

 

[geraldthehamster]

I did something similar to what you're suggesting. My rafters were 3x3 inches. I battened them out with additional 3x3, to make 6x2. Then fixed 100mm Celotex between, and 25mm under. This was the quantity specified by building control about 6 years ago. The foil on the layer placed over the whole lot provides a vapour barrier.

Do you have access to the space above the ceiling? Ideally 300m of rockwool above the ceiling will provide adequate insulation. If you don't have access, it might be worth removing the ceiling at the same time as you remove the boarding from the rafters, to do this properly. Likewise with any dwarf walls you may have at the bottom.

The space behind your insualtion needs to have continuous ventilation. This means vents in the soffit at the bottom, and ideally also some kind of venting at the ridge (I used 3 vented ridge tiles). That way you get free flow of air, to carry away any mositure penetrating from above, or finding its way through your vapour barrier...

Cheers
Richard

 

[shamusdb]

Thanks Richard

The 3mm plyboard is basically up one side across the ceiling and down the other side so i'll be taking it all off. I wasn't planning on putting anything in the roof space above the ceiling but probably just doing the same thing to the ceiling as the sides. Is this ok or are you saying it might be easier just to stick some 300mm rockwool up there?

When you talk about the foil on the layer, are you talking about the foil that will be on the insulated plasterboard? Would there not also be some foil on the GA4000 (for example) as well

I will need to look at the venting in more detail.

Would i then need to tape over the joins and seal the internal join at the floor where the insulation meets?

 

[geraldthehamster]

Thanks Richard

The 3mm plyboard is basically up one side across the ceiling and down the other side so i'll be taking it all off. I wasn't planning on putting anything in the roof space above the ceiling but probably just doing the same thing to the ceiling as the sides. Is this ok or are you saying it might be easier just to stick some 300mm rockwool up there?

When you talk about the foil on the layer, are you talking about the foil that will be on the insulated plasterboard? Would there not also be some foil on the GA4000 (for example) as well

I will need to look at the venting in more detail.

Would i then need to tape over the joins and seal the internal join at the floor where the insulation meets?

You could put Celotex between the ceiling joists.

I'm talking about the foil on the outside of the Celotex board. I put 25mm Celotex over (OK, under, from where you're standing) the rafters, and screwed normal plasterboard over it.

I'm not clear what this internal join with the floor looks like. Is there a vertical dwarf wall coming up from the floor, before the ceiling slopes to follow the line of the roof?

Cheers
Richard

 

[shamusdb]

I have actually got dwarf walls however i'm going to insulate the whole roof from top to bottom and create cupboards behind the dwarf walls.

If i insulate the roof then at some point, starting from the top the roof the insulation will meet the floor (or joists of the ceiling below). Its that point i'm asking about and whether i need to seal the join?

Thanks

 

[geraldthehamster]

I have actually got dwarf walls however i'm going to insulate the whole roof from top to bottom and create cupboards behind the dwarf walls.

If i insulate the roof then at some point, starting from the top the roof the insulation will meet the floor (or joists of the ceiling below). Its that point i'm asking about and whether i need to seal the join?

Thanks

OK, so you'll come down to the first floor ceiling joists, and the top of the ceiling plasterboard. Cut the Celotex to fit, and seal the join with expanding foam. Probably not crucial but a good idea. It's quite a good idea to seal all the gaps between Celotex and rafters in the same way, as it's hard to get an exact fit, and this ensures it's air-tight.

Cheers
Richard

 

[shamusdb]

Thanks for the advise Richard is very much appreciated.

Just to recap. 100mm between extended rafters and 25mm on top (or combination to achieve depth of 125mm) then plasterboard on top of that? Seal all joints and joins as appropriate. Check ventilation and fit ridge vents and soffit vents if not already fitted?

Cheers
Shaun

 

[geraldthehamster]

Thanks for the advise Richard is very much appreciated.

Just to recap. 100mm between extended rafters and 25mm on top (or combination to achieve depth of 125mm) then plasterboard on top of that? Seal all joints and joins as appropriate. Check ventilation and fit ridge vents and soffit vents if not already fitted?

Cheers
Shaun

That's what I did. Not saying there aren't other ways to do it

Cheers
Richard