Insulation advice.

[p4rky86]

Hi newbie here. Not sure if this is in the right section but,

I do have some diy experience but I’m looking for some advice on insulating the roof of my house. Last year I had foam put in the loft which is shown by the red lines. Problem is the rest of the roof has no insulation as the foam didn’t get sprayed that far down. I was tempted to get them back and do the job but it was over a year ago now and it’s only after putting a socket in I have seen and felt the cold in the yellow area.

I was thinking of the following options as it is stud work and room in roof style.

Pir board cut and siliconed in place on the vertical part and the try and put some on the sloping roof part or do I go for mineral wool, but then it may fall out of the vertical part?

I am conscious to keep the air flowing as the soffits are vented.

Any opinions on what’s best? I have an 8 month old baby and want the room to be warm for him.

 

[^woody^]

Rigid PIR is more efficient than any quilt, but may be more difficult to fit. But worth it.

If you have spray foamed the top section, then you don't need too/ can't maintain ventilation to it.

I would lay quilt to the ceiling (300mm) and PIR to the vertical wall - 100mm between and at least 25mm across the studs if you can.

Have you read up on spray foam insulation, the horror stories, maintenance risk and implications if you come to sell?

 

[robinbanks]

Have you read up on spray foam insulation

How did you go from recommending PIR/PUR (foam) that is foamed into place between rafters, to decrying use of spray foam between rafters?

 

[robinbanks]

Pir board cut and siliconed in place on the vertical part and the try and put some on the sloping roof part or do I go for mineral wool, but then it may fall out of the vertical part?

This isn't a "try and put some" affair. Installing insulation effectively, particularly rigid board, is more like "get it right, as in forming an effective continuous thermal envelope, or it will largely be a wasted exercise"

If you're looking to insulate the room in the roof, the window of which is pictured, then you'd either have insulation on the vertical yellow line joining up with insulation on the floor level (blue line in my doodling attached) which itself joins up with the cavity wall insulation (pink line on my doodle), or you'd have insulation that joins from the existing sloping insulation to the existing cavity wall insulation. Doing both wouldn't offer significant advantage

Rigid board is harder to work with than quilt/wool and requires more attention to detail to seal out draughts which diminish its effectiveness. Wool is quite forgiving and malleable, and self expands to awkward shapes well so while board is thermally better (100mm of PIR/PUR is worth around 140mm of polystyrene and 170mm of wool) wool can create better enough of an install to diminish the difference

 

[^woody^]

How did you go from recommending PIR/PUR (foam) that is foamed into place between rafters, to decrying use of spray foam between rafters?

The way spray foam is applied leads to it encapsulating the rafter timber on three sides, so any moisture from the upper surface, broken felt, nail holes etc is trapped in the timber and does not freely evaporate - leading to mould and timber rot.

Rigid insulation board between timber is not so enveloping, and fitting this between and across the vertical eaves wall, whilst being on three sides of the timber, the location has a completely different risk profile to rafters.

 

[p4rky86]

I managed to get a good hole cut to have a look and this is what it’s like. Would 50mm pir board be ok to back onto the plasterboard or would I be better using 100 wool?

Thanks

 

[Mr Chibs]

50mm PIR is same as 100mm wool.

 

[robinbanks]

That wool that is fitted to the sloping part; can you feel a breeze blowing into this small space you've photographed, coming from the gap between the wall and the wool?

 

[robinbanks]

50mm PIR is same as 100mm wool.

Depends on the wool, but this is a reasonable rule of thumb; PIR 's thermal transmission is 0.022, wool varies from 0.032 to 0.044 so 50mm PIR aligns with ~75 to 100mm of wool.

Wool is certainly easier and faster to install than rigid board, which needs careful cutting for a tight fit, or cutting loose and foaming in. Board insulation may shrink slightly post install which lessens its effectiveness.

In an application like this, if it were determined that insulating the back side of the dwarf wall were the sensible thing to do (it may be that this was intended to be warm roof in which case insulating the wall is pointless once the warm roof fail is corrected) I'd consider wool between the studs and then board over the studs.

 

[p4rky86]

That wool that is fitted to the sloping part; can you feel a breeze blowing into this small space you've photographed, coming from the gap between the wall and the wool?

Yes there is a breeze through here, which is why it is cold in my little boys room.

I think ideally I need to go up the sloping bit but I don’t think I can get pir or wool to go up there and allow air to flow as well, as the gap is too tight. I don’t have the option of taking the ceiling out and doing it that way either. The studs are 75mm of wood and using 100mm I’m either going to have to extend the stud or using netting and then I’m probably getting to the price of a board of pir

 

[robinbanks]

Can you do us a diagram of what you've worked out in terms of the existing construction? Is it the case that you have some amount of wool between the rafters, pushed up against the tiles/roofing membrane with a gap below, and this is all the way up over the top of the loft conversion's ceiling plasterboards too?

 

[^woody^]

Quilt does not deal with draughts

 

[robinbanks]

If that were true there would be no point using it in the typical application (resting on the floor of a loft)

The way wool insulation works is by inhibiting air movement; it is the trapped air that does the insulating. If a draught were capable of easily disturbing the trapped air then the insulation wouldn't insulate

 

[^woody^]

If that were true there would be no point using it in the typical application (resting on the floor of a loft)

The way wool insulation works is by inhibiting air movement; it is the trapped air that does the insulating. If a draught were capable of easily disturbing the trapped air then the insulation wouldn't insulate

It is true.

The plasterboard ceiling stops draughts, the insulation insulates.

Draughts through quilt are not the same as air movement around the quilt (unless it's a hurricane), however the more air movement in a loft above quilt, the less it will perform.

However, remember in the context of this thread, using quilt to stop draughts is not the best thing.

 

[p4rky86]

Can you do us a diagram of what you've worked out in terms of the existing construction? Is it the case that you have some amount of wool between the rafters, pushed up against the tiles/roofing membrane with a gap below, and this is all the way up over the top of the loft conversion's ceiling plasterboards too?

I have attached a picture of what it is like. The yellow part is the area which I’m not sure what to do. As you can see from the other pictures I think I’d struggle to get 2400mm length of pir board on the sloping part to meet the spray foamed section. This is why I was only going to insulate the knee wall. Then again this won’t stop all the cold but I’m hoping it would help a bit.

There is some fibreglass insulation probably 10mm thick just under the felt. This goes up to the spray foam on the slope and that’s it. There is probably a 100mm gap between that thin insulation and the ceiling plasterboard on the slope as I looked up.