Spray foam for Dormer Crawl Space?

ravendude

Hi,
We recently moved into a dormer bungalow, approx 15 years old. .

We find it loses heat upstairs fairly quickly, and yes I know now dormers are hard to heat. But we'd like to improve in whatever way we can.
We have had the cavities done, with bonded bead and also the attic bumped up to slightly over the current regulations.

This leaves the dormer crawl space and slopes as weak points.

The crawl space has approx 10 to 15cm fibre wool behind the knee walls and we had the space above the knee walls bumped up to above current regs. I read somewhere that the cold air movement through the crawl space can sweep all the warmth out of the fibre insulation? - and this is the biggest problem with dormers (along with the slopes obviously). Is there any credence to this? I can feel the cold wind bellowing in there when I go in there, so it seems to make sense.

The slopes seem to have approx 10mm of insulation fibre board, but it would seem hard to do much with this.

Someone said to me that one solution would be to get the roof part of the crawl space insulated with open cell spray foam, effectively sealing the area for the most part against cold air movement, any thoughts? Ventilation would seem a concern. But I'm told the open cell foam should be fine, and anyway the whole roof rafters are not being sealed, just the part in the crawl space.

Thanks for any info!

muffler

There are quite a lot of threads on this topic - see here with a fairly recent one here and a well debated thread here on insulating the crawl space

batman1

I also had the same problem and I also looked at sprayfoam. However, having done loads of research and getting some great advice on this forum, I decided against it. It's a good job I did because I had to hae a velux window repaired which would have been a nightmare had the foam been in place.

In the end I went for cellulose. The company made the knee walls 'thicker' by extending the studs out (every 3 or so) and then stapling on a membrane outside it. This was then pumped with cellulose from the floor of the space to the slope. I think the term is 'scrimming'!

The floor of the crawlspace was then filled to 300mm. The attic was also done with 200mm on top of my existing fibreglass.

The benefit of the ceelulose is that it can be moved aside and replaced if you need to get in the spaces for repairs or whatever. Once the foam is in, it's in for good. And that's aside from all the condensation issues etc.

The cellulose worked out about 50% cheaper too, including the knee wall extension.

ravendude

That might be a good option batman1.

I think my biggest problem is the wind howling through, blowing the heat out.

Looking at some of the other threads above (thanks muffler)
I might start with a DIY job by sealing the gap where the roof meets the soffit/fascia with plywood and tape. I think this even on its own would make a huge difference, as I am sure the cold wind is blowing all the heat out of the insulation that is there.

I could then engage a company to beef up the knee wall insulation as you have describe.
We have loads on the knee wall floor space in and over the joists as we got this and the overhead attic done last year.

ravendude

@batman1: Did you notice much of an improvement by beefing up the knee wall insulation ?

shedweller

Noob question. Isn't there meant to be ventilation passing over the insulation to remove condensation? But having said that, i see how this would blow away any heat stored or passing through the insulation! Should there be a membrane over the insulation to allow moisture out but not allow wind through the insulation?
This reminds me of part L regs where there must be a great big 4 inch hole in the wall of each room. Despite everybodys drive for efficiency!
It may have changed so correct me if i'm wrong!

Ush1

shedweller wrote: »

Noob question. Isn't there meant to be ventilation passing over the insulation to remove condensation? But having said that, i see how this would blow away any heat stored or passing through the insulation! Should there be a membrane over the insulation to allow moisture out but not allow wind through the insulation?
This reminds me of part L regs where there must be a great big 4 inch hole in the wall of each room. Despite everybodys drive for efficiency!
It may have changed so correct me if i'm wrong!

Unless the roofing felt is breathable there is supposed to be a 50mm gap between the insulation and felt.

I see your point about blowing away heat though. Always seems to be a conflict between ventilation and airtightness/insulation.

ravendude

shedweller wrote: »

Noob question. Isn't there meant to be ventilation passing over the insulation to remove condensation? But having said that, i see how this would blow away any heat stored or passing through the insulation! Should there be a membrane over the insulation to allow moisture out but not allow wind through the insulation?
This reminds me of part L regs where there must be a great big 4 inch hole in the wall of each room. Despite everybodys drive for efficiency!
It may have changed so correct me if i'm wrong!

So, that might be my idea blown out of the water due to the 5inch gap requirement, - I was thinking plenty of air would still get in.
My felt is not breathable from what I know, - the house is 15 years old so I expect not.

Hmm, the membrane thing might be the answer. I might look into that. (Sorry Batman1, I just notice your point about membrane now also, - do you know the type?)
Any idea what kind of membrane should be used for this purpose (covering the felt), and where they can be got?

We got our insulation bumped up last year, - you would have thought the contractor would have known this!

(Thanks everyone, lots of useful info here!)

Ush1

To the best of my knowledge:

You should leave a 50mm gap(not 5 inches) between the insulation and non breathable roofing felt. The soffit of the eaves should be ventilated allowing air in and up along the 50mm gap and then out at the ridge via vent tiles.

The insulation usually has an integral or added vapour control layer which prevents warm air vapour getting through to the insulation and beyond. The vapour control layer is always on the warm side. I think when all joints of this are sealed and the plaster is over it, it is considered airtight?

I presume the reason for not having a membrane on the cold side is that you would be trapping warm air in, stopping it from escaping and creating dampness. So it seems that the vapour layer may stop vapour getting through but still allow air to pass?? But that would make it not air tight?Yeah, kinda confused.

ravendude

Ush1 wrote: »

I think when all joints of this are sealed and the plaster is over it, it is considered airtight?

Sorry I'm getting a little confused now, it might be down to my lackof familiarity with terminology etc.

So, in my crawl space I have foot or so of rolled wool insulation on the floor(ceiling joists) and about 100mm or so on the knee wall backs. I would hope to seriously bump up this insulation on the knee wall along the lines of what batman1 suggests above.

I was thinking the membrane was like a breathable, but damp proof cover or sheet that would protect this insulation from the cold wind blowing through (and blowing our the heat held within the insulation).

I'm unclear on the plastering bit, Is the plastering taking it a further step?
Also, what do you mean by cold and warm side? I would have thought just one side (the outward facing) of the insulation was exposed.
Perhaps we are talking about slightly different thing here.

Ush1

ravendude wrote: »

Sorry I'm getting a little confused now, it might be down to my lackof familiarity with terminology etc.

So, in my crawl space I have foot or so of rolled wool insulation on the floor(ceiling joists) and about 100mm or so on the knee wall backs. I would hope to seriously bump up this insulation on the knee wall along the lines of what batman1 suggests above.

I was thinking the membrane was like a breathable, but damp proof cover or sheet that would protect this insulation from the cold wind blowing through (and blowing our the heat held within the insulation).

I'm unclear on the plastering bit, Is the plastering taking it a further step?
Also, what do you mean by cold and warm side? I would have thought just one side (the outward facing) of the insulation was exposed.
Perhaps we are talking about slightly different thing here.

I think using a breathable membrane on the cold side(exposed) of the insulation would be okay but does the membrane allow air to breath in both directions? Be curious to how much it would help.

batman1

AFAIK, the 'membrane' used behind the knee walls is mainly to build up the wall thickness. It's not airtight by any means. In my case the studs were 90mm behind the knee walls. By using the method I described it allowed 200mm of cellulose to be pumped in at a high density between the back of the knee walls and the new 'membrane'. It's put in at a higher density from the top of the knee wall to the bottom of the joists/top of ceiling downstairs. This has reduced the draughts considerably. All the insulation that was already in the crawlspace was pushed in between the joists, under the upstairs floors rather than drag it out through the house.

sydthebeat

Ush1 wrote: »

Unless the roofing felt is breathable there is supposed to be a 50mm gap between the insulation and felt.

I see your point about blowing away heat though. Always seems to be a conflict between ventilation and airtightness/insulation.

ush, youve said this in a few different posts now.

Can i ask why you have this opinion? how do you overcome the obvious non compliance with building regs?
could you add to this thread as its a topic im very interested in

ravendude

sydthebeat wrote: »

ush, youve said this in a few different posts now.

Can i ask why you have this opinion? how do you overcome the obvious non compliance with building regs?

OP here, - I've abandoned the idea of blocking the gap with plywood now due to this reason. Looking now at a solution using a membrane and cellulose pumping as described by batman1 above.

The point being made I think is that we're supposed to make the house airtight, but ventilated. There is an obvious conflict.
I guess the trick is, to get the balance just right somehow. Obviously the breathable felt is a much better situation to have.

Ush1

sydthebeat wrote: »

ush, youve said this in a few different posts now.

Can i ask why you have this opinion? how do you overcome the obvious non compliance with building regs?
could you add to this thread as its a topic im very interested in

To be honest syd it's just what I've always been told by Kinspan or Xtratherm.

Is there a rough formula as to how much ventilation is needed per room in an air tight house? Most engineers I've talked to have just said put a vent in each room.

Seems a bit, I dunno, not precise.

sydthebeat

Ush1 wrote: »

To be honest syd it's just what I've always been told by Kinspan or Xtratherm.

Is there a rough formula as to how much ventilation is needed per room in an air tight house? Most engineers I've talked to have just said put a vent in each room.

Seems a bit, I dunno, not precise.

yes there is, its covered in the building regulations...

im not quite sure why you are asking this in this thread though ???

thunderdome

I could be confusing things as I have the same issue in a 1.5 storey house. I am looking at a way of improving the slope section and windows box areas, there is mineral wool in the space already with wind blowing across into the attic space. I have thought of 2 options, remove the tiles and felt from the outside and spray foam, using the venting plates to maintain airflow or use EPS Board between the joists sealing with tape maintaining the 50mm gap for air to flow into the attic. The air section should be sealed from the living area below and insulation should retain the heat below. The foam would probably be better for the window sections etc
Another option is to leave the wool as-is in the slopes and use insulated board on the internal, less messing and no cold bridges at the joists but I don’t know how well it will perform and we loose a bit of space inside, in this case the house is not finished, it is plastered though so it’s a pity to have to start over existing work.
Any thoughts.



Mod edit: I have changed your text size above. Please use the default size 2 fonts. Cheers

Grassy Knoll

https://www.boards.ie/discussion/comment/83888625#Comment_83888625

I know this is an old thread, but interested in hearing what you did. I have a dormer ‘Box’, knee walls and crawl space is well insulated but the slopes and top piece over the ceiling is not great with mineral wool. Dilemma is do I try to reslab from inside - issues over ceiling height, fittings etc or would I have the roof opened on the outside and get contractors to spray foam from there. Costly I presume, but is it a good job?