Insulation behind 'knee walls' in dormer. Options?

batman1

I am in the process of upgrading insulation in our dormer and would appreciate any opinions.

The main problem I have is how to insulate behind the dormer room walls, i.e. the crawlspace. It has access, albeit tight.

The floor area in the crawlspace will have 200mm rolled insulation fitted across where the joists have already got 100mm in between. That's the easy bit...

The 'knee walls' have glass wool packed in between the studs.

I am considering fitting insulation slabs across the back of the studs. This would be very time consuming and messy as all slabs would have to be cut in order to get them into the space, which means a huge jigsaw puzzle inside the space. However If done right it should be a good job.

Could anyone recommend a type of board that would be easy to cut etc, and what thickness I should get?

I could get the space insulated completely using spray foam. I don't know much about it and would be keen to hear any opinions.

CJhaughey

Does it have to be rigid insulation?
Could you use foil backed quilt and fix it to the back of the knee wall studs with screws and battens?
It will be fiddly but no worse than trying to fit rigid boards in that space.

jameshayes

I had this problem, was scratching my noodle for days... in the end I worked out how much the materials would cost and then priced someone to do the job and it was in and around the same price... (friend of a friend discount)

worked out great, he was in and out in a day and there was no mess to clean up, snug as a bug since

batman1

What type of work was done and was it much benefit?

After talking with several people it seems the problem is not the insulation, but rather airtightness between the floors. If I was to go the route of fitting rigid insulation then I would have to ensure that it would fill the gaps between the joists, i.e. the space between the ceiling downstairs and the floor upstairs because apparently this is where the heat loss occurs most due to cold air flowing into this space and taking any heat generated within the house out the sides again. The same would go for fitting the foil.

I am now thinking of getting the knee walls and joists(floor) of the crawl space spray foamed out as far as the wall plate, leaving the eaves clear.

Could anyone recommend such a job?

goat2

batman1 wrote: »

What type of work was done and was it much benefit?

After talking with several people it seems the problem is not the insulation, but rather airtightness between the floors. If I was to go the route of fitting rigid insulation then I would have to ensure that it would fill the gaps between the joists, i.e. the space between the ceiling downstairs and the floor upstairs because apparently this is where the heat loss occurs most due to cold air flowing into this space and taking any heat generated within the house out the sides again. The same would go for fitting the foil.

I am now thinking of getting the knee walls and joists(floor) of the crawl space spray foamed out as far as the wall plate, leaving the eaves clear.

Could anyone recommend such a job?

you hit the nail there, i have done the knee height walls of my dormer, but the biggest problem i was having before i started this job is, there was loads of air coming through the floors upstairs, the wind was coming in under floorboards and freezing the floors upstairs, also the heating pipes going to radiators were being cooled by this also thus making it hard to heat the place, and i got the rigid board and put at intrance between floor and downstairs ceiling, then i used expanding foam to seal around it so there is no longer air getting in under floor, this the downstairs is far warmer also due to this,
dormers are hard to insulate, but i did find the cans of expanding foam handy in awkward places, i used the rolls of insulation and the rigid board, just did my best to make the place comfortable,
i am now reaping the benefits, with a nice warm house,
i also sealed around all windows outside, it has also helped,

mac3

Hi all, having same problems with my former, have noticed the breeze thought the floor upstarts, tried blocking between joists with fiberglass from the eves, how ever if that space under floor is made airtight, is their not a chance of wood rotting going on over a long period of time?

goat2

it is my guess that if the air was able to get up through the boards it will still circulate between floor and ceiling in the sealed off part due to the boards on floor not being airproofed so it,
what i found was that the hot water pipes were running along inside the footwall and there they were getting really cold due to those huge draughts coming in, so i have insulated those also and made sure they were covered well with insulation, not i find that my radiators heat up faster and hotter,
and the heat lasts longer in the home, also those pipes that are running along under floor and above downstairs ceilings are now adding to the heat in the home, i feel the floors upstairs are warmer,
i dont know if you have opened the wall yet to see what is back there, but you may be in for the same fright as i got,
the old insulation had flattened a bit and had come in about four inches all round from the wall, which meant the downstairs rooms were suffering due to not enough insulation over the ceilings also,
the only thing i have to ask now is, how do i get to seal around upstairs windows, as i find a little air coming in at windowsill, i am hoping to now seal that too,