Improve insulation in dormer

Standard Toaster

Our dormer built circa 2001 needs attention to the insulation. 2800sq ft. What is there is 100mm fiberglass as far as I can tell, and there's a good chuck missing, from the crawl space at the front mainly. The dormer part (split into two areas upstairs) has the walls insulated fine, it's just between the joists that need attention. (for now!)

I'm going to have to lift the floor upstairs in both areas as I can see there's insulation missing once I crawled up there and peered down between the joists.

I've a thermal imaging camera which also confirms the same from below deck

So, the job ahead is to pull up floor (sh!ty laminate/foil underlay) and the chipboard and add/replace insulation where needed.

Plan is to use 200mm Knauf insulation (there's a 200mm/8in spacing between the joists, I have maybe 10rolls from another job) under the flooring upstairs and use the same in the crawl spaces.

Is there any suggestions here to increase the insulation R value?
Things I thought about:
-Use "better" insulation where I'm limited to 200mm?
-Double up at 90degress in the crawls spaces to 400mm
-Replace the chipboard with boards that have insulation adhered to it (does that exist?)
-Raise the floor upstairs allowing me to use more insulation.

Any thoughts on the above?

mickdw

Before you do anything, consider airtightness plan as insulating while there is wind blowing through the place is a waste of money.

Standard Toaster

mickdw wrote: »

Before you do anything, consider airtightness plan as insulating while there is wind blowing through the place is a waste of money.

Thanks Mick.
Could you expland on that a bit?
I assumed allowing good ventilation in and around the crawl spaces was crucial.

Also, I'm by no means knowledgeable, I think alot of this is just common sense so trying to diy it.
Plus, the satisfaction of doing it myself.

mickdw

Standard Toaster wrote: »

Thanks Mick.
Could you expland on that a bit?
I assumed allowing good ventilation in and around the crawl spaces was crucial.

Also, I'm by no means knowledgeable, I think alot of this is just common sense so trying to diy it.
Plus, the satisfaction of doing it myself.

Yes you will need to continue to allow the ventilation into side and top attics if that the way it has been constructed.
Airtightness line should follow the line of the insulation to ensure no random air entering the heated part of the house.
Dormer style houses are hard to seal after construction however you need to go look at all the critical areas mostly in side attics.
Once you are satisfied that you have closed off the worst of the air entry points, insulate fully after.
The first place to check is to the base of what I call the purlin wall.

rustynutz

I have a dormer also, same size as yours and was in a similar situation to you a couple of years ago, what I did was:

1. Upgraded the 150mm of insulation that was stuffed between the studs of the dwarf wall by getting 100mm of insulation (used to insulate industrial ventilation ducting with foil backing) and added it behind the insulation in the crawl space with foil facing towards the cold side, from the top of the purlin, to the top of the floor joists. Then I taped all the joints (as best I could) with foil tape. Then I topped up the 200mm of insulation between the joist in the crawl space, with another 200mm. None of this was easy due to restricted access, I would have preferred to have used rigid board but couldn’t physically get it into the void.

2. Like you, I had only 100mm of insulation between the joists of the sloped part of the roof, and as the ceilings were boarded and skimmed I agonized before deciding to add another 50mm insulated board to the underside of the existing finished ceiling. Now I have been told since that this may not be a good idea as it is claimed moisture from condensation can get trapped between the existing skim and back of the new board, but thankfully that was 6 years ago and I haven’t had an issue, make of that what you will.

3. I sealed every joint between the base of the dwarf wall and osb board subfloor with mastic, before laying the finished floor, as well as placing a gasket on the base of the skirting against the finished floor, and bedded the skirting in mastic when installing. I also sealed around window boards and void space access hatches.

I done all this in stages over a couple of years and I have to say the sealing of the joints was the most noticeable difference, followed by the dwarf wall insulation upgrade ( which also helped with air tightness due to the foil back). What I would like to do next is pump the block wall cavity as it currently only has 50mm low quality insulation, but most people I have spoken to haven’t noticed any great difference after doing it.

Standard Toaster

Spot on rustynutz, exactly the type of advice I was looking for.
Yeap I too wouldn't be able to use rigid board in the crawl space without busting open a wall, I'd have no way of getting it in there. (crawlspace doors are 30inches wide, they themselves need replacing with an insulated solution)

I didn't even consider sealing of the joints, being the most noticeable difference for you it's definitely something I'll factor in.

Yeap will take me a while to complete this but sure there's no craic in paying someone to do it!

I might upload some pics of it's current setup when I get a few mins.

Thanks again.

Standard Toaster

https://youtu.be/0fjiztow4BI?t=78

Did you seal up like this chap (at 1:18 in video) or is that overkill?