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WorBry 08-04-2012 01:16 PM

Insulation under overhang
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I'm new to the forum.

I have a property where, at the rear of the building, the ground floor overhangs entrance to the basement and garage. In the cavity of the overhang the previous owner merely installed (maybe 10 years ago) what looks to be black fiberboard sheathing loosely secured with a few wooden batons.

Photo attached below.

I'm looking to improve the insulation for the rooms above. My wife is of the mind to have the cavity sprayed with polyurethane foam (which we are considering for the garage ceiling), but I have some reservations (cost included) especially with it being on the exterior - damp exclusion etc.

Any suggestions what I might economically put in there and, if necessary, cover it with?

P.S. - Property is in Quebec, Canada. Winter temperatures typically -15oC and below.

joecaption 08-04-2012 06:13 PM

Take a piece down and look to see if there's not already insuation behind that board.
Not sure you could get someone to foam spray a job that small.

mae-ling 08-04-2012 06:23 PM

Overhangs have a couple of issues.

1. Air Flow.
The warm air from the inside needsd to warm the floor. To do this we use insulation stops to keep the insulation down an inch or two.
So in a 10" space use insulation designed for a 8" space, with 2" above that can be kept warm against the bottom of the subfloor.

2. Insulation.
To increase the amount of insulation add styrofoam to the bottom of the overhang. 2" is about R10 and makes a big difference.

You will need to open this area up from outside, do the needed work and then close it back up and use unperforated soffit or whatever is your choice to cover it all in

WorBry 08-04-2012 09:34 PM

Thanks for your reply.

Yes, I think I do need to pull down the sheathing and have a look at the structure.

I hadn't considered the possibility that there might be continuity between the inside sub-floor air space and the cavity itself and I guess it is critical to make that determination. I was working on the assumption that it would be sealed off from the interior and that insulation only served to retard heat exchange between the sub-floor immediately above and the outside air - in which case it needed to be air tight - either spray foamed or maybe caulked foam board - and possibly including some vapor barrier.

Gary in WA 08-08-2012 12:54 AM

No gap to fiberglass insulation, gap OK with foamboard and fiberglass;

No vinyl flooring above. Cover with plywood, no vapor barrier required, has to dry to inside.


WorBry 08-08-2012 08:10 AM


I've yet to go back to the property to check what might be under there other than the sheathing.

WorBry 08-15-2012 05:21 PM

Just visited the property and had a look what's in the cavity; and it is just that - a cavity about a foot deep with rendered (parged, as thet say here) concrete on all sides - the 'ceiling' of it appears to be just an extension of the concrete ceiling of the basement and garage (not sure how thick). So there's no continuity with the sub-floor air space to consider - whatever air is circulating in the overhang sub-floor is above the concrete slab.

The cavity has just been stuffed with fiber glass with the sagging sheath board holding it up. A strip of what looks to be regular styrofoam has been glued to the 'inner' (house) side of the cavity, but there's nothing on the other side or ceiling.

I'm not exactly sure how the fiber glass has been up there (10 years at least) and how long it's good for, but I'm thinking to line cavity with insulation foam board (2", or 1-1/2" enough), stuff the fiber glass back in, and knock up some more batons to close with plywood. Does that sound reasonable?

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