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bootpdx 01-21-2008 03:17 PM

Insulating exterior floor?
I know I am not going to do a great job explaining this one, but here goes. In my living room is a small window alcove (5 ft X 3 ft) - meaning the floor of this alcove hangs out over the foundation. So outside, it pops out of the house -follow me? Meaning the floor hangs out roughly 2-3 ft from the foundation and about a foot from the ground.

Now that we some freezing weather here in Oregon, I notice the floor in the alcove is very cold. From the outside I can't tell if there is insulation in this floor or not, but the temperature of the floor tells me there isn't.

Finally- my question - Can I just attach some foam board to the underside of the exterior floor? Will this do anything to insulate or do I have to remove the old wood (that I can see), insulate, then reinstall wood? The wood on there now is old planking, should it be plywood? Since this is a overhang, it doesn't get rain, just cold. This is all made more difficult by the fact that the PVC gas furnace vent runs right below this exterior floor.

Thank you!

ToolGuy 01-21-2008 04:17 PM

You explained it very well. What you have is a catilevered floor.

Puting insulation on the outside isn't likely to do much, unless it covers any openings between the boards. I would remove the boards and insulate it properly. Use craft faced insulation, making sure the paper is facing up toward the floor, the exposed fiberglass facing down toward the exterior of the floor. This will help a little toward providing a vapor barrier. You'll probably get R-19 for this. You can add more "unfaced" insulation under the R19 to get probaby up to an R-30 value. But don't try to fit more than there is room for. Packing it tight or crushing it will loose the insuating properties.

I don't see anything wrong with reinstalling the old boards. But if they are damaged during removal and not re-useable, it's easier to replace it with exterior grade plywood.

bootpdx 01-21-2008 04:23 PM

Thanks Toolguy for your detailed response and for summing up my floor in one word. I have already found some resources online using your terminology. A bit to chilly to attempt this project now, but I appreciate the insight.

Once again this forum proves its importance.


travelover 01-21-2008 05:08 PM

I have a bay window, which is similar, in my house and I found that I could access it from the basement. It was not properly insulated and I did the 6" fiberglass thing, as described. Big difference.

glennjanie 01-21-2008 05:14 PM

Welcome Bootpdx:
If the cantilever is over dirt you could use some fiberglass vinyl panels to underpin it; just stick the material in the ground about 2" and fasten it to the outside wall. Be sure to extend the furnace vent out past the underpinning. This is a quick, down and dirty (temoprary) method to fix the problem and you can revisit it next spring or summer under much more pleasant conditions.

bootpdx 01-21-2008 05:26 PM

Thanks. This cantilever actually comes out over a recessed basement window and cement. I have thought about sealing the whole thing up, but since it is the only window in the basement I haven't wanted to get rid of the only natural light source. The window well actually makes it easier to work on the underside cuz without it me and my beer belly (all those Northwest microbrews) would not be fitting underneath.

The PVC gas vents certainly are going to make it tricky, but I think with some ingenuity I can get some plywood around them.

Thanks everyone for your input.

kok328 01-21-2008 06:39 PM

Leave out the boards and replace it w/plywood. Caulk the perimeter to keep out the draft.

bootpdx 01-23-2008 12:31 PM

Good point, kok328, thanks. Is there a standard thickness of exterior grade plywood that should be used on something like this? Or is it whatever fits?

Kerrylib 01-23-2008 02:36 PM

I would suggest going with whatever matches the thickness of the individual boards that are currently closing this space over. First guess would be 3/4" material.

Dangros 02-08-2014 11:16 PM

I have the same problem but with a kitchen extension. It's also dry b/c the deck extends past that as well. I can easily crawl under the deck to install something but what kind of insulation? There is already plywood there.

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