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fothrof4 12-20-2011 10:17 PM

Insulate floor on above grade addition
I bought a home in Crownsville, MD that has an above grade addition. Half of the addition is the back portion of my master bedroom and the other half is the master bath. The addition is supported with 6x6's and is roughly four feet above the ground. Our home has forced air heating and our primary heat source is heating oil. Currently I have to run a space heater in the bathroom and bedroom to warm them. I thought of running duct work to the addition but the duct work will be exposed outside and the air would cool before entering the rooms (located about 30 feet (outside) from the heat source). I would like to insulate the underside but was told I risk mold issues because the cool air from inside trying to seep out below the insulation would cause moisture. Is there a vapor barrier I could use? I currently do not have funds to replace the floor and install radiant floor heating.

Are there any options available to me?


Windows on Wash 12-21-2011 07:44 AM


There are several options to your scenario that do not create a mold issue or require the removal of the floor for radiant heat.

Warm air carries more moisture than cold air and will likely being trying to diffuse to the top side of the home.

The easiest solution to your issue is spray foam. You could do rigid foam but that is more time intensive for a more questionable seal if not done perfectly.

AGWhitehouse 12-21-2011 09:23 AM

I agree with WOW that spray foam installed from the exterior beneath the floor is the best option.

As for heating: They do make retrofit radiant floor systems that can be applied to the underside of floor sheathing. Though not as efficient as a system applied directly below the floor finish, it still does help. I wouldn't recommend radiant floor heat for rooms with carpeting though, as the padding acts as an insulator and blocks the heat transmissions.

An electric retrofit system:

A hydronic retrofit system:

I couldn't find an actual product for the hydronic, but the pic is what you would want. You'll need the aluminum channels to help distribute the heat energy across the floor system.

If you have a large area to heat, I would highly suggest the hydronic system. It is much more efficient than electrical. If you do not have a hydronic boiler, you can add an on-demand heater just to serve this loop.

AndyGump 12-21-2011 09:55 AM

You mean that your floor has NO insulation whatsoever?


Windows on Wash 12-21-2011 10:04 AM

fothrof4 12-21-2011 12:29 PM


NONE.....the floor is very cold and the room is extremely cold after a nice hot shower.

WOW and Washington,

Thank you for the great information. I will do more research on your posts and hopefully get something going soon.

Thanks again,

AndyGump 12-21-2011 12:46 PM

Well if the floor is over a vented crawl space why not just install non-faced, R19 batt insulation?

It is fairly inexpensive and can be done DIY.


fothrof4 12-21-2011 02:20 PM

attached a picture of the underside of addition
1 Attachment(s)

The space below is not a crawl space. It is elevated about four feet above ground. My basement door actually exits under the addition. the stairwell goes about halfway and then once at the top of the stairwell you have about short distance before you are out from under the addition. I just stuck some old insulation between the floor joists for now until I find the right way to do it.

AndyGump 12-21-2011 02:31 PM

Oh yeah, get some fiberglass insulation under that thing.

Not likely to have any condensation problems under there.

I would say un-faced and wire it in there. You can probably find individual lengths of wire for this purpose at a building store near you.


AGWhitehouse 12-21-2011 03:35 PM

If you are going the fiberglass route be sure to cover it with an air barrier of some kind. Plywood would be best, but tyvek is minimum. Air currents through an un-faced batt will negate almost all of its R-value.

AndyGump 12-21-2011 03:43 PM

AG is probably right, as open as the area is you would be better off with kraft faced or another barrier of some sort as he suggested.


fothrof4 12-21-2011 03:48 PM

Thank you both very much

AGWhitehouse 12-21-2011 03:56 PM

I wouldn't do the kraft faced as that is a Class II vapor retarder and those should only go on the warm side in this application.

Windows on Wash 12-21-2011 08:00 PM


unfaced batts with tyvek or housewrap barrier.

Make sure they are installed tight to the underside and slight over compression on them will ensure they maintain contact with the floor above.

Gary in WA 12-21-2011 11:14 PM

Faced batts for your area. Housewrap (not a vapor retarder or barrier) on the joist bottoms would still leave you with a floor that insects/rodents could get in, is exposed to the cold ground (radiation factor) and outdoor temperatures with a lot of moisture from the Bay less than two miles away, and could still condense on your framing.

Treat the floor as a wall. I would:
1. air seal any holes going through to the rooms above (to stop the natural stack effect);
2. foam board (2") the rim joists and air seal with canned foam;
3. insulate any HVAC or plumbing pipes;
4. use cellulose or f.g. with a facing vapor retarder, paper, not plastic, held up with wire stays as said;
5. 1-1/2" foam board to the joist bottoms- taped at seams;
6. plywood on foam board with proper length fasteners.

Find your Zone below the map;
Find your floor insulation:

R-19= 6-1/4 thick
Or stop the ground temp. radiation with foil-faced foam board and f.g. insulation (not held up tight to the floor- figs.6,7). Except where the vinyl bathroom floor is:


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