insulating basement walls partially below grade

fixrite

I need to insulate my basement walls. These walls are full height with 4 ft being below grade. I have been told that I only need to vapor barrier the top half of the walls. This does not make sense to me. The inspector (home grant inspector) stated this to me. I asked how to seal the 6 mil vapor barrier and he said just let it hang. He stated that the concrete foundation wall needs to breathe.I am shaking my head trying to figure out a way to do this right. I can't supply pics yet as I need to know how to do this right so I can create a budget for materials, so I don't end up with half a job done and no more money. Any help would be greatly appreciated.

Bob Mariani

Finally an inspector that gets it right. Yes... the below grade needs to dry to the inside. A vapor barrier like plastic will only block moist air in the insulation and collect mold. But you can glue rigid foam to the concrete, which keeps the warm inside air from condensing on the colder concrete wall. Leave a 1/2" space before your wall to allow space for the wall to dry out. Then add insulation in the studs. Kraft paper will add a vapor retarder, which the code requires, but still allow some air flow to move into the space allotted and the wall can not dry out.

fixrite

ok, Normally when I have installed a vapor barrier I have used acoustic sealant on the sill plate to seal the vapor barrier. How do you use a vapor barrier for half of a wall? That is what I cannot get my head around. Would I have to install a cross brace with the insulation and vapor barrier stopping where the ground outside begins?

concretemasonry

Canada infamous for the insane, magic red line on the inside of a basement at the grade. Apparently, they have not discovered that building materials transfer the heat in all dimensions and NOT directly through the wall. Some day, they will take time to look at the interior surface temperatures of the wall and see they do not vary too much once you get away from the leaky (air infiltration) wood construction.

Dick

Bob Mariani

I did not notice you were in Canada. Actually it is recommended that in very cold climates the poly is used on the entire wall. As posted air moves through the concrete wall from the moist soil but also up the wall from the footer. Possibly installing a foam blocking at the area where the grade changes may help block the changing air flow. Not as cold here and I do not need to deal with this as you do. Did the inspector have a solution to this situation? Did you ask him? They are there to help and may offer the info you need.

Peeps

Yes the walls do need to breath but at the same time just hanging the vapor barrier is not sufficient enough. It should be sealed properly or it will cause for moisture thus the wall would fail over time. You may also want to check with a contractor about the building codes for your area you cannot always rely on an inspector. You may want to know for yourself what is going on.

n0c7

To add to your confusion, here in Alberta I've only seen insulation and then the full wall vapor barriered. No foam board on the inside of the walls. Some guys seal the sills, some guys seal every stud, others don't seal at all. The basement of our 2004 house was framed and insulated with vapor barrier but not sealed. This was acceptable to the city.

You're going to get so many varied opinions on here from everyone in different climates until your head is spinning. Take Peeps advice and ask your locality/municipality what they want.

HandyPete

I live in the north of Québec (for what it's worth). The first and foremost way to finish those walls is with spray foam, you get great insulation and the humidity questions are solved.

The next "best way" is rigid foam sheets (pink or blue). Around here the guys hold the sheet against the wall, take a 1x2 furring strip and hold that too, then they drill a small pilot holes and nail away to the wall. The drwall goes over that and your done! (NO vapour barrier).

The "old fashion way" was to put black construction paper against the wall from top to about 3" from the bottom. Frame 2x3 verticaly and fill the voids with fiberglass batts. Finish the wall WITHOUT a vapour barrier and cut 2" x 6" horizontal holes at the bottom of the drywall to let air circulate behind the drywall.

Have fun,

_pete (an insane canadian)

__________________
---pete

n0c7

Better hope that foundation is straight.

Like wise. Hard to type with stitches.