Can I insulate between my floor and basement?

tuckertdogMay 9, 2006

I've been to several sites and most claim you should not insulate the floor joists that are exposed in the basement, but rather insulate the walls, etc.

My basement is dirt and cement with only 6' head room in most places. There's a 5'x6' recessed cement structure with 2' cement walls (3" high on the dirt side) in the middle with a furnace on blocks and a hole with a sup-pump and around that is what looks to be a dirt floor. The walls are thick cement that have had a layer or 2 added in the past. Between 2 furnaces, 2 water heaters and 2 oil tanks plus all the hot air ducts and plumbing AND the new addition of a huge water treatment system for the well, I have no plans of using the space for anything beyond what it is. I can hardly stand down there! And the walls are mostly inaccessible for any insulation plus wall covering to enclose it. Without a boiler, there is hardly any extra heat generated by the hot air furnaces or anything else that might float upstairs.

I need to cut the cold! It's spring and lovely to know I won't need air conditioning this summer as it keeps about 10-20 degrees colder in the house then outside...but what about when it's -6 out there!! As I type this, my hands are frozen while it's a sunny 65 degrees outside!

What are the drawbacks to insulating the first floor joists down in the basement? Which way would any moisture barrier go? Can I just use some of the Styrofoam boards that might let me skip having to seal it in? With all the ducts, I can't drop a ceiling or I will end up with a crawl space!

BYW this is the 1875 federal with foundation problems!

Any info or ideas are appreciated!

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Yes, you can insulate between your floor joists, but .....

it's not the most cost-effective first insulation project if you're starting from no insulation. The best is the attic, and then make a huge effort to seal up air infiltration throughout the house (caulking, mostly).

But you have another problem with your basment (aside from the squirrely foundation issues) and that's moisture vapor rising from the dirt. That you interupt with a good moisture retardant material laid directly on the dirt and well sealed at the foundation.

You could also do an air infiltration barrier under the floor (without moisture control); staple it just under your joists it will help prevent cold air getting into your living space. It doesn't insulate, just blocks pressurized air from coming up. But I wouldn't do that until you had blocked moisture vapor at the soil level.

The insulate the walls (of the cellar) first recommendation may not really apply to your situation, and anyway I have read here some comments suggesting that it might do some damage. (I have no opinion about that, I'm just passing info I've seen here.)

But first, you need to get the bulge/settling problem sorted out and at least know what you must do. After that you can decide whether that project is going to happen soon, or will be put off indefinitely. Then it would be time to decide what else you want to do in that space: Moisture barrier on soil, infiltration barrier under joists and/or insulation between joists. If you chose to do it all, I would square away any electrical or plumbing changes (including insulating any water/drain pipes), insulate between joists, install air infiltration barrier and finally install the moisture barrier on the floor.

In addition you need to think through your furnace/boiler arrangements. You may need to insulate the pipes or air ducts, as well. If you create a thermal break between the unheated cellar and the heated living space, you can create problem with your heating plant. I have no advice there as I don't have central heating.

If you're just "itching" (LOL) to get some insulation installed, do the attic first, and soon, as the temps up there will get unbelieveable in just a few weeks.

Or you could take a wait and see approach and resign yourself to having not the most energy efficient house while you experience a full year's life in the building and can make better choices based on what find. There's good counsel in taking a go-slow approach to old house care. Less of a risk of an inadvertent bad choice.

BTW: my personal choice for between the joist insulation is rigid foam sheets.


    Bookmark   May 9, 2006 at 12:30PM
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Molly offers sage advise. I've got a brick floor in my basement which is essentially a dirt floor. I have mold problems with it and have temporarily put a moisture barrier on the floor - just some thicker plastic. That, and a case of caulking are the two must-do house-warming projects for any new old-house owner with a dirt floor! That has increased the heat in the winter and decreased the humidity in the summer and the cost couldn't have been close to $50!

    Bookmark   May 9, 2006 at 3:49PM
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My grandmothers house had a dirt floor. Dug it out more and put in a cement floor.

    Bookmark   May 10, 2006 at 2:32PM
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Thanks for all the ideas and the info. I'll check out the rest of the house and see what's up in the attic.
And I'm still trying to find out about this crazy foundation before I do anything!!

    Bookmark   May 16, 2006 at 7:54AM
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As I understand it, foam should never be used w/o a fire-retardant material covering it, like sheet rock. One reason for not insulating a bewteen basement and house is to prevent pipes from freezing. My basement is insulated on outside with foam covered by stucco, but had unsealed cracks letting in cold air, which infiltrated into walls of house. I solved a lot that by caulking to seal gaps all around the house between top of foundation and floor- really amazing what a difference that alone made. Then I went around and sealed every little opening I could find anywhere, such as around windows. My house became much more comfortable with almost no drafts.

I do have wet basement, due to street drainage issues. This is slowly being corrected. Until then, moisture barrier over damp areas and sump pump keep the problem at least under control. Once the street issue is fixed (on the town books), I am going to replace damaged part of concrete floor and intall proper moisture barrier. Dampness hasn't kept me from using the basement, which is a daylight with big slider and nice workspace for studio. In summer I run dehumidifier down there.

Dayle Ann

    Bookmark   May 16, 2006 at 8:24AM
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