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Community and Q&A

Insulating an unheated basement / cellar

Chris Kim | Posted in Green Building Techniques on

Hello everyone,

Thank you for the great site and apologize in advance for lengthy first post.

I have a small room (6’x20′) in the basement that is currently unheated and uninsulated under covered porch. Ceiling and walls are concrete and there is no visible water penetration. There is exterior door separating this room from the rest of the finished basement (conditioned/insulated)

I am in Chicago area (zone 5A) and the room stays pretty much 75 -40 degrees throughout the year but I noticed pretty high humidity levels (70%) during the summer months, with sun blazing down on the front porch that is facing south.

Dehumidifer helped a lot but I am wondering whether adding some insulation on the ceiling (directly under porch) would help with humidity level and ensure my water main that is located in this room will not freeze?

The walls of this room are more or less underground but I guess I can extend insulation about a foot down from the ceiling.

A few questions for you experts:

1. What insulation do you guys recommend for the ceiling? Could I place XPS panel or fiberglass on to wood framing on ceiling or do I need to remove wood framing to avoid rotting due to condensation?

I want to make sure condensation doesn’t build up between concrete porch /wood framing /insulation.

2. Would I need faced batts (vapor barrier) that is facing the warm side?

3. Also could adding insulation actually negatively impact humidity level and or temperature?

4. Should i wrap the water pipe in this room to minimize any chance of freezing? House is 3 years old and went through winter of 2013 without issues but I would like to not take any chances.

Many thanks in advance!

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  1. GBA Editor
    Martin Holladay | | #2

    This room should be insulated. I'm not a fan of fiberglass batts, especially in a basement.

    I advise you to insulate the ceiling with a continuous layer of foil-faced polyisocyanurate (at least two inches thick), installed as a continuous layer on the underside of the joists. Tape the seams and the perimeter of the rigid foam to make the installation as airtight as possible.

    This article will tell you what you need to know about insulating the walls: How to Insulate a Basement Wall.

    If this porch ever gets wind-driven rain or snow, I hope that there is a waterproof membrane (roofing) between the porch floorboards and the plywood subfloor.

  2. Chris Kim | | #3

    Thank you both for the quick reply and valuable info.

    Martin - a couple quick questions

    1. Could I install the insulation on top of plywood on the ceiling? Should I be concerned with any moisure build up between the two layers? Would it be advisable to continue the insulation about a couple feet down to below ground level?

    2. Great point about moisture from wind blown rain etc. I recall seeing plastic sheeting during the pour but no additional waterproofing has been done. Then again I haven't had visible moisture on the ceiling. Could be there but plywood may keep it contained. Any way to confirm other than just visual inspection?


  3. GBA Editor
    Martin Holladay | | #4

    I have no idea what is above the plywood on the ceiling -- I assume either porch floorboards or a concrete slab. Why do you suggest that you want to install insulation on top of the plywood? Is there even access to the plywood from above?

    Basement walls should always be insulated, all the way down to the footing, especially in your climate.

  4. Chris Kim | | #5

    Sorry for not being clear but the plywood on the ceiling was needed for the concrete pour for the porch above. With wiring that has been done I was hesitant to remove the plywood. Some have suggested that I need to remove plywood before installing insulation....

  5. GBA Editor
    Martin Holladay | | #6

    So I still don't understand why you are suggesting that you might install insulation on top of the plywood. Maybe you mean "under the plywood"?

  6. Expert Member
    Dana Dorsett | | #7

    If the concrete is exposed to rain wetting & snow accumulation you probably don't want to install foil-faced polyiso under the joists, since that would inhibit the plywood & joists from drying. You might even have sheet poly between the plywood & concrete (or not). If there is, foil facers create a moisture trap with the poly. If there is no poly, you still have direct wetting of the plywood from both capillary draw and high vapor drive when the sun hits even slightly damp concrete. Either way, it has to be set up to dry toward the interior. You don't much care if the plywood rots away, but you probably need the joists for structure(?).

    You can get a pretty reasonable thermal break on the framing by tacking up 1.5" wide strips of cut up foam board, then installing full-depth rock wool batts that extend from the plywood down to the bottom of the srips, and installing a "smart" vapor retarder between that and the interior gypsum.

    Insulating the exterior foundation walls with foil-faced polyiso would work though, and is highly recommended.

  7. Chris Kim | | #8

    Yes I meant under plywood.

    Also what are your thoughts on insulating water pipe in this room assuming I didn't insulate the ceiling? I thought it would be an added protection but I also thought perhaps this room would also act to protect the pipe from freezing?

  8. Chris Kim | | #9

    One thing I didn't mention is that I would like to keep this as storm / storage cellar. Keeping humidity low was just my paranoia with mold. Temperature remains fairly reasonable at high 40 to 75 degree range even without any insulation in this room. The rest of the basement is fully insulated with separate hvac zone.

  9. GBA Editor
    Martin Holladay | | #10

    It would really be helpful to know what type of waterproofing membrane, if any, is under the concrete slab. This is not a great detail -- the slab is a floor that must also act as a roof, and you don't want a damp slab to be in constant contact with the plywood. Ideally there needs to be a bulletproof layer of roofing above the plywood.

  10. Aj Builder, Upstate NY Zone 6a | | #11

    Your builder/architect should have a detail, make a call.

    Not being there, I say rip down the wood form work, and use rigid foam walls and ceiling.

    Another option is too leave well enough alone. Summer humidity is normal. Mine is the same as yours and I have no issues. I run a dehumidifier very rarely . Just try not to store paper based items or clothing in the basement even though I break that rule and in my present basement all is ok if not on the floor directly.

  11. Richard Beyer | | #12

    Foundation Insulation Effectiveness: Basement building science

    Pat Huelman, Cold Climate Housing Coordinator with the University of Minnesota Extension discusses the issues associated with insulated foundation walls.

  12. Expert Member
    Dana Dorsett | | #13

    "Temperature remains fairly reasonable at high 40 to 75 degree range even without any insulation in this room."

    A 40F min-temp is NOT a reasonable temp for a room that has plumbing in it. If the heating system were down 2 days during a cold snap the rest of the house would be fine, but the risk of the pipes freezing is pretty high.

    Insulating the roof & walls to even R8 would couple the the space to a large ~50-55F thermal mass (the deep subsoil temp in the Chicago area), and coupled with the heat loss from the rest of the house to that space would let it ride through a cold stretch without much risk at all.

  13. Chris Kim | | #14

    Would pipe insulation help just as much if I decided not to do insulation on the ceiling? I may have be mistaken about 40s. I have been seeing low 50s last couple weeks.

  14. Nick T - 6A (MN) | | #15

    Insulating the piping very well (1.5"-2") would allow for less heat loss to the cold space - and the conductive gains from the adjacent ground and adjacent heated space would transfer into the piping/water.

    Heat-tape that automatically maintains 35-40deg would also be added under it if desired. (will only help with power of course...) - could plug it into a thermostatic cube outlet as well.

  15. Chris Kim | | #16

    Thanks Nick. I guess what I do not know for Cera in is whethe my cellar is warmer than ground and insulating actually goes against what I am trying to achieve?

  16. Nick T - 6A (MN) | | #17

    That is why insulating the ceiling would be your best bet - isolating the cellar from the outside. Currently you have around R-1 to R3 worth of insulation with the concrete and plywood. Unless that porch gets lots of sun - and holds that heat somewhat - you end up with 1/6 of your surfaces is essentially outside temperature.

    By covering the walls/ceiling with rigid foam board of some sort you will go a long way in keeping it warmer (in turn keeping basement wall warmer slightly), and reduce the vapor/moisture that is allowed into the space. Reducing condensation during spring/summer, reducing frost in winter...etc.

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