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braverichard 02-12-2010 10:54 AM

Over insulate basement?
Hi all,

I've been here for about 3 months now, just reading and learning. I'm in the middle of the long (hopefully not too long!) journey of finishing my basement. I am spending a considerable amount of money over the typical basement finish job just to try my best to achieve a basement that will feel like the main floor in terms of humidity levels and overall dryness, smell, etc.

Somethings about the house: construction completed about 20 months ago, last summer the basement was very dry, right now in the winter it is still very dry. I have a sump pump in there and from the tests I've done it looks like I have a dry basement all in all. The basement is a daylight basement, the northside is entirely underground and the south side is 40% underground. The east and west sides are about 60% underground as well.

In the summer it stays colder than the main floor obviously but not the 60F cold people have come to expect of basements. It was usually about 68F. Perhaps it is because it is not fully underground.

I have applied two coats of drylok on all concrete walls to seal out moisture. There's also a sealer applied on the outside of the foundation walls when the house was built so the walls should be well sealed in total.

My question is: should I insulate the concrete walls or not? I've read that heat escapes through those walls so you will be better off insulating them for the winter, but in the summer the heat that escapes gets trapped by insulation, so will this make the basement very hot? I'd like to see if I can hear from anyone else who has done this. I have R-13 on the walls above the concrete and will be doing R-19 on the ceiling and R-10 on the concrete walls. The ceiling insulation further means that the heat which naturally used to rise up to the main floor will be trapped in the basement so I'm just really curious about what the effects of all this will be. I've already purchased all the R-10 polystyrene foam boards I need for the concrete walls but would like to hear from people to help me in making a final decision on whether or not to use them. They can always be returned.

Thanks for reading!

Scuba_Dave 02-12-2010 11:04 AM

Where are you located ?

braverichard 02-12-2010 11:07 AM

Kansas City, MO.

We're smack dead geographically in the center of the USA with no major water body around to help moderate temperatures, so we get winters in which temps can fall below 0 F winters and summers with daily highs between 85F and 100F.

jlhaslip 02-12-2010 11:44 AM

Insulation will slow down heat escaping in thecwinter and also slow down heat from entering in the summer. Works both ways.
Ever live in or visit an old style 'trailer' in the two seasons? If it is cold in the winter, it will be hot in the summer. Guaranteed.

Insulation is a good thing.

braverichard 02-12-2010 11:48 AM

True but if you think about it, there won't really be any heat coming in on the concrete walls in the summer from the gound as the ground stays cool all year round.

jlhaslip 02-12-2010 11:56 AM

No, but the part above ground lets the heat in/out seasonally, and yes, there is heat lost/gained from the part buried.
You have to go a few feet under ground to reach a constant year round temperature. Here it is about 8 feet that maintains a steady 55 deg F. Not sure about where you are.

braverichard 02-12-2010 04:41 PM

Well just about 3 inches of the concrete is exposed above ground, the rest of it is underground. Our frost depth here is 36 inches, so that pretty much tells me that the first 36 inches of our ground generally has no R-value as it loses all its heat and freezes in winter. So that means heat can also come in through it in the summer. Makes sense, I guess I'll proceed with my original plans.

That aside, anyone here who has ended up with a "too hot" basement as a result of insulation? Just curious, thanks.

meboatermike 02-12-2010 07:24 PM

:eek: too hot from insulation :no: No

VelvetFoot 02-12-2010 08:02 PM

Maybe less mildewy in summer.

Gary in WA 02-12-2010 09:21 PM


Location R-value required:

Good read:

Don't forget the rim:

Or the thermal/capillary break under the p.t. sill plate: No air space at foam/wood wall.

Be safe, Gary

wnabcptrNH 02-13-2010 06:41 AM

Spend the money now and insulate the walls.

2" insulation directly on the concrete with all joints taped (use building wrap tape).

Then put in your walls, insulation, vapor barrier, and drywall.

Dont forget to seal your rim joints with either spray foam, or ridgid with all joints sealed around the ridgid. You wont regret doing all this.

Chemist1961 02-13-2010 05:31 PM

I would vote for insulating the basement walls but Not the ceiling, perhaps the floor. I would only insulate the basement ceiling if I were trying to reduce noise.

Insulation works year round. 100 year old homes with 16" walls were cool in summer warm in winter. try this site

braverichard 02-15-2010 08:02 AM

Thanks for the replies.

I will be insulating the ceiling as well as doing many other things to it for soundproofing.

I will insulate the concrete walls as advised, thanks.

My rim joists are already insulated with R19 fiberglass. I do intend to use ridgid foam board over that fiberglass and great stuff to caulk the corners and create a moisture-proof seal. I didn't get to do this to the rim joists over the bathroom (already completed) but I will be certain to do it to the remaining joists.

braverichard 06-20-2010 07:37 PM

Just an update.

So far I've done the following:

-Insulated concrete with a total of R23: R-10 of foam board insulation which is also a vapor barrier directly on concrete, unfaced R13 on top of it. Seams taped up with Tyvek tape.

- Above grade walls insulated to R-13 in most places. Some spots where I furred out the walls I have anything between R23 and R40 depending on how much space is in there. All R-13 covered with 6mm vapor barrier. Seams taped up with Tyvek tape.

-Rim joists insulated with R-10 of foam board insulation for insulation and vapor barrier, edges of foam board filled with insulating foam ("Great Stuff")

I have not insulated the ceiling yet but so far so good. The temperature stays very cool and the humidity always between 55-61 no matter how high it is outside. I do have air returns already in the basement so I'm sure some of the humidity is being removed by the A/C. The humidity always stays higher in the basement than in the main floor though, something I don't understand why. Also curious to see how much cooler it stays down there after the R-19 goes up in the ceiling.

VelvetFoot 06-20-2010 08:10 PM

Cold air holds less moisture than warm air, so the relative humidity will be higher in the basement.

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