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Over insulate basement?

10K views 26 replies 11 participants last post by  jklingel 
#1 ·
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!
 
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#4 ·
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.
 
#6 ·
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.
 
#7 ·
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.
 
#10 ·
#11 ·
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.
 
#13 ·
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.
 
#14 ·
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.
 
#17 ·
If I understand right you built a straight wall ground up over your half exposure ?

Then in front of the framed portion you stuffed full of ins.and then added a vapor barrier.

Holy overkill.

The above ground framed portion should be 2x6 and already insulated to probably r19 or better depending on what they used.

The "vapor barrier" is the house wrap ( which is really a vapor retarder so your walls will "breathe").

I believe using the 6mm vapor barrier on the interior is a big mistake.

The walls need to breath.

Higher humidity is probably from the concrete wicking some moisture from the earth ( normal).

I believe basement moisture should be under 50% with these type walls ( foam/glass) according to building science ( less chance for mold to develop)

You may wish to use a dehumidifier in the basement along with the A/C as needed.

If its not hot enough to cycle the A/C much then the dehumidifier picks up the slack.

Money may be better spent on acoustical tile ect. rather than fiberglass as far as sound reduction goes ( ceiling )

If you have lots of carpeting upstairs you may not have any noise issues , hardwood/ceramic floors yes.

I see no advantage to ceiling insulation from an efficiency standpoint when the basement is living space.
 
#19 ·
Actually you're wrong on several counts...
-My above ground portion is framed by 2 x 4 and was stuffed with faced R-13. I removed the "faces" and replaced them with a continuous sheet of 6mm vapor barrier. House wrap is not a vapor barrier. It is a water barrier but moisture/water vapor can still go in and out of it so the wall breaths through it.

-I didn't state that I put a vapor barrier over the fiberglass on top of the foam insulation on the below grade portion. Just fiberglass, no vapor barrier on it because the foam insulation is a vapor barrier itself.

-Soundproofing-wise, don't sweat it. I'm doing many more things to soundproof the ceiling, including using much thicker drywall to add mass (a total of 1-1/4" thickness), decoupling the ceiling from the joists using clips and hat channels and then using a sound dampening chemical between the drywall. For what I am doing, adding fiberglass in the joist cavity is absolutely essential to achieve complete results. (see http://www.soundproofingcompany.com...s/?/library/articles/neighbor_noise_ceilings/ )

The moisture must be coming from the concrete floor which isn't sealed. I sure hope so. I'm doing a lot of different things here to ensure that I won't need a dehumidifier. Looks like I am pretty close to getting to 50%, I'll probably be there once the floor is sealed. It makes much better financial sense to spend a little more now with a one time cost than to just do a "contractor grade" basement finish with an on-going cost (electricity for dehumidifier).
 
#20 ·
OK gotcha !

Sounds like ya got it all figured out.

Many exposures around here use 2x6 on the basement level ,hence R19 or in

my case BIB insulation at R23 ( I'm further north ).

I'd tell ya more but I just remembered I have to organize my sock drawer.
 
#21 ·
I actually furred out some of my walls so I could get R-19 in them. I honestly believe they should just do all walls with 2 x 6s for additional insulation. R-19 traps 72% more heat than R-13, it would easily pay for itself in no time. Oh well, builders have their structural engineers design stuff to just meet code, never to exceed it. :mad: When I had my house built I couldn't even get them to make changes and pay more. They simply wouldn't do it due to the additional costs of submitting new plans to the city, etc.
 
#22 ·
http://www.buildingscience.com/resou...r_barriers.htm

Type in Vapor in the search bar and look for






Research Reports

RR-0410: Vapor Barriers and Wall Design


You may want to read this article from the above link.



As energy prices climb you'll probably see less stick and more SIP & ICF

I'll bet plenty of builders would jump though hoops to please you in this economic climate.
 
#24 ·
Richard, If your relative humidity upstairs is 41% at 74 F and your downstairs temp is 68 F then the downstairs relative humidity will be 50%. That difference is not due to moisture coming from the floor or walls. It's strictly a temperature effect.

Hopefully your builder used poly under the slab to prevent moisture coming through it. So while you might be getting some moisture through the slab please be aware that as long as you have the basement cooler than the upstairs then it will be a higher relative humidity. There is no reason that your basement should be a different temperature than the upstairs. Any difference in temp is entirely due to inadequate balancing of your heating/cooling system. Fix that and you may find the relative humidity upstairs is the same as downstairs.
 
#25 ·
Well I don't believe my builder used a vapor barrier under the slab. I have yet to ask, but a house from the same builder just got a flooded basement last week (their sump pump after barely keeping up with the torrential rains we have been getting lately finally burnt out and they had no backup) and the owner of the house said they could literally see watter bubbling up from the basement slab. If it is confirmed to not have the barrier then I'll have to use a concrete sealer on it.

The game hasn't been called with respect to my house, for one thing, I have air returns open in the basement but the supplies are closed for now. I'll know how things are for sure once the construction is complete.

As for temperature differences and balancing, I don't believe a perfectly balanced system is attainable. While I can stop heat from rising from the basement up to the floor above using fiberglass, I can't stop cold air from the main floor from sinking into the basement. So the basement will always stay colder unless I am adding some heat to it, no matter how much I run the fan which circulates the air in the entire house between both levels. The aim is to have a very small temperature difference, say 2 F instead of 7 F.
 
#26 ·
"I didn't state that I put a vapor barrier over the fiberglass on top of the foam insulation on the below grade portion. Just fiberglass, no vapor barrier on it because the foam insulation is a vapor barrier itself." ---- only if it is 8" thick or more.

http://www.buildingscience.com/docu...sheet-310-vapor-control-layer-recommendations

http://www.buildingscience.com/docu...rol-for-new-residential-buildings?full_view=1

http://www.quadlock.com/technical_library/bulletins/R-ETRO_Value_of_Basement_Insulation.pdf

Did they use a sill-sealer? http://www.greenbuildingadvisor.com...ressure-treated-sill-plates-and-building-code

Be safe, Gary
 
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