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Old 10-28-2009, 11:18 AM   #1
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Insulating a basement


I have read many different replies regarding insulating the basement walls and not. Also I have been told not to install a vapor barrier. One source I talked with said, that I should just build the walls 1" away from the poured concrete and than install the drywall. I have already studded out my basement, leaving a 1" gap from the pour walls. Do I even need insulation in the cavity ? Do I apply any vapor barrier ? Do I now cut pink or blue foam and stick it within the studs ? Help, I also live in the Chicago area.

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Old 10-28-2009, 09:45 PM   #2
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Insulating a basement


There's so many ways because each persons basement situation is different. The common way is to cover the walls & floors with XPS foam insulation (that's eXtruded PolyStyrene), and from there you can do what you want (put up the regular walls on top or the xps can have channels in it for 2x4 so you can screw your drywall into it). Just make sure you get the XPS kind... you don't want polyisocyanurate nor expanded polystyrene (styrofoam). XPS is impervious to water, moisture, and is a barrier as well. Polyisocyanurate does not handle moisture well and not recommended for basements, while expanded polystyrene (usually white styrofoam) the beads have gaps between them that take in water and it is not as waterproof. For basements XPS is the way to go.

You can either get tongue & groove XPS, or install each piece 1/4" apart and then fill in those gaps with polyurethane expanding foam and cut off the excess. That insures there's no gaps or air leaks. Or put it tight and use mastic tape on the seams.

By code, the foam must be covered for fire reasons with drywall, and attached with mechanical means to the concrete. You can't just use glue to hold it against the wall.

This method assumes your basement doesn't have water, that requires XPS with channels so the water can drip down and flow in channels and get to your pump.

The XPS prevents condensation in summer when hot humid air comes in contact with cool concrete.

Okay, absolutely you want to insulate with the pink/blue foam (which is XPS) but you want it right against the cement and then seal any seams. You may be able to do that without taking down your studwall, and don't forget needs to be fastened by mechanical means.

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Last edited by Piedmont; 10-28-2009 at 09:57 PM.
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Old 10-29-2009, 07:49 AM   #3
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Insulating a basement


I have poured concrete walls. All you have to so is make sure the fireblocking is in place. I had mine spaced out from the a bit with nothing between the concrete and the 2 x 4. I then used r-13 and a vb. in one area i used r-19 to fill all the way to the concrete due to a little more spacing (it happened this way due to how i ligned up some walls with the floor trusses above).

Both the city inspector (framing, insulation, hvac) and the electrical (state inspector) told me how well of a job i did and dont usually see this quality of work from a DIY'er. The electrical even told me i needed to install one more light and gfi and since i did the rough-in so well he said that he doesnt need to check it until the final.
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Old 10-29-2009, 08:03 AM   #4
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Insulating a basement


There's two issues you should think about where you are: insulation and air movement. You don't have to have either one, but your confort will be enhanced if you do.

(a) Insulation: since you have already studded out a wall, putting rigid boards onto the conrete wall might be a challenge without cutting them and that is, IMO not necessary. If you have put the studs 16" or 24"oc then just get unfaced fibreglass batts and fill the cavities with those. R20 or something like that...that will keep the heat 'in'.

(b) next you want to manage, and reduce, air movement because the air carries your heat out along with humidity. So staple a 6mil continuous plastic vapour barrier onto the studs, and tape up every joint so that no air can get from the inside to the wall. Don't forget the sill plates openings... That means using acoustic sealant to stick to vb on the studs making a seal between the room and the wall. Any mistakes here can lead to mould problems (caused by excessive air movement) no matter what the insulation is.

(c) then cover that with wallboard.

The foam boards stuck to the concrete are very good; spray foam is the best but sometimes pricey. This is a method that applies to the Chicago area just as it does to areas north of you. Not to every area - so someone reading this living, say, in Altanta might disgree. As he should.
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Old 10-29-2009, 11:10 AM   #5
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Insulating a basement


I'm really surprised at the others advice. The 2 things you have to worry about is condensation and moisture penetration from the foundation. With the condensation point moved into foam, you don't have to worry about it it's impervious. The other is moisture penetration through the foundation walls. Once again foam is impervious and fastened to the wall seals against it. At least having the wall 1" away helps with the 2nd scenario but does not prevent the 1st.

When you put up XPS, you move the condensation point inside the XPS foam where it's impervious to water and you're set. Not putting it up, you're moving the condensation point inside the fiberglass where it is not impervious to water so it will condensate and eventually start dripping water inside your insulation. Doesn't matter how good you make the fiberglass insulation look, looks don't move the condensation point out of the fiberglass.

Do me a favor, look up basement insulation fiberglass. There you will see article after article of why fiberglass in basements is a terrible idea and also read stories of users asking how to insulate it because they used fiberglass and it got infested shortly afterward with mold/mildew and had to rip everything out. Clicking the images, you'll see countless basements being just XPS or XPS with fiberglass on top. That's fine (and cheaper), XPS with fiberglass on top the condensation point will still be within the foam and the foam protects the fiberglass from moisture seeping in through the walls. This system does depend that your basement is dry (no drips or flooding).

To answer your question if it is a dry basement use a sealer 1st (like Drylok), then put down a layer of XPS foam (in your case probably 1"), then if you want put up fiberglass between the stud walls or not, and drywall. By code the XPS must be attached with mechanical means and it has to be covered with fireprotection like drywall.

Last edited by Piedmont; 10-29-2009 at 11:38 AM.
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Old 10-29-2009, 12:32 PM   #6
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Insulating a basement


Look, Piedmont, I expressly said that the insulation profile I outlined may not suit every region...it suits me where I am in a "cold" region and I assume that Chicago is about the same. The method I described is what we do up here - sometmes not always - but is what I have in my house.

I don't know where you are from and I have no idea what vapour profile your house has nor what is recommended where you live. Each climate and each house represents a unique construction challenge that should address the variety of ways moisture can harm a dwelling.
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Old 10-29-2009, 01:01 PM   #7
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Insulating a basement


I have a besement in western NC. The walls are currently covered in just drywall with no insulation. I want to rip out the drywall and insulate because it gets cold. Any suggestions for this situation?? Whats the best method?
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Old 10-29-2009, 01:44 PM   #8
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Insulating a basement


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Originally Posted by organick View Post
I have a besement in western NC. The walls are currently covered in just drywall with no insulation. I want to rip out the drywall and insulate because it gets cold. Any suggestions for this situation?? Whats the best method?
My suggestion, for starters, would be to read this thread.
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Old 10-29-2009, 02:19 PM   #9
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Insulating a basement


A basement in western NC...well, I'm hardly an expert on insulation but I can tell you that you probably have the exact opposite problems than I do: and the main one is humidity. Up here we often get 30% RH levels inside the houses whereas you probably have 70%...so you guys have to deal with air conditioning and dehumidification, and prevent air from getting in. Just the opposite from us.

So first, you're going to have to look at the house and its systems as a whole: what sized AC unit do you have and is is approprately sized? what do you mean "insulate from the cold?". What are your walls made from?
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Old 10-29-2009, 02:56 PM   #10
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Insulating a basement


"The best foams to use have a perm rating of greater than 1 perm for the thickness used. This means limiting extruded polystyrene insulation to less than 1-inch thickness for walls (more than 1 inch thick and they do not breathe sufficiently) and making sure that the rigid insulation is not faced with polypropylene skins or foil facings. Additionally, since foams need to be protected from fire, and this is often done with gypsum board only latex paint should be used on interior gypsum finishes (since it breathes)." From: http://www.buildingscience.com/docum...gs?full_view=1 http://www.buildingscience.com/docum...ts?full_view=1 http://www.rd.com/57548/article57548.html
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Old 10-29-2009, 04:29 PM   #11
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Insulating a basement


DOE website link http://www.eere.energy.gov/buildings...s/db/35017.pdf

seems to be what piedmont said but

I am just concerned with the issue of if the wall studs are directly on the foundation walls then the xps does not really do its job as the moisture can still come through the wood studs. Also even if it is a few inches off the block( or whatever) wall how do get a tight seal with the xps ???
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Old 10-29-2009, 04:31 PM   #12
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Insulating a basement


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My suggestion, for starters, would be to read this thread.



I read the thread and it is conflicting and based on location which is why i asked.
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Old 10-29-2009, 04:48 PM   #13
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Originally Posted by ccarlisle View Post
A basement in western NC...well, I'm hardly an expert on insulation but I can tell you that you probably have the exact opposite problems than I do: and the main one is humidity. Up here we often get 30% RH levels inside the houses whereas you probably have 70%...so you guys have to deal with air conditioning and dehumidification, and prevent air from getting in. Just the opposite from us.

So first, you're going to have to look at the house and its systems as a whole: what sized AC unit do you have and is is approprately sized? what do you mean "insulate from the cold?". What are your walls made from?


Well not necessarily. We are in the mountains so we don't get the southern heat and the humidity is def way less than on the coast. I think its still the same issue....not losing heat in the winter. I just want to make sure I don't end up with mold issues.
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Old 10-30-2009, 08:02 AM   #14
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Insulating a basement


Well, there you go, organick, at least we're narrowing down the problem and the choices you have where you are...so no high humidity but mould issues and heating/insulation issues nonetheless.

Well one thing is for sure, you don't need a vapour barrier like some of us have, but you may need a vapour retarder - that is, if it one is called for. And as I said, insulation may be required...but the type and thickness may be different from what we need. You have tricky situation of controlling where the drying of the basement walls goes to: to the inside if your inside is drier than the outside say in winter - or the reverse...You may have more concerns with water coming in after a rainfall, where the gutters take it and how much if any outside water is coming in from underneath - as opposed to worrying about air infiltration etc.

Wood shouldn't touch concrete; concrete is naturally highly alkaline for a number of years before it becomes just alkaline so no studs directly against the wall, and preferably foam underneath the bottom of the wall.

As far as I know fibreglass is inert and therefore doesn't support life forms such as mould and mildew. Given there's mould everywhere and that fibreglass can become moist, that doesn't mean mould won't grow on it - but it won't eat it they way it eats the paper off drywall panels and wood. Mould neeeds only a few things to thrive: food and humidity being paramount. Provide no food and controlling the humidity with good ventilation are just as good recommendations up here as would apply down there...
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Old 10-30-2009, 09:14 AM   #15
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Well, there you go, organick, at least we're narrowing down the problem and the choices you have where you are...so no high humidity but mould issues and heating/insulation issues nonetheless.

Well one thing is for sure, you don't need a vapour barrier like some of us have, but you may need a vapour retarder - that is, if it one is called for. And as I said, insulation may be required...but the type and thickness may be different from what we need. You have tricky situation of controlling where the drying of the basement walls goes to: to the inside if your inside is drier than the outside say in winter - or the reverse...You may have more concerns with water coming in after a rainfall, where the gutters take it and how much if any outside water is coming in from underneath - as opposed to worrying about air infiltration etc.

Wood shouldn't touch concrete; concrete is naturally highly alkaline for a number of years before it becomes just alkaline so no studs directly against the wall, and preferably foam underneath the bottom of the wall.

As far as I know fibreglass is inert and therefore doesn't support life forms such as mould and mildew. Given there's mould everywhere and that fibreglass can become moist, that doesn't mean mould won't grow on it - but it won't eat it they way it eats the paper off drywall panels and wood. Mould neeeds only a few things to thrive: food and humidity being paramount. Provide no food and controlling the humidity with good ventilation are just as good recommendations up here as would apply down there...

mold will def grow in fiberglass insulation. I've remodeled enough bathrooms to asure you of that. Secondly from what I have read you always want to be able to dry to the inside on below grade walls because soil will always be higher moisture content.


I'm thinking xps on the block then unfaced in the wall cavities.

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