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Insulating a basement

7K views 27 replies 12 participants last post by  concretemasonry 
#1 ·
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.
 
#2 · (Edited)
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.
 
#3 ·
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.
 
#4 ·
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.
 
#5 · (Edited)
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.
 
#6 ·
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.
 
#9 · (Edited)
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?
 
#10 ·
"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/docu...rol-for-new-residential-buildings?full_view=1 http://www.buildingscience.com/documents/digests/bsd-103-understanding-basements?full_view=1 http://www.rd.com/57548/article57548.html
Be safe, Gary
 
#11 ·
DOE website link http://www.eere.energy.gov/buildings/building_america/pdfs/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 ???
 
#14 ·
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...
 
#15 ·
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.
 
#16 ·
Oh, for sure, you see mould in and on fibreglass - but fibreglass isn't of itself a food for moulds. Other moulds or mildews on the other hand are...Yet, this doesn't preclude using fibreglass in shower stalls however because now we "waterproof" showers using the same "vapour barrier" concept - as we now know the problem.:yes:

And sometimes you do want moisture infitration and drying below grade. In cold zones and in certain types of exterior construction, you don't - as in my particular case. We limit it...but up here a bigger concern is what is called air transport in the articles.
 
#18 ·
Was reading all these posts and we just studded one wall of our basement to put kitchen cabinets on. We used treated 2x4 studs but they touch the block. This wall is next to a crawlspace that the ceiling will be insulated in and all is below grade except about 3' or so that is inside the crawlspace. We didn't put a moisture barrier because it caused condensation and were planning on just using densboard (paperless drywall that uses fiberglass instead of paper) Are we doing this right? You sound very smart and have obviously researched this a lot. Any tips? Should we also use foam?
 
#24 ·
from what i have been reading the DOE recommends that all basements be insulated with a ridgid foam against the walls. Correct me if I am mis-reading that...

The other thing is I would be careful using all pressure treated to frame a wall. When it dries that wall might be as wavy as the ocean.
 
#19 ·
Joan, it depends where you are, and what your house is made of...there's no one correct answer because there are too many variable that affect your particulat structure...
 
#22 ·
Hi YM...missed you too! have been around but not as much on this forum. Hope you're doing OK!
 
#25 ·
I am also in the design phases of finishing my basement. I talked to our chief building inspector (I work for a city) and asked him what he would recommend for insulating. This is what he told me (note: we are located in the Chicago area, age of house is 7years old, poured concrete foundation).

He said that the XPS boards are nice, but overkill and not very often used in our area. He recommended just framing 1" off the concrete, and using regular fiberglass batt between the studs. He said that putting the insulation all the way to the floor will make little difference, and that in an 8' basement, you only really need to insulate the top half, since the ground below about 4' of depth is a pretty-constant temperature. However, since insulation isn't very expensive, you can do the whole thing for piece of mind.

He also said that if there is a concern about moisture, put a poly barrier between the concrete wall and framing. (which would keep any moisture away from the wood and insulation). Again, he didn't think this was absolutely necessary and another "piece of mind" step.

Prior to any of this (at this point another building inspector joined in) they both recommended using a sealer, even something as basic as killz, on the walls and floor. The other inspector recommended scraping/chipping anything away from where the concrete walls join the floor and sealing it with a polyeurothane caulk. Caulk any cracks and expansion joints. Also caulk and cover sump pits and other areas where radon gasses can seep into the basement. This will help mitigation efforts, if you need them.


Not that it's the best, most correct, or the way I'm going to go about doing it, but I thought I'd share what I was told.
 
#26 ·
Putting the insulation to the floor makes little difference??? That makes not much sense. The gap would obviously allow for easier airflow and heat lose. The R value of insulation is resistance so no insulation would mean extremely easy airflow.

I am not an expert but from my research it seems that many people highly recommend avoiding batt insulation in a basement because of the potential for it to hold moisture.
 
#27 ·
The issue that comes with fiberglass is that it dosn't mold itself, but it hold moisture and it is and insulator wich promotes mold to grow on the framing and surfaces around it. I own a remodel business here in Michigan and what we see is mold growing in walls where there was a continual high moisture content in the walls behind the framing, especially without an air gap. If the basement was a relatively dry basement there wasn't much of a problem if they left an air gap behind the wall. One thing that you have to remember is that the insulation usually is a secondary insulator rather than a primary insulator. The foundation wall will be the primary insulator. In Michigan it isn't necessarily against code not to have a vapor barrier but some inspectors like them and some don't. It is really a personal preferance here. I would determine the vapor berrier issue by assessing which side of the wall the moisture is coming from. If it is from the inside humidity I would skip the berrier because the moisture will penetrate through the drywall and fiberglass and hit the vapor berrier and get trapped between the berriers and insulation and that is a breeding ground for mold. If the moisture is coming from the foundation wall then a berrier would be smart.
 
#28 ·
You never know how fiberglass was stored, handled and treated on the job site. The dirt and foreign material reduce the insulating value. Even without moisture the fiberglass and some other materials (non moisture repellent materials) lose insulating value. If moisture (as little as 1%) is present it can reduce the insulating value dramatically. This reduction in the insulation reduces the temperature in the insulation and can lead to condensation on the fibers, which will not the removed since it is in a "vapor retardant" environment or there is little air circulation or movement to cause the moisture to be removed even though it is not absorbed, but is held physically. This coupled with any adjacent materials or the contamination of the the fiberglass and support mold that will hold more moisture and grow, depending on the temperature and any increasing moisture supplies.

Because of the flooding of several feet of water during Katrina, many ceilings had insulation that held enough moisture from the humidity to cause the weight of the "drywall" and fiberglass celing systems to collapse after the water receeded. Some people did not even bother to renail the studs to the plates with new corrosion resistant fasteners, so the walls will be questionable in a few years. Some even drywalled over the old damp insulation to add insult to injury.

Dick
 
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