Over the last few years I’ve written many articles about different basement insulation topics. Readers have left hundreds of comments and questions about specific basement insulating topics. In an effort to streamline all of that information I decided it was time to create a comprehensive basement insulation guide. This guide summarizes the basic concepts in one single source to make your search for information easier.
If you take the time to understand the basic concepts of insulating basements you’ll be able to develop an insulation system that provides both warmth and comfort without creating an unhealthy environment. This may sound simple but the reality is that hundreds of thousands of basements are insulated incorrectly and lead to serious mold problems. I hope you’ll find the following information informative and helpful on your next basement renovation project.
Probably the single most important concept when insulating basements is the vapor barrier. Once you understand vapor barriers in basements you’ll be able to insulate your basement without causing a serious mold problem.
In my one of my previous articles, Basement Vapor Barrier, I pointed out that you need to realize that concrete walls (or concrete block) are a huge sponge full of water vapor. Even concrete walls that “appear” to be dry are always full of water vapor. Without going into some serious chemistry and physics just take my word that on a microscopic level concrete has millions of tiny voids full of water.
Once you understand that all foundations are full of water vapor you start to realize that the primary objective of the vapor barrier is stopping moisture from the foundation from entering into the finished framing where it might lead to mold. As you can see in the adjacent sketch the basement wall is insulated in a fashion that forces the drying to the outside of the foundation.
Ultimately basement vapor barriers are best accomplished with closed cell foams, either spray foam or foam board products. Definitely go check out the Vapor Barrier for Basements article for more details.
Another really important aspect of insulating basements is the rim joist. Rim Joist Insulation is often overlooked when finishing basements and it can be a crucial component of a properly insulated basement. The Rim Joist Insulation article discusses several options for properly insulating your rim joist. Today most quality insulation contractors recommend only using foam insulation products for the rim joist.
Fiberglass just doesn’t do a good job insulating this critical area and it’s prone to mold problems. As you can see above I’ve insulated my rim joist using foil faced poly-iso insulation board sealed with spray foam. Another great option is having the rim joist completely spray foamed. It’s worth pointing out that I DO NOT recommend spray foaming the rim joist with open cell foam. While some may argue that it’s not a problem I’ve personally seen many issues with it and cannot recommend it. For more detailed information check out my Insulating Rim Joist article for more details.
I can’t tell you how many poorly insulated basement walls I’ve seen over the years. Almost all of them are due to the use of fiberglass insulation installed directly against the foundation wall or up against poly that’s directly against the foundation. If you learn three things from these articles it should be this:
If you read my article on Basement Insulation you’ll see literally hundreds of questions from readers trying to insulate their basement. Over and over readers continue to ask if they can put fiberglass insulation against the foundation wall because they think the wall is dry. The answer is simple, NO! Then they ask if they put up some poly first would that be ok? The answer is simple, NO! Lastly I get the question what if I apply DryLok then fiberglass and again my answer is NO!
What my video on how to insulate your basement walls:
While the poly and DryLok might stop moisture from leaving the foundation and hitting the fiberglass it won’t stop warmer moist air from the finished basement from contacting the cold surface of the foundation. If that happens the water vapor in the warmer air will condense and turn into water. If that happens it will be trapped in the fiberglass and possibly start mold growth.
In my opinion there are two (and one modified version) ways to insulate basement walls and they are:
As you can see in the adjacent sketch one method for insulating a concrete floor involves a layer of foam board insulation followed by sleepers and a plywood sub-floor. The foam board insulation provides a great vapor barrier and a quiet floor surface.
Other modifications of this detail include eliminating the sleepers and attaching the plywood sub-floor directly to the foam board. Regardless of which detail you choose the keys involve sealing the foam well and being sure to have a good connection to the concrete.
My biggest piece of advice for insulation basements is to take your time and do your research. If at all possible completely eliminate fiberglass insulation from your plans. And lastly take your time and do a good job. A shoddy basement insulation project will likely result in serious problems down the road including potentially dangerous mold growth.
If you follow this advice I’m sure that your basement renovation project will be a success.
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View Comments
Hello - Thank you for all the useful information. I live in PA and am experiencing frost/dampness in our basement, much like others have posted. My house has a walk out basement and the only framing in the basement is on the outside wall. My concern is that we will not be able to get a vapor barrier between the studs and the plywood. Is this a problem? What suggestions do you have? Thank you in advance for your response.
-Carrie
Carrie - Not sure I know what you mean. Can you explain which portions of the basement have framing and which portions are just foundation walls?
Todd,
Thanks for the website!
Four questions:
1. What's the best way to finish around a basement window?
2. My house has 4 feet of insulation wrapped with vapour barier. Should I remove this and foam the whole wall?
3. Does it make a big difference to use foil faced or the same foam used on the wall for the floor joists?
4. I was considering using a material like superseal(rubber membrane) on the floor and finish with laminate over top? Or, tile the whole floor. I've read that as long as the laminate is 12 mm it can be placed over this type of subfloor. Any suggestions?
Thanks
Steve
Thanks for visiting and the kind words!
1. Most basement windows are best dealt with by installing an extension jamb around the existing jamb to extend it out to meet the new wall surface.
2. First off have you cut open that insulation and examined it? If not that's the first thing you should do. Secondly, you need to make a decision about what the costs are if a mold situation develops after you've remodeled the space. It's always the situation of do it now or do it later.
3. Foil faced foam typically has a different R value (see http://www.homeconstructionimprovement.com/foam-board-insulation-values/), improved radiant barrier properties and likely better vapor barrier properties. There are some publications that indicate reduced R values for foil faced polyiso with drops in temperature so that's a concern. Foil faced insulation does offer a better flame resistant rating if you're leaving it exposed. Ultimately these are all pros and cons that must be weighed.
4. When it comes to flooring I say following the manufacturers recommendations. The rubber membrane sounds like it might work well if the laminate is ok on top of it.
Good luck.
Thanks for the response.
The basement window sweats around the perimeter of the sill. The window is a white vinyl product. Should I cover the sill with an extension jam? What material is best for the jab? Why is it sweating so much?
Thanks,
Steve
Steve - It's sweating because the frame is cold enough that when water vapor (moisture in your basement) hits the cold frame it condenses into liquid. The same thing happens in finished living space in houses when there is too much humidity in the air. See this article for more information: http://www.homeconstructionimprovement.com/winter-window-condensation/
I would build wood extension jambs that can be painted or sealed. The important thing is to control humidity in that space.
Todd,
we have basement walls that are not the same depth from top to bottom, the wall is flush with the framing for about 3 feet then a "Shelf" then the wall is bumped out about 1 cinder block to the celling of the basement.
My questions is how do we insulate with foam board? Do we make it flush with the framing or cut it out to fit each section and glue it in place?
Your site has been a huge help!
Thanks!
Diane
Diane - I would fit it to the foundation wall, glue and seal well. Thanks for the kind words. Good luck.
Todd: I purchased an 85-year old home in Windsor Locks, CT in October 2009, with a partially-finished, but very cold basement. The house is a 2 1/2 story, on a 24' x 28' foundation. The below-grade portion of the foundation is approx 4' of concrete, and the above-grade portion is approx 3' of cement block (up to the rim joists.) The below-grade concrete walls are quite a bit colder to the touch than the above-grade cement blocks. The house has a hot water, cast-iron radiator heating system, and there is one radiator in the basement, but it's insufficient to overcome the significant cold. I plan to insulate the walls and rim joists per the instructions in your articles but have the following question: in the interest of saving a few bucks, will I accomplish my purpose of warming up the area by only insulating the below-grade portion of the foundation? Thanks for your help.
Mike - The answer isn't all that straight forward. First off think of un-insulated walls as having a window that's very cold to the touch. The more windows you have the colder the room will potentially be. Sure you'll save some money but it's likely that the money you do spend is not working very efficiently. The reason the "blocks" feel warmer is they are hollow and air is a fairly good insulator.
If it were my home I'd bite the bullet and insulate it all. It will make the room MUCH warmer and MUCH dryer as you'll be locking out the moisture that the foundation walls hold.
Good luck.
Todd - that's pretty much what I thought. Thanks for your quick reply. I've been doing a lot of research on these whole issue and I wanted to tell you that your site(s) and the info you provide are the most helpful I've found.
Mike - Thanks for the kind words. I truly appreciate that and hope you come back often. We give away some great tools on the site and try to keep the site up to date on a regular basis. Happy New Year.
Todd,
I am going to start finishing my basement in the near future and I have a question regarding foam insulation. I decided to do foam insulation vs fiberglass but it is very expensive. I went to my local home improvement center and noticed it is $30 plus per 2" 4' X 8' sheet. I have come across EPS styrofoam sheets of the same size for $17 a sheet which is faced with foil on both sides.
My question is can I use the EPS styrofoam sheets to insulate my basement?
Scott - I would not. See http://www.homeconstructionimprovement.com/foam-board-insulation-values/ for some basic information on the product. EPS is NOT CLOSED CELL and will therefore allow water vapor to move through it and also become trapped in it.
I would search for several different brands of XPS, Dow, Owens Corning, etc for the best pricing.
Scott, HD has 1 1/2 owens corning for 19 per sheet. Also if you have a Menards anywhere close they are a few cents cheaper. I found a Menards grand opening sale that had 2" Owens Corning for $20 a sheet, that was $8 cheaper than HD.
Todd,
I have a poured concrete basement. We are planning on finishing soon. We have dryloc on the walls already. Is there any issue with putting the blue board over top of dryloc. If not what would you recommend attaching it with.
Thanks,
Jason
Jason - That's not an issue.
Good luck.
Todd,
Looking for 2 in. Polyisocyanurate Rigid Foam Insulation and I am having trouble finding this thickness. I live in St. Louis, MO. Any idea where I can find it? Lowes & Home Depot only carry 1/2" & 1".
If not can I use 1 in. Polyisocyanurate and put 1" Polystyrene behind it to get the 2 inches of thickness?
Thanks
John R
I would use 2" XPS for the basement walls, 2" polyiso is good for the rim joist.
Not sure you'll find it as the Box Stores, but most large lumber yards carry it. If this is for the rim joist I would just use two layers of 1".
Good luck.
Todd,
I plan to use this same method but I have a few quick questions.
One, the thicknees of the rigid foam HAS to be 1.5" to be a vapor barrier? I was thinking that I would do 1" foam, frame the wall and then to unfaced within the studs but sounds like the 1"will not be enough of a barrier. Is this correct?
and 2.I have the pony wall and I plan to make a ledge in some areas. Is it ok to us a paper backed insulation like krafts on the walls about the pony wall?
You really do need a minimum of 1-1/2" of XPS foam in order for it to work as a vapor barrier. 2" is even better.
Fiberglass will work fine above on the studded walls.
Ok thanks Todd,
I understand.
I was trying to avoid having to go thicker with the xps. That stuff is not only expensive but hard to find at 1-1/2 and 2".
The big box stores around here don't carry it. I found it in one place and it was 28 bucks a sheet. Ouch.
thanks again for your help
Good luck. The cost is definitely cheaper than a nasty mold problem later on.
Todd,
I have found some 2" "GreenGuard" xps.
Its not Corning or Dow. Its "GreenGuard" and the board is actually green. I've gone to their site and it looks like this is the good stuff. Like I mentioned, its not Blue or Pink. Do you know much about this brand?
I haven't used it, however, I did check out their website and it's really the same type of product.
Todd,
I live in MD and have a fairly wet basement. I treated the walls with Drylock about 5 years ago and its basically falling off and not doing a great job. I patched several cracks with hydraulic cement but those areas seem to still let in a little bit of water. My questions is I had a "Mr Basement" trench system installed around the entire perimeter of my basement. Between the trench, Sani-dry dehumidifier and 2" foam board do you think finishing the walls with drywall is a bad idea? I am sure water will still come in through the block walls but my hope would be that the foam board will divert the water to the floor trench channel and the dehumidifier will dry the space enough.
Any help would be appreciated,
Ted
Did the Drylock come off after the trench system? Honestly if that's the case I think your basement is one of those that I wouldn't recommend finishing.