Insulating Basement Walls
Best Methods For Insulating Basement Walls
I’ve written several articles on insulating basement walls and evolved some of my methods over time. Basements provide for a very challenging insulation problem. Basements are a challenge because of high moisture levels and cooler temperatures. In this article I’d like to summarize some of my experience with insulating basement walls.
Basement Insulation Reference
Most of my methods are based on information from Building Science.com. They have a really great publication that you can download called: “Renovating Your
Basement” Research Report – 0308, 2003 (revised 2007) by Building Science Corporation. This publication is really concise and full of great information.
Understanding Basement Insulation Problems
I can’t tell you how many finished basements I’ve seen with wood framed walls directly against the concrete and the cavities of the framing filled with fiberglass insulation in direct contact with the concrete foundation walls. This type of details almost always leads to a serious mold problem and obviously a potentially risky health concern for your family.
Most people don’t understand the microscopic composition of concrete and often overlook the significant moisture levels present in concrete. I often have people say to me “the concrete walls are very dry”. I hate to be the bearer of bad news but ALL concrete has significant levels of moisture present in it. If you were to look at concrete under a microscope you’d be surprised to see how many small voids or “pores” exist. Concrete actually acts like a sponge and those pores contain lots of water. So you MUST assume that your concrete is fully saturated and full of water in order to properly insulate your basement walls.
Insulation Materials Are The Key
The key to successfully insulating basement walls is selecting insulating materials that stop moisture movement and prevent mold growth. Basements are the perfect location for foam type insulation products. Cellulose is also an option for basements but it’s not a product that I’ll cover here. I wrote Insulating A Basement with Cellulose which uses both foam and cellulose to insulate a basement for a Energy Star Home.
Spray Foam
Spray foam is probably the best possible insulation material for basements and damp areas. Closed cell spray foam is perfect for locking “out” the water vapor that inherintely wants to migrate from your damp basement walls to your finished basement rooms. Spray foam offers several benefits including additional structural stability, great vapor barrier, easily covers pipes, wires and other utilities and it provides exception R values. The problems with spray foam are it’s substantial price tag (although this will get better with time) and it’s a very messy task. However, if you can afford the cost and mess then this is the ultimate insulation product.
Foam Board
The next best basement insulating method is using foam board products. This is the method that we use most often and it’s the method that most DIY folks can handle. It’s also quite a bit cheaper than spray foam so it’s an attractive alternative. There are lots of foam board products on the market so I suggest you read Foam Board Insulation Types and R Values for a quick education on the different products available.
The key to using foam board is choosing the correct thickness and sealing it properly to create an effective vapor barrier. If you’re just going to use foam board then you’ll most likely need 2 to 4 inches in thickness depending on local energy code requirements. I recommend sealing all the joints with Tyvek (or similar) house wrap tape. You can also use “Great Stuff” spray foam in a can to seal around all your utilities and also along the bottom of the foam board. For more information on using foam board I recommend you read How To Insulate Basement Walls with Polystyrene Insulation.
Hybrid Foam & Fiberglass Insulation
The last method that I’ve used is a hybrid system of foam board and fiberglass. This method is the least expensive yet I believe it will perform well in basements that don’t have visual signs of water infiltration. This method should NOT be used if you have a history of water, even small amounts.
In this method you’ll be installing a layer of foam board, sealing it as noted above. Then you’ll frame a wall (wood or steel as I don’t believe there is much difference) in front of the foam board. Finally you’ll install fiberglass insulation in the stud cavities. Again the key here is to come up with the proper R value based on local energy codes. Most references say that you should NOT use a vapor barrier over the fiberglass insulation in this method. I’m not sure if I agree with that but it’s certainly a gray area.
Vapor Barriers
There have been tons of questions about whether or not to use a vapor barrier. The discussion is complicated so we wrote another article about the topic. Please check out: Vapor Barriers For Basement Insulation.
Basement Insulation Summary
The bottom line is not all basements are acceptable for finished space. Too often people try and “force” a finished basement when it’s not practicle due to flooding and water problems. All of these methods assume you don’t have any serious water problems in your basement. The key is using foam insulation effectively to separate the damp concrete or block from framing and insulation products that promote mold growth. I hope this article will help you decide which basement insulation method is best for your home.
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Todd:
Great site and thanks for your help. Would welcome your guidance on whether or not to: (1) place the XPS foam insulation directly on the foundation walls or to leave a 1/2-2 inch gap to channel occasional water seepage due to cracks in the wall; (2) place the foam directly on the header joist — i.e. from base plate to the ceiling; and (3) Build in place additional features to channel occasional seepage water away from the insulated foundation walls. Details follow.
I live in a 1930′s corner row house in Washington DC with a cement basement and brick foundation walls. In the basement is about 7 feet tall with 8 inch header joists/joists. The header joists that about the foundation wall jut out about 2 inches from the wall. The basement is damp and along one wall we occasionally (every 3 years or so) get some water seepage coming through cracks in the foundation wall/bricks that leads to minor basement flooding (less then 1/16 an inch) which I quickly vacuum up. I have sealed the cracks from the inside with hydrostatic cement, but we still occasionally get water seepage when it rains a lot or a neighbors pipes bust.
Given the above and based on your guidance/blog, on the walls that have water seepage I was planning to leave a 2 inch gag between the wall and the XPS foam. I was also going to run the foam all the way from the bottom plate and over the header joist. I was also going to rest the foam on a 3/4 inch composite board bottom plate in which I was going to rout 1/8 to 1/4 channels every 4 feet or so to allow any flooding water to seep out into the basement where I can then vacuum it up. I would welcome your advice on the three questions/issued presented above. Thanks.
Rob – Sounds like you’ve given this some thought. Your plan seems reasonable but I’m wondering if you might want to consider having an interior french drain installed around the perimeter first. Then any water that seeps in can be collected by the drain before it runs out onto your floor. If you leave the gap be sure you secure the foam really well. I recommend using Great Stuff Pro for the adhesive. It works very well.
Thanks. Am considering putting in a french drain. Should I run the foam all the way up to the wood ceiling (i.e. over the wooden header joist)?
Rob – The more you insulate the better. It’s always a matter of what’s practical.
I was wondering if formaldehyde free fiberglass that JM sells is any better than the standard stuff? They claim its more mold resistant and fairs better in basements then the old stuff. I’m in the process of building a home theater and originally planned to stay with all xps. However fiberglass is said to help improve acoustics.
Not sure if its worth the mold risk?
Chris – I’d stay away from fiberglass unless you’ve install foam first as a barrier. All fiberglass insulation is now formaldehyde free as far as I know..sounds like marketing glory :)
I have xps against the foundation already. My basement is also partially above ground as I mentioned in another question to you before. I put 2 inch xps in these studded above foundation areas. I’m just wondering if r10 will be enough for chicago area.
John Mansville makes it out like their insulation is the only formaldehyde free stuff on the market. They also say it has some sort of epa approved mold inhibitor. Then again the history of mold problems with fiberglass insulation deters me.
Many of the fiberglass companies sell formaldehyde free. Mold inhibitors are just that, inhibitors, it’s certainly better than none but my experience is that they don’t work well. Not mention once the insulation gets wet it loses it’s R value and will likely not dry out.
R10 is probably at the low end. However, it really depends on the energy code in your area. Some of them are based on the entire house as a whole, so if you’ve got good insulation and windows you might be able to get away with a lower value in the basement.
Thanks for the tips. Unfortunately, I had already finished half the basement before I found it. So my question is about what I already have done. I painted the concrete with a thick concrete sealer paint then proceded with a wood stud frame and fiberglass insulation. Considering I sealed the concrete do you think I will have a problem?
I don’t have a damp basement and water runs away from the house pretty good. I was aware of the porous concrete and water vapor. Do I need to tear it down to avoid a mold problem?
I am also painting the floor to completely seal the concrete.
Thanks for the help
Todd – The chances of having a problem are quite high if you live in a cold environment. The reality is that the concrete wall will be cold, any moist air that gets in the wall will hit that surface and condense, the fiberglass will trap it, and mold will grow. At the very least I would not recommend fiberglass against that wall.
Hi,
I live in a split foyer, in northern Virginia. The basement, with the exception of the laundry area in finished. I have two rooms, a bathroom and a recreation room in the basement. The two rooms and and bathroom have drywall on them and are as livable as the rooms I have upstairs. The rec room is surrounded by a thin paneling, which apparently are hung directly on a 1×2″ stud installed by wide face against the wall. The rec room always has a musky smell and, if we leave the house for 1-2 days, upon return we smell something like a dead animal (this has been going on for a long time, so it is not actually that!). Anyhow, I’ve been looking at removing the paneling and installing drywall. The contractors, whom I have seen, have suggested applying a hydraulic cement against the concrete blocks to seal any minor issues (once the paneling is removed), applying some kind of cement sealant (like drylok) on the concrete, followed by installation of plastic sheeting, frame, (may be insulation – although not strongly suggested), and dry wall…
What do you think about drylok or other liquid applicators on the net, which claim to penetrate concrete up to 4 inches to close all the pores?
My concern and problem is with humidity and not much with heat / cold. I recently installed a dehumidifier, which has remedied the smell issue, but dries the air too much and is too noisy.
PS. The previous owner installed a french system. But, I have no water sipping in and the pumps have not had to work in almost 12 years I have lived here.
Thank you for your advise.
Joe – Sounds like you’ve done some of your homework already. Obviously moisture is an issue after hearing about your dehumidifier experiment.
Drylok and other similar products use some interesting science for such claims. While I have no doubt that those products can seal the surface of concrete I’m almost certain they cannot penetrate 4 inches into concrete just based on the pore structure of concrete. At any rate they can make a difference in water vapor, just don’t expect them to lock out water that’s under hydrostatic pressure because it’s not going to happen.
So, if this were my basement and I lived in VA I would recommend installing an inch of closed cell foam board followed by strapping and drywall. The foam makes a good vapor barrier (you would use Drylok plus the foam) and it would help insulate the space during extreme temperature swings. This may seem like overkill but it’s money well spent considering today’s energy prices and the possibility of mold/mildew.
Good luck.
Thank you, Todd.
When you say “closed cell foam board”, what type of board are you referring to? Lowes have many types of board that appear to match, but prices vary greatly and I am not sure whether the difference is simply the extra cost of brand recognition.
http://www.lowes.com/pd_15099-46086-143096_4294858106_4294937087?productId=3099161&Ns=p_product_price|0&pl=1¤tURL=/pl_Insulated%2BSheathing_4294858106_4294937087_?rpp=30
http://www.lowes.com/pd_14546-46086-202631_4294858106_4294937087?productId=3050997&Ns=p_product_price|0&pl=1¤tURL=/pl_Insulated%2BSheathing_4294858106_4294937087_?rpp=30
http://www.lowes.com/pd_15358-10477-3/4X48X96+R-TECH_4294858106_4294937087?productId=3014194&Ns=p_product_price|0&pl=1¤tURL=/pl_Insulated%2BSheathing_4294858106_4294937087_?rpp=30
http://www.lowes.com/pd_304089-210-304089.0_4294858106_4294937087?productId=3122445&Ns=p_product_price|0&pl=1¤tURL=/pl_Insulated%2BSheathing_4294858106_4294937087_?rpp=30
Thank you
For your application any of those will work. They all have different pros and cons, but mostly you’re dealing with the differences between higher R values and some with radiant barrier foil. Take your pick based on what you want to spend.
Thank you! ;-)
This is a great site, Todd. I’ve read quite a few comments, and I apologize if my questions have been covered and I missed them.
I’ve read “Basement Insulation Systems”, a research report, (C)2002 by Building Sciences Corporation. Their diagrams indicate foil-faced polyisocyanurate rigid foam (or XPS) for insulating the rim joists and the top of the concrete foundation wall, then XPS rigid foam on the concrete wall face.
Would you see any objection to using the foil-faced isocyanurate entirely…including on the walls? The basis of this question is price and availability. My product sourcing research concludes that the isocyan is actually cheaper than equivalent thickness XPS, and has better availability in my area (southeastern Mass). I’m planning to install 2″ foam over the concrete, covered by a 2×3 partition of drywall…with no further insulation and no vapor barrier.
Secondly, what is your preferred method/product for “sticking” the foam to the concrete walls? I believe I read that you use Great Stuff Pro as an adhesive for XPS foam (and also to seal seams and edges against wood/concrete/penetrations). Will this product work as well on foil-faced polyisocyanurate? And if not, what alternative product would you recommend?
Regards, Gregg
Gregg – You did some great research….and your proposal is right on track. The Great Stuff Pro will work nicely. Good luck.
Thanks, Todd. I’ll advise with a progress report.