Basement Insulation Guide
Written by Todd Fratzel.
Guide To Insulating Basements Properly
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.
Insulating Rim Joists
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.
Insulating Basement Walls
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:
- NEVER install fiberglass insulation against a foundation wall.
- NEVER install a layer of poly against a foundation wall followed by fiberglass insulation.
- NEVER install moisture barrier type paint (DryLok or similar) followed by fiberglass insulation.
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:
- Spray Foam – Spraying foundation walls with a good closed cell foam is the best approach in my opinion. While this method is expensive it does provide the best vapor barrier and tight insulating system.
- Foam Board – If you want to save some money and do-it-yourself then I recommend insulating your basement walls with foam board insulation. The basement wall insulation detail using foam board creates a good vapor barrier along with an insulation layer that won’t promote mold growth.
- Foam Board & Fiberglass – In some situations you can create a hybrid approach that uses a minimum layer of foam (1-1/2 inches) to create an effective vapor barrier followed by some fiberglass insulation for increased R value. This approach can be used to save some money while maintaining the basic concepts that are needed to prevent mold problems.
Insulating Concrete Floors
Lastly I want to share an article that I wrote about insulating concrete floors. There are numerous ways to insulate a concrete floor and help provide a nice warm finished surface. If you can afford the extra height that an insulated floor requires the payback is surely worth the effort.
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.
Final Thoughts On Basement Insulation
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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