Thermal Break for Steel Support Beam
Hello! I am working on a full high performance/passive house-ish retrofit of a small 24×40′ single story house. It is quite the learning experience, but a manageable size.
The original (1945) wood support beam in the basement is badly cracked and will be replaced by steel beams and posts.
The beam will be embedded in the top row of cement blocks (see photo of current wood beam location).
I am hoping someone could direct me to, or help me with, thermal breaks surrounding the beam. In retrospect, I wish I had looked into something like an LVL beam instead. I’m concerned with the thermal impacts of having steel running end to end in my house.
i.e. Is there a specific rubber or mat that can sit between the beam and the floor joists above? Should I be concerned for the steel to concrete connection?
Any guidance or direction is appreciated, thank you!
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You're not likely to have any huge problem with the steel sucking heat out of your house. The wood is, in effect, already a "thermal break", so there is no need to set joists on anything to isolate them from the steel. The best thing you could do would be some exterior rigid foam over the exterior of the part of the wall where the beam is. Even some 1/2" rigid foam (about R3 for XPS, a bit over R2 for EPS) would help. A piece 2 feet square would get you most of the benefit, but bigger would be better. You could also inset the girder just a bit so that it doesn't go all the way to the exterior of the wall, then put some 2x stock in there on the end. Just make sure you don't compromise the bearing surface area with anything you do around the ends of the beam.
Again, don't worry about isolating things from the steel inside. Just insulate the ENDS of the steel a bit on the exterior and you'll be in pretty good shape.
One other thing: I recommend priming the steel before installation (or better yet, order it primed from the steel fabricator), then paint it after installation. You can paint it before installation but you'll probably need to touch it up afterwards. The goal is to have a coating over ALL of the steel before it's installed to prevent rust down the road and keep things looking good. I've had good luck with Rust Oleum's "Stop's Rust" and performance enamel lines for this purpose in the past.
Bill, I have to disagree with you on several counts.
1/2" XPS is about R-2.1, not R-3. When brand new it might be R-2.5 but at that thickness will age to R-2.1 or 2.2 relatively quickly.
Steel and concrete are good conductors of heat. While I agree that a 2' square of foam on the exterior would provide the most benefit, heat will travel around the foam so it should really be bigger.
With the steel in contact with the concrete wall, the steel beam effectively becomes a radiator, pulling heat from the indoor air and directing it to the cold outdoors. (I'm assuming we're talking about a cold climate.)
There are a few materials the OP could use to isolate the steel from the concrete. Depending on the size of the load and the bearing area, pressure treated lumber might suffice. An engineer would have to calculate it. There are various specialty foam plastics available, including from this company: https://www.fabreeka.com/tim-landing-page/.
I agree with you regarding paint, and holding the end of the beam in a bit. Paint helps prevent rust. Rust comes from moisture. Moisture comes from condensation. Condensation comes from warm air hitting a cold surface. Keeping the beam warm will prevent rust.
1/2" Owens Corning XPS is labeled R3. I agree with you and it seems a bit high to me too, especially considering their 1" version of the same product is only labeled R5. A 2 foot square piece can be bought from the box stores as a "handy" size, so while not ideal, it is easy to buy and easy to install.
Pressure treated lumber used as a bearing plate for a steel beam can result in corrosion problems the same way using "regular" steel nails or screws can. You'd need an interface material between the two, such as HDPE sheet or a thin piece of stainless steel. Now that I think about it, a piece of 1/8" hardboard would probably work as an interface material too, although I've never used it for this purpose. You would have the issue of the bearing surface of the wood used a squash block, but that should be a very easy calculation for any engineer to run for this project.
Personally, I prefer the "insulate the exterior of the foundation wall" method better than the "try to isolate the steel from the concrete" method. The former has additional benefits, and less potential downsides -- especially with a retrofit project. Either way, trying to isolated joists from the steel has no benefit.
Bill, This is all great advice, thank you very much!
Your question reminded me of a German product I heard about from an architect looking to break the thermal bridge with steel beams: Schöck Isokorb T Type S steel connection modules; they are insulated with expanded polystyrene. I wrote a bit about it in: this post. See the section titled “Breaking the Thermal Bridge.” It’s not the same situation, of course, but I thought you might find it interesting.
Here's what I'd be tempted to do:
- Stop the steel beams shy of the block wall, and support the ends on a steel post bearing on the inside of the footing , or a pad you pour there. If you decide to finish the basement, those posts will fit in the framed walls you will be building on the inside of the foundation.
- Tie the new beams to the foundation with a 2"x4" plate fastened to the top in the same plane as the existing sill-plate. This makes attachment to the floor joists easier too.