Replies

1. Expert Member
   DCcontrarian | Jan 24, 2023 11:24pm | #1

   Steel has an r-value so low that it is indistinguishable from zero. It's a very good conductor of heat. So having the beam against the outside wall is like having a hole in the wall with a piece of R-4 over it. Which isn't the end of the world, that's about what a door or window is and it's a lot smaller than one of those.

   Steel is so conductive that having any of it exposed to the interior is the same as having all of it exposed, so it only makes sense to insulate it if you can insulate all of it. Whether that is possible depends on how it's installed. For example, if it's below the joists and exposed on all four sides, and where it's bearing joists they're thick, spray foam would work. But if it was a flush beam with the subfloor sitting on the top of the beam, the floor is only going to give about R-2 worth of insulation so there's no point in putting more than that elsewhere.

2. Expert Member
   Deleted | Jan 25, 2023 09:12am | #2

   Deleted

3. Expert Member
   Akos | Jan 25, 2023 09:18am | #3

   The problem with steel beams is if they extend through your envelope say to support a cantilever. In this case the steel has to be carefully insulated on the outside to prevent condensation on the inside section and of course to reduce a giant thermal bridge.

   With the beam being inside the house and some rigid on the exterior, I would not worry about it.

4. Expert Member
   BILL WICHERS | Jan 25, 2023 09:21am | #4

   Assuming the floor above is conditioned space (which I'm assuming is the case), there is no need to insulate the steel beam itself. All you need to do is insulate the ENDS of the beam from the exterior outdoor temperatures. The easiest way to do this is some rigid foam on the end of the beam between the exterior sheathing and the end of the beam, or if you have exterior continuous insulation, that can serve the purpose.

   I would not box over the beam with vapor impermeable insulation, since if you keep the beam itself moist, you'll get a lot of oxidation (rust). It looks like your steel has probably been primed (hard to tell for sure from the pic), but if it hasn't, I would give it a coat of one of the Rustoleum "stops rust" enamels or primer before insulating for some extra protection. Note that most radiant barrier insulation (the "bubble wrap" stuff) is vapor impermeable, so if you use that, be sure to get one of the perforated variants that won't act as a vapor barrier. You can also limit condensation with the thick textured smear-on coatings that are usually used for cold water lines and tanks.

   I would try to get some rigid foam between the beam and the exterior and leave the rest of the beam alone. Insulating the ends is far and away the best way to go here.

   Bill

5. 
   thierry19 | Jan 25, 2023 11:51am | #5

   Thank you for your answers.

   In hindsight, I would have push back harder or at least try to insulate better than R4 on the outside.

   I'm puzzled by the comment above about risk of condensation though. If I use closed-cell spray foam, then moisture wouldn't be able to reach the steel and there would not be any condensation, right?

   Thanks

1. Expert Member
   BILL WICHERS | Jan 25, 2023 02:12pm | #6

   Correct, a fully adhered insulating material would eliminate any risk of moisture getting to the steel. If you have no choice but to insulate the entirety of beam, spray foam is probably a good option, but it will be messy to apply so keep that in mind. A layer of about an inch will get you about R6. It’s much more efficient in terms of the amount of materials needed to insulate the ends only though as mentioned earlier.

   Note that it’s probably not necessary to insulate the entire beam. If you insulate only 5-6 feet on either end you probably get most of the benefit. While steel is a good conductor of heat (relative to wood framing), only a relatively small total area is “exposed” to the exterior as the cross section of the “I” shape. That limited area helps to limit the total heat transferred through the steel.

   Bill

6. Expert Member
   DCcontrarian | Jan 25, 2023 06:16pm | #7

   I'm going to throw around some numbers.

   Let's say the beam is a W10 x 33. That weighs 33 pounds per linear foot. Steel has a density of 490 pounds per cubic foot, so the end has an area of 33/490= 0.067 square feet.

   The formula for heat flow is dT*A/R, where dT= difference in temperature between the two sides, A=area in square feet, R=R-Value. So if we assume that it's 70F indoors and 0F outdoors, the beam is at the indoor temperature, and the end is insulated to R4, we get:
   70*.067/4= 1.18 BTU/hour.

   Which is essentially an immeasurable number. I should probably just stop there. But just for fun let's look at the impact of the two suggestions so far, insulating the whole beam with spray foam, and insulating the first five feet.

   Let's say you spray the whole beam with foam and add R-6. So now the R-value between the outside of the beam end and the interior is R-10. Plugging into the formula above that reduces heat loss to 0.47 BTU/hr, a reduction of a whopping 0.7 BTU/hr. Note however that this means that the temperature of the steel beam, rather than being at room temperature, will be at about 28F, which is low enough that any exposed spots can be at risk for condensation.

   Spraying just the end five feet of the beam means that the insulation between the outside and the inside is going to be R4 plus five feet of steel. Steel has an R-value around 0.00001 (I couldn't get an exact number in US units in a quick Google search) so that spray foam increases the R-value of the assembly from 4 to 4.00005.

   I will say one thing about spray foaming the ends though. It's important to keep the exposed area low. It's common to put a steel beam in some sort of pocket in the framing or foundation. I would recommend spray foaming the very ends of the beam to insure that the wall there is air sealed and that the insulation there is full thickness.

1. 
   thierry19 | Feb 08, 2023 12:59pm | #8

   Thanks for the advice.... Sorry for the late response.

   So if I understand well, the more I insulate the beam in the conditioned space, the more I risk having condensation on exposed spots correct? So I should only insulate the ends located inside the outside walls and stop there.

   Thanks

1. Expert Member
   DCcontrarian | Feb 08, 2023 06:53pm | #9

   That would be my recommendation, yes.