HSS post down to footings
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I am in Seattle in a moderate climate. In my new home we have an HSS post in a basement that goes down and is bolted to the footings. The HSS post is 20′ tall and goes up to the 2nd story floor and above it rests a steel beam that expands the length of the building. There is no thermal break between the footing and the HSS post. So right now when I touch it, it is pretty cold. I’m pretty sure the best thing would have been to put a thermal break between the footing and the HSS post but that was not done and now everything else is in (stego wrap, insulation, rebar). Next up is the radiant and then slab pour. Should I have them insulate around the bottom of the HSS post so that it does not touch the slab? But then my logic tells me that the post it will not warm up and will be cold. Or should I leave it uninsulated, and try to wrap radiant tubing around the bottom of the post (beneath the slab) to get it to warm up. But then it will draw energy out.
Hope I am being clear enough in my thoughts.
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Replies
I'm certainly not the authority, but I'm not sure what type of insulating material you could put under a structural post like that, since most insulating materials have a lot of air in them. Point loads are difficult to deal with, aside from bearing directly on concrete and soil. There is XPS with very high compressive strength, like Foamular 1000, but you would need engineering approval for anything of the sort. I would definitely isolate your heated slab from everything under it and around the perimeter, including this post if possible, and anything else within the footprint that goes to grade. The post within and below the slab with turn into a thermal wick and will be a permanent heat leak into the ground. Soil temp under your building around these parts is likely to be in the 50s, so while it could be worse it is certainly worth minimizing, especially if you can do it with a few scraps of foam.
I am an expert, or at least a structural engineer, and while there are products that would help isolate this column, it seems it is a little late to take advantage of them. They would also need to be engineered although based on what you have described, some engineering has already taken place to design the 20 foot tall column and steel beam. Now back to the energy issue. Yes the column will feel cold because steel is a good thermal conductor. However it may not be a cold as you think. Even a 70 degree steel member will "feel" cold as our bodies are normally warmer. The column also has a relatively small surface area so there is little transfer to the air in your building. The net loss is therefore probably small. With that said, you can always wrap the column with insulation and finish it with gypsum board, wood or perhaps cork that also has good thermal resistance, and will decrease the net thermal loss as well as making the column feel warmer..
Steel at room temperature always feels cold to the touch because its high conductivity is rapidly drawing away the warmth of your hand. If you are concerned about this effect wrap the post in wood or sheetrock. In the mild climate of Seattle the small area of contact with the subsoil through the footing will have no measurable effect on the overall thermal performance of your home, nor will it result in localized condensation. It is common to wrap the base of the post with a half inch thickness of fibrous material to provide a separation joint from the slab, this will help prevent the slab cracking at the stress concentration points around the post. Do not make any special attempt to heat the post with your radiant floor tubing, this would be pointless and possibly counterproductive.
There is probably a material capable of absorbing the compressive loads and providing some measure of thermal break which could have been introduced below the post but at subsoil temperatures any warmer than permafrost the effect would not be noticable. Don't worry about it.