Insulating Below Lally Column Pads
Hello, I am a new member here and wondering if anyone can shed some light on best practices for the following. I am a carpenter who is rebuilding 1000 square feet of an old farmhouse from the ground up. We made it a full basement and just finished pouring the walls using Nudura ICF. Now comes the basement floor. I was going to insulate below the slab with 2” XPS, but wondering about insulating where the (2) 3×3 ft lally column pads are? To avoid using steel to pick up the middle of the floor system I will be using some leftover LVL, but they will require (2) posts in the 30ft span. My plan is to pour these lally column support pads at the same time as the floor, basically dig deeper where I need these and add rebar. I think 3’x3’x1’ deep will be plenty. The question is I have a well insulated basement between the ICF walls and the 2” XPS under the floor, but the floor will have (2) 9 square foot areas with no insulation. Sure the ground temp is constant year round essentially, but I would love to be able to not have a thermal looses through these little areas. I know they sell high density foam board, but I also have learned it can compress or shrink over time. Any thoughts on what I could or should do?
thanks!
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3x3 seems large to me, but anyway. Set the top of the footing level with the bottom of the insulation. You will have to do two pours.
Thanks! I did not know the exact specs of what we were doing above this so I decided to go bigger than I probably need. If I use a piece of steel to support the second floor, then these footings will only pick up the 1st floor. So with 50lb/sqft I am looking at 4800lbs per lally column.
So you are saying just sandwich the XPS will just get sandwiched between the slab and these footings to keep everything continuous? The standard pink XPS I was going to use is rated for 15-20psi. So it would take 20,000lbs to crush that foam in the 36x36” area and that is not including the fact the load will be shared by the slab near by as well? Seems like I will be more than set, no?
sb1616ne,
A variant on what Jonathan is suggesting is to pour the pad with a concrete pier the size of the tele-post base flush with the finished floor.
I'm not sure there is much more complication pouring the pads first than there would be trying to ensure the integrity of the foam and poly around the perimeter and underneath the deeper section of the slab - and you are much less likely to get cracking if the pad is decoupled from the slab. Foam is rated for a certain psi based in a given deflection, which may be more than you would want.
I am suggesting that the insulation would sit on top of the footing. Posts would go through the foam and sit directly on the footing.
Jonathan,
I was suggesting much the same thing, but with the insulation interrupted by a small concrete pier so that the posts start at the floor level instead. It would allow some flexibility as to when and exactly where the posts were set. I guess one advantage of placing them directly on the footing is that the slab would constrain the base without a mechanical attachment. It might be subject to moisture damage though.
I form up pads for decks much the same way.
You can build a 3x3x1 insulated box using Foamular 600, which provides you with an insulated footing. Another option is to pour the footing as you normally would and then use a column bearing block to create your thermal break (see link below). I have done both and the latter is a lot easier.
https://www.generalplastics.com/wp-content/uploads/2019/07/General-Plastics-Construction-Brochure.pdf
Here are a few of examples including insulated footing, an insulated footing and pier and column bearing block. The bearing blocks I used are rated at 1000 psi, and I have not noticed any settling or cracking around any of the columns including the ones with the insulated footings. Note that there is still a small thermal bridge with the column bearing block from the anchor bolts.
I would not guess at the size of the footing required, especially with the long spans you're planning. There is a good chance the footings should be 42" square, or larger. I size my own footings so I'm speaking from experience doing the calculations.
To answer your question, I prefer to insulate under footings but don't get too upset if it doesn't happen.