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Heat Loss for Raised / Elevated Slab

Heat Loss for Raised / Elevated Slab

Heat Loss for Raised / Elevated Slab

There is a plethora of discussions and debates online regarding heat loss through slabs-on-grade. I've gone over several of the studies and different models that have been used. What I cannot seem to find is info regarding heat loss through a raised or elevated slab...stem walls backfilled with compacted fill and a 4" slab poured on top. Instead of "on-grade," the top of the slab is ~28" above grade. I've attached two foundation details for what I mean, one with an uninsulated slab and one with 2" of rigid foam insulation at the slab edge and along the inside of the stem wall down to the footing.

The F-factor (exposed linear foot method) for slab-on-grade heat loss calculation assumes ground contact with a specific soil type. It seems to me that an elevated slab would be significantly different in that regard as the slab is not in contact with the "ground," but rather with the compacted fill which is enclosed in the stem walls. Perhaps the most significant difference is the fact that much of the stem wall, (and therefore) the slab perimeter edge, the fill, and some of the soil underneath the fill, are above grade and in closer proximity to the outdoor air temperature than with a typical slab on grade where only 6"-8" of the foundation is above grade. I would imagine in both cases (insulated and uninsulated), the compacted fill under the slab would still be significantly warmer than the outdoor air temperature, but by what extent and to what effect?

So how would you calculate the heat loss for the attached details? Does anyone have any thoughts on this? Any advice would be greatly appreciated.

RE: Heat Loss for Raised / Elevated Slab


Thank you for the reply. I'm well acquainted with BSC and Joe Lstiburek's work. Excellent resources. I've spent more time than I care to recount trudging through models, studies, details, etc, and while I'm certain I have not read "everything," I've read and studied a fair amount and also built a fair amount throughout my years. While I'm sure the resources are there, unfortunately, I have not been able to find much, if anything, regarding heat loss for this type of elevated foundation. It seems like it should almost be considered more like an extension of the wall since so much is exposed to the ambient temperature, but I would like others' take on this.

This project is very important and personal to me. I'm not just tossing this out to the masses with no effort. I've tried very diligently to work through it analytically using different methods and ideas, some from the dark recesses of my own think box. I was just hoping to gain some insight from how others might approach the issue, or perhaps someone could point me to a resource that I may have missed. Thank you again for the reply. Pointing one to Mr. Lstiburek can hardly be considered bad advice.

RE: Heat Loss for Raised / Elevated Slab


I had already discovered THERM from a discussion on GreenBuildingAdvisor. I'm currently learning the software.

RE: Heat Loss for Raised / Elevated Slab

I'm sorry I don't have any scientific answer but personally I would assume the slab to be a partition with 50 °F temperature on the other side (even though in the center of the building it should be warmer. I think you'll find that the load calcs do not change significantly. In practice, I think most of your heat loss/gain will be on the perimeter (especially without insulation). But again, I wouldn't expect any heating/cooling issues other than it being a waste of energy. If it was a pre-school or something where kids play on a thinly carpeted floor near the walls of the building, maybe I would look into it a little more so it's not so cold over there but otherwise I personally wouldn't really worry about it.

RE: Heat Loss for Raised / Elevated Slab


Thank you for the reply. Most of the loss is definitely on the perimeter. I just wish I had a way to get a more accurate heat loss result for HVAC sizing purposes.

RE: Heat Loss for Raised / Elevated Slab

I've modeled the slab in THERM and I can see the difference between different insulation options vs uninsulated, but I'm not certain how to place the U-factor tags in order to show the total loss for the floor/slab for a block load heal loss. Anyone have experience with THERM?

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