beopt slab insulation modelling

asked 2024-06-04 20:18:46 -0600

tkranz
11 ●1

Hello all, I'm trying to model different levels of combined perimeter and whole slab insulation. I tried selecting both whole and perimeter slab insulation and the model thinks I'm trying to do a parametric solution. I also tried adding a new option for Slab with both perimeter and whole slab insulation but it eliminates the values from one or the other categories when i save the new slab insulation definition. Does anyone have a solution for this? Also, any feedback on if the ground temperatures are reasonably modelled anyway?

Cheers

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answered 2024-06-05 09:46:22 -0600

shorowit

11801 ●200 ●9 https://www.nrel.gov/b...

What you're finding is correct, BEopt does not currently allow an option with combined perimeter and whole slab insulation. In general, the perimeter insulation will not provide as much benefit (i.e., will be less cost-effective) if you already have whole slab insulation. Can you provide more information on how common this is or why you want to model this? We can look into allowing it for a future release.

Regarding ground temperatures, BEopt uses the Kiva model in EnergyPlus, which is a 2D finite difference model. You can read more about it here.

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Hi Scott, thanks for the quick answer. In my case I can say I am building in zone 5 and zone 6. In some cases a slab on grade makes the most sense for the soil conditions. We're interested in using the slab itself as the internal floor to optimize our material use. That being the case we want it to behave as in interior thermal mass and also optimize its perceived comfort. The perimeter delta T during the winter would be quite high so perimeter insulation makes sense (and is code required).

tkranz ( 2024-06-05 12:41:57 -0600 )edit

Sub slab temps are likely in the 50s being isolated from surface temps. But that is a large surface area and would still be perceived as cold to the touch. The design concept is to have high SHGC windows allowing winter sun directly on the concrete floor, insulated around the perimeter and underneath to make a thermal mass totally in the house itself. Modeling in beopt with whole slab insulation does show great improvement over perimeter only. Oddly though the perimeter only results in lower HVAC design loads.
I do think this is becoming more common, have read numerous articles doing so.

tkranz ( 2024-06-05 12:45:55 -0600 )edit

Also in modelling the cases in beopt I am getting a result I don't understand. The cases with full underslab insulation result in significantly lower yearly energy use than perimeter slab insulation. But those cases also output higher HVAC Heat Design Loads than the perimeter insulation cases with other conditions unchanged between those cases.

tkranz ( 2024-06-05 12:58:05 -0600 )edit

Hmm, I'm starting to think that we're talking about two different things. See this image. Are you trying to use the perimeter insulation options in BEopt to model what's labeled as "gap insulation"?

shorowit ( 2024-06-05 16:04:21 -0600 )edit

No, but that is a decent representation of what we're considering. I am trying to model with both perimeter insulation and under slab insulation (whole slab) both to get some numbers around different levels of insulation as well as for more accurately sizing HVAC. Realistically we will do gap insulation as well to isolated the slab from the foundation so it doesn't feel cold at the perimeter. An assumption I am making is that even with significant perimeter insulation heat from indoors would transfer through the slab into the soil under the building.

tkranz ( 2024-06-05 16:17:18 -0600 )edit

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