# Parametric runs for underslab insulation

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Hello, WFH equesters!

I would appreciate any insight on modeling underground surfaces from anyone that has tried modeling continuous insulation as is common on Passive House projects, especially in radiant slab heating applications.

I want to compare alternative thicknesses for continuous horizontal underslab insulation for both radiant slab heating and non-heated slab applications. Continuous underground insulation is becoming more common, particularly in Passive House projects or from passive-house-inspired architects. DOE-2.2 and 2.3 emphasize slab edge effects in the documentation and seem to ignore potential heat transfer from surfaces that have a path of travel greater than 4' between the surface and ambient air conditions. The wizards have only one option in the dropdown menu for continuous slab insulation - R10 continuous. Any underground slabs that have no exposed perimeter are given an R-100 insulation layer that results in near-zero heat transfer in the simulation.

All tips, musings and even non-sequiturs will (likely) be appreciated.

~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP
Senior Energy Engineer

T: (585) 698-1956 F: (585) 325-6005
bbishop at pathfinder-ea.com www.pathfinder-ea.com
134 South Fitzhugh Street
Rochester, NY 14608

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I think the answer I needed basically is: use F-factors.
To simulate various thicknesses of continuous (fully insulated) slab insulation for a slab-on-grade building with heated (radiant heated slab) and/or unheated slabs:

1.) Find the F-Factor for the R-value of the insulation you want to simulate in ASHRAE 90.1, Table A6.3.1.
2.) Determine the perimeter length by adding the widths of all exterior walls.
3.) Multiply the F-Factor by the exposed perimeter length.
4.) Divide this value by the total floor area of the slab.
5.) Take the inverse of this value. You now have a number with the same units as R-value. Use this to edit the R-value of the insulation material in the underfloor layers, which is used in the underfloor construction for all the slab surfaces included in the area and perimeter calcs above. (Don't just use it for slab surfaces with perimeter exposure. Use it for all slab surfaces within the boundary of the exposed perimeter.)

Suggested edits or clarifications to this methodology are welcome.

Thanks all!
~Bill

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