I asked this a couple of days ago on the HAP users forum and no one has responded so hopefully I'll have better luck here. I am working on a very large church complex (45,000 sqft) with about 80 spaces. The architect intends to use 24" of R-10 insulation in the vertical plane which should be an F-factor of 0.54 for the proposed building. According to Table 5.5-5 in 90.1-2007, the base building needs an F-factor of 0.73 and no insulation is required since the floor is unheated and it's a non-residential space. I don't have a good handle on how to properly represent the F-factor in Carrier's HAP since it only allows a slab U-value, edge insulation R-value and exposed perimeter length. Has anyone determined a good way to do this? And do I need to calculate a different U-value/insulation combination for each space? It seems to me all of the spaces are going to have the same slab and edge insulation, so they should all have the same inputs other than their exposed perimeter.

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Julia Weatherby

President

Weatherby Design & Co. Engineers

LEEDuser Basic Member

67 thumbs up

Fri, 04/29/2011 - 16:24

Hi- I don't normally use F-factors, as I use Carrier HAP to calculate loads. I agree that the spaces should all have the same inputs (floor U and edge R) other than their exposed perimeter. (Or you might adjust the U-value for the floor if some spaces are carpeted and others are bare concrete, for example.) I would model the proposed building with an edge insulation of R-10 and a floor slab U determined by adding the R-values for an indoor horizontal air space with heat transfer in the downward direction, average carpet or flooring you have, plus the concrete slab itself -- then taking the inverse to get U instead of R. So then the tricky part is what floor U and perimeter R to use for heat transfer equivalent to F-factor of 0.73 in the base building. From what I read on the this bulletin board link, http://lists.onebuilding.org/pipermail/equest-users-onebuilding.org/2010-October/006392.html an F-factor of 0.73 is equivalent (at least in E-Quest) to a 6-inch thick uninsulated concrete slab with no edge insulation. So you could try using an edge R of zero and a floor U based on 6-inch thick concrete and see what result you get. Maybe do a single zone test case with a building the same area to perimeter ratio as your base building, and compare it to a manual F-factor calculation to see how close the heat loss is. Then adjust the floor U-value to get the two heat loss numbers as close as you can. This bulletin board post: http://lists.onebuilding.org/pipermail/equest-users-onebuilding.org/2010-October/006391.html says you can convert F to edge R by multiplying the F-factor by the perimeter of the building, then dividing by the floor area of the building. Then take the inverse of that number in order to get R-value instead of U. But that seems to oversimplify for the application you have with the Carrier HAP program that also uses a floor U-value. You could also try contacting Carrier software's help desk to find out what they recommend as input for the base building 0.73 F-factor.

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