# F-Value for Uninsulated Slab

| Posted in General Questions on

Hopefully a quick easy question…

Manual J 8th Edition (pg. 348) list the F-Value for a slab on grade with no insulation (heavy moist soil) as 1.358. There’s no distinction in the actual F-value between heated (radiant) and unheated slabs. The difference is in the formula (temp delta +25 for radiant floors). Makes sense, but…

ASHRAE Standard 90.1 in appendix A table 6.3 lists an uninsulated unheated slab as 0.73 and a heated slab as 1.35. Curious about the very substantial difference in F-values. I’m sure I’m just missing something. The notes state that the base assembly is a 6″ slab poured directly on the earth and the thermal conductivity of the soil for the stated F-Values is 0.75 Btu/h·ft·°F. I suppose this could account for some of the difference, but not for the entirety. Even for uninsulated on light dry soil, Manual J gives an F-value of 0.989.

CoolCalc uses 1.358 as the F-factor for an uninsulated slab on grade floor for the heat loss calc. So I assume that’s right since it follows Manual J.

Anyone know what the ASHRAE number is for and why it’s so different?

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### Replies

1. | | #1

I'm sure there's an explanation that I'm missing. Anyone??

2. | | #2

Bumping this to see if anyone has an explanation.

1. Deleted | | #5

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3. | | #3

Sorry Jason can't answer yours but I have similar question regarding the slab calculation.

I did not have F value data so I calculated a U value for my proposed slab on grade with R20 insulation.
For my 1344 ft^2 slab Manual J calculates a 4973 BTUH load for the C6 design condition (70, -4)
So from this it appears that manual J is using the full design delta T to calculate slab heat load.
Shouldn't the calculation be based upon a smaller delta T because the ground will not be at the outdoor air temp of -4 deg F?

1. | | #4

For slabs, Manual J uses F-Value instead of U-Value for the calculation. It is based on linear feet of exterior wall and not sqft of the total slab. So it accounts for the fact that the primary slab heat loss occurs at the slab edge. While the ground is warmer, the outdoor air at the slab edge is -4°F (assuming design conditions).

The F-Value data is in Table 4A on page 348 of Manual J 8th Ed.

For example...

For my proposed build, I have a 52' x 40' rectangle. So 2080 sqft, but 184 linear feet of exterior wall. The F-Value for an uninsulated slab on my soil type (heavy moist) is 1.358 and my HTD is 43.

Heating Load HTM = F-Value x HTD
Heating Load HTM = 1.358 x 43

Heat Load = HTM x lin.ft of ext wall
Heat Load = 58.394 x 184

F-Value for my soil type with R10 slab edge insulation that extends 3ft below grade is 0.481 so the calculation would be...

Heating Load = 0.481 x 43 x 184

So adding 2" of rigid foam at the slab edge cuts my Manual J heat loss by 6939 BTUh.

This is my understanding, so hopefully some others will check in and correct me if I'm wrong. What I still don't understand is what I asked about in the original question...why the large discrepancy between ASHRAE and Manual J F-Values?

Another thing I don't understand is why heat gain is not accounted for through the slab. Manual J apparently doesn't even take the slab into consideration for heat gain calcs. This seems odd to me, and I haven't really found a good explanation. I can understand that the area near the center of the slab is probably mainly adiabatic, but what about the edges?

Jason

4. Deleted | | #6

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5. | | #7

Can anyone shed some light on this? I'm still curious as to why the F-factors between Manual J and ASHRAE are so drastically different. It would be much appreciated if someone who knows could help. Someone has to know...

I also found this old study on the AIVC site that looked at slabs in hot climates (Atlanta, GA and Albany, GA). The chart on the final page shows 33 BTUs/hr per linear ft of exposed edge for uninsulated slab which would be a F-Value of ~0.72, inline with the ASHRAE number.

https://www.aivc.org/sites/default/files/airbase_4798.pdf

6. | | #8

Dana,

If you're listening, perhaps you could weigh in on this? It would be greatly appreciated.

7. | | #9

Anyone?

8. | | #10

/bump

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