Masonry Magazine June 1987 Page. 30
Results showed annual basement heat loss predictions by the models varied by a factor of two, despite claims that the models had been "Verified by field test results." Addition of floor losses to wall losses to calculate an overall basement heat loss increased the uncertainty, while most models predicted wall losses fairly closely.
Modeling and prediction discrepancies can lead to significant errors in the economic analysis of insulation options. A variance of $700 per year between models can create a considerable uncertainty when the overall cost of installing insulation is compared with the anticipated energy savings. The installed cost of typical foundation insulation systems varies greatly, thus reliable construction cost data and accurate calculation for local climate conditions are extremely important in correct decision making.
The authors concluded that additional research is needed to uncover the fundamental elements of disagreement between the models. Four models predicted basement heat loss with some confidence, including the Shipp Model used by ASHRAE 90.2P researchers to set foundation insulation standard levels.
Parker, of the Northwest Power Planning Council, prepared an ASHRAE technical paper that refines and extends the F-Factor method now found in ASHRAE Fundamentals Handbooks. Building on the simulation work of Shipp, whose foundation heat loss model is considered to be one of the best, Parker simulated heat loss conditions of full basements having one foot of wall above grade and seven feet below grade. Both masonry and treated wood types were analyzed. The F-Factor, a perimeter "proportional heat loss coefficient" was developed from computer run results of annual heat loss estimates. Temperatures for the walls were evaluated by integration of hourly temperature differentials in a cumulative distribution function. The final values of F were determined using the annual computed energy use for the specific foundation type divided by the overall heating degree hours, assuming a heated basement.
Parker checked the F-Factor calculation results using computed analysis results and found good agreement except in climates of less than 2500 heating degree days (Base 65F). Errors occur because no single set of coefficients can provide total coverage of all climate types. This may be because in the South, average ground temperatures are closer to average air temperatures than in the cooler northern areas where the ground may be 4 to 7 degrees F warmer than the average annual ambient air temperature. Solar effects upon soil heat flux are also more pronounced in southern areas. This method has been shown to be quite accurate compared to full-year simulations in climates with 3000 or more heating degree days.
Predicting Basement Heat Loss
Parker provided the following method which is very similar to typical heat loss analysis methods described in ASHRAE Handbooks. To predict basement heat loss:
QbFxPxHDD x 24 Hours
Where:
Q The seasonal basement heat loss estimate
F= The applicable value from the coefficients in Table I selected by actual R-Value (Linear interpolation is permitted and R-2 is the uninsulated case)
P The perimeter of the basement in feet
HDD = The seasonal total heating degree days at the thermostat setting (Base 65F data may be used for purposes of comparison). Local annual heating degree days are available from The National Weather Service or from ASHRAE Handbooks.
By first solving for the product (Px HDD x 24 hours) corresponding to local conditions, repetitive calculations predicting energy savings for different foundation types and insulation strategies can be performed using the F-Factor Table. Also, the product of Fx P is the "Earth Contact" equivalent of the above grade U x A heat loss value for an envelope component exposed to ambient air temperatures.
Sample F-Factor Calculation
Problem Statement:
Determine the annual difference in heat loads between two concrete block foundations, one with plain block and the other of identical construction with R-10 extruded polystyrene insulation added to the exterior.
The masonry basement walls are eight feet high with uniform wall insulation the full height (the plain block wall is "uniformly" insulated). Dimensions of the basements are 30x40 feet. Both are located in a locale which averages approximately 7,200 heating degree days (HDD) per year.
Solution:
Refer to Table I and select the proper construction Type, in this case BA.
Extract and record the F-Factors for the uninsulated case (assume R-2) and for the insulated case, R-12.5. Interpolate the F-Factor for R-12.5 as shown below:
F (R-12.5) =
F (R-10) + F (R-15)
2
.88 +.78
2
= 0.83
Since R-12.5 is midway between R-10 and R-15.
Next, solve for Px HDD x 24, which in this case is 2(30+ 40) x 7200x24 24.192 million (Ft. degree days).
From this it can be determined that changing from the uninsulated wall, F= 1.69, to the insulated wall, F=0.83, results in an energy savings of approximately 20.8 million BTU/Year.
If the R-10 insulation cost $1200 to install, what would the payback period be? If a 70% efficient natural gas furnace is being used at a cost of $6.50 per million BTU of fuel, $193.14 worth
TABLE I
F-Factor Perimeter Heat Loss Coefficients
for Climates With 3000 HDD or Above
(Btu/hr-F-linear foot perim)
| System | Actual R-Value |
|---|---|---|---|---|
| | 2 | 5 | 10 | 15 | 20 |
| BA | 1.69 | 1.08 | 0.88 | 0.78 | 0.71 |
| BB | 1.78 | 1.38 | 1.24 | 1.17 | 1.12 |
| BC | 2.53 | 1.38 | 1.03 | 0.88 | 0.78 |
| BX | 1.60 | 0.98 | 0.78 | 0.69 | 0.63 |
Legend:
BA: Full basement with uniform wall insulation
BB: Full basement with half wall insulation
BC: Shallow basement (more than 30% of wall exposed)
with full wall insulation
BX: Full basement with full wall insulation and an R-5 insulated
slab