6.   WHOLE HOUSE FOUNDATION INSULATION LAYER THERMAL RESISTANCE TRADE-OFF OPTIONS

6.1    Trade-off Envelope Heat Transport Physics

In order to assess the trade-off requirements for permitting R-5 exterior insulation in place of the R-10 optimum as determined in chapter 4,  a parametric analysis of the impact of foundation wall system R-value on whole house performance was undertaken.  As reported previously (Go1985(a), Go1985(b)), envelope advective mass flux (as a result of ventilation and infiltration) is the largest component of the whole house envelope heat loss.  Hence the whole house effects of foundation insulation R-value variation of necessity need to be determined as a function of envelope advection in order to appreciate the relative impact of such changes that tend to be small in comparison with the much larger advective losses.  The basis for the trade-off strategy is that the whole house energy consumption with R-10 foundation wall insulation at a given rate of infiltration and ventilation without any trade-offs should be exactly the same as the whole house energy consumption with R-5 exterior insulation and the trade-off.

Based on table 2.1, the design infiltration rate for the MN-CALC house is 0.31 cfm/ ft² of conditioned space at a 50 Pa envelope pressure difference which thus defines the upper limit for the parametric variation of infiltration rate.  The infiltration rates at 50Pa are converted to infiltration rates at a service pressure difference of 4Pa and then dynamically varied in the simulation as a function of wind speed (using the DOE2 infiltration model coefficients).  In terms of N1104 (Mechanical Ventilation Systems) of the proposed MN energy code, the required continuous ventilation rates range from 40 cfm for 1000 ft² of conditioned space (including the basement) to 113 cfm for 6000 ft² of conditioned space.  Thus the ventilation rate is varied parametrically from 0 cfm (that is, infiltration only) to 120 cfm.  It is assumed that the infiltration and ventilation components are decoupled (that is, the ventilation is always perfectly balanced, that is, volumetric inlet flow = volumetric outlet flow).  While this yields a conservative estimate of the envelope advection losses, it is not accurate as the ventilation and infiltration are always coupled though the interior pressure that is a determined by fan pressure/flow and control characteristics as well as by external wind hydrodynamic pressure, internal bouyant flows (that is, the stack effect) etc.  Two ventilation strategies are examined, namely exhaust only ventilation and energy recovery ventilation. 

The simulations are run for R-5 and R-10 foundation wall system continuous uniform insulation levels.  The above-grade wall exposure at 18% is higher than the 10% nominal minimum (grade 12" below the top of the foundation wall system everywhere) in order to account for fairly shallow berms on either side of the lookout wall in the MN-CALC house.  The HVAC system flow-rates are balanced with the R-10 insulation so that the the first floor and basement diurnal average temperatures (that is including the effect of heating season thermostat setbacks) are approximately the same.  This enables the decrease in basement temperature as a result of the reduced foundation wall insulation to be assessed (since the thermostat that is located in the first floor zone maintains the same first floor zone temperature regardless of foundation insulation).  Note that only the winter heating energy consumption is simulated and the exercise is performed for a Minneapolis climate only as the underlying physics are climate variation independent within the context of the Minnesota climate range.  The results of the simulations are shown in figures 6.1 through 6.4.

Figure 6.1

Figure 6.1 shows the variation in relative seasonal heating energy demand as a function of ventilation and infiltration rates.  The normalization energy demand is that for which both ventilation and infiltration are zero.  The dependent energy demand (ordinate) is unconventionally plotted along the x-axis while the infiltration and ventilation rate abcissae (independent variables) are plotted on the left and right hand y-axes respectively.  As the infiltration is increased to the maximum of 0.31 cfm/ ft², the seasonal heating demand increases to from 1.0 to 2.108 for R-10 and from 1.043 to 2.155 for R-5, amounting to a 111% increase in demand for both insulation levels.  The R-5 decrease in foundation wall insulation accounts for 4% of the whole house envelope energy loss at zero infiltration and ventilation but only 2.2% at 0.31 cfm/ ft² infiltration and zero ventilation.  Adding exhaust only ventilation increases the relative energy demand further to 2.692 and 2.746 for R-10 and R-5 insulation respectively.  Thus adding 120 cfm of exhaust-only ventilation increases the demand over the infiltration datum by 27.7% for R-10 and 27.4% for R-5, and, the impact of the R-5 foundation insulation decrease is reduced even further to 2.0%.  Replacing exhaust-only ventilation with an ERV reduces the net energy demand increase by 5.2% for both insulation levels, a five-fold decrease compared with exhaust-only ventilation.  With an ERV, at 120 cfm, the R-5 foundation insulation decrease still accounts for just 2% of the whole house demand.

These data point to the inescapable conclusion (that confirms the results in the literature cited above) that foundation insulation is a relatively insignificant component of the whole house envelope heat loss, particularly when compared with infiltration and, to a lesser extent, ventilation.  Thus the whole-house trade-offs required to compensate for reduced foundation insulation on a full basement are small.  An ERV offers an order of magnitude greater energy savings compared with increased foundation insulation (roughly 22% compared with 2%) at a maximum ventilation flow rate of 120 cfm, a savings advantage that decreases proportionately as the ventilation flow rate is decreased.

The primary trade-off strategy considered in this study is an increase in furnace efficiency beyond the current national minimum of a 78% AFUE (annualized fuel usage efficiency).  The required furnace AFUE variation is shown in figure 6.2.

Figure 6.2

Figure 6.2 is roughly the inverse of figure 6.1.  In this case, the ordinate plotted along the x-axis is the minimum furnace efficiency.  As the energy demand increases with infiltration and ventilation, the energy loss from the R-5 reduction in foundation wall insulation decreases as a fraction of the whole house energy demand and hence requires a decreased amount of compensating furnace AFUE increase.  Thus, as the infiltration rate increases from 0 to 0.31 cfm/ ft², the required AFUE offset for using R-5 insulation decreases from 81.3 % to 79.7%, or 3.3% to 1.7% more than the 78% base.  Increasing the exhaust-only ventilation rate from 0 to 120 cfm minimally reduces the AFUE offset further to 79.6%, while an ERV essentially has no difference on the required AFUE with infiltration alone since the incremental ventilation demand is so much smaller than that with exhaust-only ventilation.  Thus, the trade-off required for R-5 foundation wall insulation in a Minneapolis climate is small in terms of furnace AFUE increase at nominal operating infiltration and ventilation rates.

Figures 6.1 and 6.2 also show that in all cases an ERV may be used as a trade-off rather than a furnace AFUE increase.  In fact, at a given prescribed ventilation rate, the energy saving afforded with the ERV far exceed those required by the R-5 reduction in foundation insulation.

Lastly, the impact of reducing the foundation wall insulation from R-10 to R-5 on the basement temperatures is shown in figures 6.3 and 6.4 for the living and utility rooms respectively.  The ventilation rate for these simulations is set at 80 cfm which is a little higher than the prescribed value of 70 cfm for the MN-CALC house.

Figure 6.3

Note again that  the temperatures plotted are the diurnal averages that include thermostat setbacks.  R-5 insulation does produce a detectable decrease in average temperature that reaches a peak of about 1.5-2 °C at mid-winter in the living room.  The maximum decrease observable in the utility room is slightly larger at about 2-2.5 °C. 

Figure 6.4

Thus the larger thermal impact of reduced foundation insulation is in decreased occupant comfort rather than in increased whole house energy consumption.  If the calculations were run under conditions in which the HVAC was rebalanced to eliminate the average temperature decrease for R-5 insulation, then the whole house energy consumption would increase slightly and so require a slightly lower furnace AFUE trade-off.  In practice, HVAC installations are balanced for the installed foundation wall insulation, and hence the trade-off simulations for the code recommendations reported in section 5.2 below are based on the HVAC system being balanced for each insulation R-value.

6.2    Full Basement Foundation Wall System Insulation Thermal Resistance Trade-Offs

1. Calculated as the above-grade area of the foundation wall system divided by the total area of the foundation wall system above the interior surface of the foundation floor system.
2. Corresponds to the entire foundation wall system being within the energy trade-off zone for the MN-CALC house that has an attached 2-car garage.
3. Corresponds to the portion of the foundation wall system beneath the attached 2-car garage plus 50% of the balance of the foundation wall system being below grade.
4. Corresponds to the entire foundation wall system being above grade with the exception of the portion beneath the attached 2-car garage.
5. Minimum baseline furnace AFUE.
6. HRV/ERV design sensible effectiveness = 82%.
7. In these cases, there is no potential for trade-offs elsewhere.

Table 6.1 shows that the R-5 decrease in foundation wall system exterior insulation produces increasing furnace AFUE's as the above grade exposure increases with an overall maximum of 88.4% in St. Cloud.  This is still well below the 92% AFUE's routinely possible with the current generation of gas-fired furnaces.  As stated in the notes, at this level of foundation exposure, the entire basement wall would be above-grade, and hence would require a frost-footing in order to meet structural requirements.  Nevertheless, the data are included for completeness.  Typical basements with normal above-grade exposure would fall in the 10 to 47% bracket requiring a trade-off furnace AFUE of 84% with R-5 exterior insulation that is very reasonable with available equipment.  With R-10 insulation in the same exposure bracket, an 80% AFUE furnace is adequate in all cases which is more or less the AFUE of standard furnace equipment currently installed.

The AFUE trade-off required with ERV or HRV ventilation instead of exhaust-only ventilation are shown in table 6.2.  In this case, as alluded to above, with the exception of R-5 exterior insulation in the 47 to 87% above-grade bracket, the required AFUE is actually less than the 78% minimum, and even in the high above-grade exposure bracket, the AFUE is no more than 79%.  Thus, using an ERV or HRV will produce significantly more whole-house energy savings than those required to offset the use of R-5 exterior full basement foundation insulation and these savings offer the potential of allowing trade-offs elsewhere in the foundation envelope.

6.3    Stem Wall Slab-on-Grade Foundation System Insulation Thermal Resistance Trade-Offs

1. Minimum baseline furnace AFUE.
2. HRV/ERV design sensible effectiveness = 82%.

The highest furnace AFUE required of 79.5% occurs in St. Cloud with exhaust only ventilation that is easily met using commonly installed 80% AFUE furnaces.  The AFUE with an ERV/HRV decreases to less than 69% in all cases, illustrating the considerable energy savings possible using such equipment at the prescribed ventilation rate.

6.4    Unvented Crawl Space Foundation Wall System Insulation Thermal Resistance Trade-Offs

The furnace AFUE trade-offs for unvented crawl space foundation wall systems shown in table 6.4 does not have the complexities of the above foundation configurations.  In this case, the required continuous ventilation rate also is 50 cfm (100 cfm total) and the AFUE's with both exhaust-only and ERV/HRV ventilation are reported.

1. Minimum baseline furnace AFUE.
2. HRV/ERV design sensible effectiveness = 82%.

With R-5 exterior insulation, once again, an 80% AFUE furnace will suffice with exhaust only ventilation, while ERV/HRV ventilation offers considerable additional energy savings as a trade-off strategy.

6.5    Closure

The simulation results confirm that furnace AFUE is a viable and practical trade-off mechanism for allowing a decreased exterior foundation insulation level of R-5 compared with the optimum value of R-10 determined in the analysis of chapter 4.  Using an ERV/HRV as alternative to a furnace AFUE trade-off offers the opportunity for very significant whole house energy savings.  These results also serve define what should be the current standard of optimum residential energy conservation in Minnesota that is practically achievable with existing technology, namely:

• continuous R-10 foundation insulation without any thermal breaks
• a furnace AFUE of 92%
• ERV/HRV ventilation.

With all these strategies in place, the overall energy conservation savings would be significantly larger than those estimated for the MN-CALC house.