The Home Envelope: Does It Really Matter In A Heat Loss Calculation?


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We know that the building envelope is important, but how much does infiltration really matter in a heat load calculation?

I recently had the pleasure of talking to a group of HVAC professionals at the ACCA 2014 Building Performance Forum about the importance of understanding “what is going on” with the building envelope (or enclosure as some say) and why measuring infiltration is so important when sizing a system for replacement. My industry colleague, Joe Medosch, and I used real world data from an actual Wrightsoft Right Draw and J project. We were able to show how the different elements of the building envelope (including infiltration), affects the heating load, sensible cooling load, and latent cooling load of the calculation.

Over the course of my involvement in the Home Performance industry for the past five years or so, I have long since valued the importance of the building envelope and the impact of infiltration on the true heating load. However, seeing the impact of different inputs in black and white was an eye opening experience even for me. We took what we assumed to be a significantly leaky home and observed the heating and cooling load results of changing the values of several input fields (infiltration and attic insulation). The results of our calculations will lead to the actual sizing of replacement HVAC equipment. The size of this equipment will affect the efficiency of this home and effectiveness of this system for its entire life.

The real world example we used was a four bedroom home built in 1981, 2 stories above grade partially over a conditioned basement, partially over a vented crawlspace. The attic had an R-7 insulating value, the walls had R-13 batt, the basement foundation walls were uninsulated and the crawlspace had R-19 batt in the crawlspace ceiling cavities for the most part, (although sagging and missing insulation in some spots made the crawlspace insulation’s effective R value less than its installed R-value). The air infiltration measured using a blower door with the house depressurized to -50 Pascals was 6,490cfm.

In wrightsoft’s software there are three methods to define the amount of air infiltration in the heating and cooling load calculations – 1) simplified, 2) detailed and 3) blower door measurement. The best choice here may seem obvious but we often see technicians and designers who are still unsure if the choice really matters. The simplified method (most common) allows you to rate the infiltration as loose, semi-loose, average, semi-tight, or tight. These ratings are defined in the ACCA Manual J 8th addition manual. Using the detailed method you “list each of the building components, its construction quality (using the tight –loose rating), and the area or count (depending on the component). The leakage factor will be looked up and used to calculate the leakage area. The leakage areas are then added together and used to complete the infiltration calculations.” The blower door method allows you to enter the actual infiltration of the home as measured by a blower door. You are able to using a single point test (most common), or a more accurate multi-point test. The infiltration is entered in cfm (cubic feet/min) at a chosen depressurization. The most commonly used value is cfm50 or cubic feet per minute with the building depressurized to -50 Pascals.

Using our real world example, we ran the load calculations using first the simplified method (rated as “loose”) and then the blower door single point test method (using the measured infiltration of 6,490 cfm50). Using the simplified method our heating load was 60,247 Btuh sensible cooling load was 28,914 Btuh and latent cooling load was 6,049 Btuh (2.9 Tons total cooling). Distributed proportionally through each building element that affects the heating and cooling load, the assumed infiltration (“loose”) accounted for 36% of the total load. The measured blower door infiltration calculation told quite a different story. The heating load was 84,879 Btuh, the sensible cooling load was 32,952 Btuh, and the latent cooling load was 11,805 Btuh (3.7 tons of total cooling). This is a heating load difference of 24,605 Btuh and ¾ ton cooling load. The infiltration in this home accounts for 64% of the total load when measured with a blower door! This full furnace size and AC size difference from making a guess to doing a test! Notice on the cooling load, roughly ½ ton of the difference was in the latent load. This is the part of the load that accounts for moisture that is introduced into the structure (carried by air). The discrepancy between using “simplified” input vs. measured blower door reading, could lead (in this and many other cases) to a grossly undersized (or oversized!) AC unit as latent capability is a major consideration when selecting a properly sized piece of equipment.

The results of this example should make one very leery of using anything but a measured infiltration value when using Manual J for sizing a replacement piece of equipment.

To further show the impact and importance of infiltration, let’s look at what would happen to the load if we change just one factor by increasing the attic insulation from R-7 to R-50 and compare that result to instead reducing the infiltration by 40%. When we upgrade the attic insulation to R-50, we reduce the heating load by 7,167 btuh, the sensible cooling load by 6,592 btuh, and the latent cooling load by 350 btuh (.57-ton cooling load reduction). However, when we reduce the air infiltration by 40%, taking the blower door value from 6,490cfm50 to 3,894cfm50 we see a heating load reduction of 16,304 btuh, a sensible cooling load reduction of 2254 btuh, and a latent cooling load reduction of 3627 btuh(.49 total cooling load reduction).

Attic insulation is often touted as the best medicine to reduce the heating and cooling load, but as we can clearly see from this exercise, infiltration reduction (also known as air sealing) can have just as much (if not more) impact as insulation alone. Another thing that becomes clear here is that while insulation alone does little or nothing to reduce the latent load, the air infiltration reduction provides roughly the same cooling reduction but greatly reduces the latent load. This is important in the comfort, health, and durability of the home. Moisture can lead to building durability issue as well humidity issues, which is one of the leading causes of allergy and comfort issues in a home.

Insulation is a crucial component of the building envelope and knowing the exact values is essential to having an accurate heat load calculation. As we saw, infiltration can affect the envelope and load just as much as insulation can. Infiltration can and will proportionally affect every component of the building envelope and heat load calculation. It is equally important to know the exact infiltration value. This can only be achieved by using a blower door to measure the infiltration.

Home performance means to understand the building and it is systems as a whole, then create a road map to make a home healthy, comfortable, and efficient. When performing a heat load calculation we must not only look at all the elements of the building enclosure to design a correctly sized heating, cooling and distribution system for a home, but also understand how the elements interface with one another to affect the ultimate comfort, efficiency, and durability of the home.

Jeremy Begley

Posted In: Building Performance, Residential Buildings

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