Why does my model show negative savings for air sealing and insulation measures?
Written by Adam Stenftenagel, CEO of Snugg Home.
NOTE: This article dives into the specifics of why negative savings shows up. However, the quickest fix is probably to turn on Window Venting, so check out this article first. The other primary reason to see negative savings for Air Sealing is if you have added a new mechanical ventilation system that uses more energy than is saved from the air sealing itself.
As you probably know, there are 3 types of heat movement: conduction, convection, and radiation. When we air seal a house, we reduce the heat loss through convection, or air movement. By adding insulation to an attic or walls, etc, we are reducing the conductive losses. Radiation is the solar gain that we get through windows, where the sun actually adds heat to the inside of a home. Radiation can also come through opaque surfaces, but the rate is lower.
During a heating season, the insulation and air sealing do a great job of keeping heat from escaping to the much colder temperatures outside. And depending on the SHGC, shading, and orientation of our windows, we might get more heat through our windows in the form of radiant gains than we lose through the windows when the sun isn’t coming through them. In a typical cold climate, a heating season has a fairly high delta T (difference in temperature) between the inside of the house and the outside. Say the inside temp is 70 and the outside is 40… that’s a 30 degree difference.
Now switch to a hot climate. You might have an outside temperature of 90 degrees and an inside temp of 75 degrees. That’s only a 15 degree difference. Heat always moves towards cold, so the outside air is not working as hard to get into the home as it was trying to escape in the winter. The insulation and the air sealing definitely help to keep the outside heat from making it into the home, but those improvements are not nearly as effective in the cooling season as they are in the heating season. This is much more common in temperate climates, but the general point is that cooling delta T is smaller and most of the cooling load comes from solar gains.
Now let’s add the radiant gains into the picture. If you have a lot of glass that’s exposed to the sun, say, unshaded south or west facing glass, then you’re going to get a whole lot more heat through that glazing than you are getting through the insulation and air leaks. Basically, you are cooking the house through those windows. If we had less insulation and air sealing in that situation, you would allow more heat to escape to the outside when the temperatures flip the other direction (now 85 inside because the house was cooking, and 70 outside because the sun has set and the air outside is cooler than the inside). This means that the cooling system has to work even harder than it did before the extra insulation and air sealing.
So when you look at the metrics screen in Snugg Pro and you have only done air sealing and insulation, you could see a significant savings in therms (for the heating season), but you could likely have negative savings in kWh because the AC had to work harder. If your heating system was a heat pump, you probably would not notice a negative number of kWh at all since they cooling losses are offset by the heating gains.
Now, I’m not advocating that we do less air sealing or insulation because of this. Typically, actual dollar savings still exists because the therm savings outweighs the cooling savings. But in cooling dominated or mild climates, this might not always be the case.
ASHRAE Standard 140, better known in its application as the HERS BesTEST addresses this scenario. The test allows for a negative savings when comparing heating usage in an poorly insulated home to a well insulated home. The test site is in Las Vegas, where there is actually more conduction driven cooling load than in most other regions of the country. In regions with lower summer temperatures the effect discussed above could be even greater.
(Look at results row 26 L120-L100. L120 is the insulated home and L100 is the base home. The "range max" of 0.68 shows that slight negative savings are allowed in the range.)
So, what can you do about it? Well, the first thing you should consider is turning on Window Venting in the Window's section. Only do this if you know that the homeowners open their windows in the shoulder seasons when it's nice out. Here's an article about that. Next, make sure your model is correct in a few places. The biggest issue is to look at the solar gains through the windows. Is there a lot of glazing on the south and west sides of the home that are unshaded? Do the windows have a low-e coating or not? (You can hold up a lighter to a double pane window and look at the 4 distinct reflections of the flame. If one of the flames is a different color than the rest, then there is a low-e coating.) If the windows are exposed, then look into improvements like solar screens, low SHGC windows, or adding trees or other shading devices. Those kinds of improvements will typically flip the savings in the other direction. Also, if you install a super high efficiency cooling system, you could also see savings flip the other way.
Also check out this article on Window Venting: https://snuggpro.com/help/article/window-venting