Overheating from south windows

Submitted by Ted on July 22, 2012

Andrea and I started sleeping in the house two nights ago, so some things that I've been noticing about the heating profile of the house are starting to become clear. When we finally got the HRV running a few weeks ago, I was disappointed to notice that we don't have the kind of evenness of temperature between the upper and lower stories of the house that we were hoping for. My first panicked theory about this was that the HRV wasn't working. Panicked, because the tubes are all nice and snug behind walls now, so if we got it wrong, it's going to be a real pain to fix. The good news is that I don't think the problem is the HRV, although I'm contemplating one tweak to the vent layout which we could do without any wall surgery. When we talked with Peter Schneider about overheating, he reported his experience with some of the houses that Efficiency Vermont has designed up in Charlotte, Vermont. These all have lots of south-facing windows, and Peter hasn't seen problems with overheating in the summer. Based on Peter's reports, we were kind of hoping to get away without doing one of the features we've got on the plan: active shading on the south-facing windows. If you look at the drawing of the house at the top of the blog page, which is a view of the south face, you'll see that the lower windows have what looks like louvers on them. These are part of the solar shading plan—the idea is to install them in spring and keep them around until fall, and then take them off and stash them for the winter. These louvers will prevent high-angle light from making it through the window in the summer and heating the interior space. By installing them on the outside, we minimize heat gain inside—hopefully most of the heat that is generated when sunlight is absorbed by these shades will re-radiate into the outside air, rather than heating the inside of the house. Why is Peter seeing different results? I don't know, but I have a theory. I think the windows in the houses in Charlotte may not have the same solar heat gain coefficient as ours, and may reflect more high-incidence light, while letting low-incidence light in, so that they gain more heat in the winter than they do in the summer, even if the same amount of light is hitting them. The way our house is set up, we have a heat pump indoor unit on the wall downstairs, right in the center of the house. We have nothing upstairs. So the reason I was worried that the HRV wasn't working is that one possible interpretation of the data is that we're cooling the house adequately, but the HRV isn't doing its job in redistributing the heat evenly throughout the house. If that's the case, it's kind of a big problem. But having just spent the morning sitting down next to the windows with the heat pump off, I have a different theory. For most of the morning, it was nice and cool downstairs, but as more sun came in and the day went on, it started to get hot, just as it is upstairs in the afternoon. In other words, the heat coming in from the windows is heating the upstairs and downstairs evenly; the reason that it feels hotter upstairs is because the cooling effect of the heat pump completely counteracts the heating effect of the windows. I've confirmed this by turning the heat pump back on. So this gives me some real confidence that when we get around to building and installing the louvers, we will stop experiencing overheating in the south part of the house. I'm still a little tempted to add one more supply vent on the south side of the house upstairs, but that's something I'm hoping to have a chance to debate with the guys at Zehnder. If it needs to be done, it's a really easy fix, because I can do it up in the utility loft, which doesn't have a finished floor.