NC climate. New Build. Compromise with SHGC for rebate?
My name is David. I have finally gotten permits for a new build in Raleigh.
The house design has windows on all sides with a back oriented to the West. The large glazing is to the South with a L shaped design with garage door to the rear. Each southern window has overhangs designed to block summer sun. My count is about 15 South, 8 North, 8 East (5 foot porch shades 3) and 3 West.
In NC, we have a code + program called HERO. Duke energy gives a rebate on new construction based on avoided kwh but you must meet the HERO code. The code looks pretty close to Energy Star. The required SHGC is .25 or less. The lowest cost window option that I am looking at (Anderson 100 – all casement/fixed) has a SHGC of .28. I am waiting for the quote with better SHGC but as everyone knows, window pricing is a pain to get.
If I skipped HERO code, I would still get the best East and West windows. But, in my mind I am only paying more for Southern windows to satisfy the code and I am losing heat gain in the winter. The rebate is between $2000 and $4500 with the higher amount with solar hot water. I have not decided about the hot water but $2500 from the utility would be a factor. Of note, $2000 of that could be captured with a heat pump hot water heater. I do feel like solar has a significantly greater longevity.
My goals are environmental for sure but finances are not ignored. I have a PV quote in hand for $15k net for a 10 kw system. Otherwise I only pay $.10 a kwh and therms are $1. Current wall plan is 2×6 with cellulose. Slab. Foamed roof line. Minisplit and conventional heat pump.
So the first big question (of many) – Is there any tangible benefit from low SHGC windows in my North and well covered South windows?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Even though you have the overhang to block the sun in the summer, that will not be effective for most of the day. In addition, sunlight reflected from the ground and other surfaces will also illuminate/heat your south side in the summer, even at noon.
So, though you will get heat gain in the winter from your south facing windows, you will get significant heat gain in the summer, which will be a major issue in Raleigh. I suggest you have a Manual J run to see how different SHGC windows affect your plan.
I suspect it will show you are better off following Duke's recommendation and taking the rebate.
Appreciate the response. I too am concerned about the reflection but I also know that the one size fits all approach to SHGC is counterproductive. So take these factors into account also.
We spend 3 times the energy on heating here vs a/c. On average.
A PV array will completely cover the electricity use for 9 months out of the year. Hence a winter kwh is worse than a summer kwh. Given our large solar installation here in NC, this will be the reality for the foreseeable future.
I have seen the shade diagrams and I don't agree that the shading will "not be effective for most of the day". Certainly in June, there is 100% shading for almost the entire day.
Manual J's are of course expensive but that is a good idea. My issue is that the Manual J is not really a good model for calculating paybacks of various options.
It isn't Duke's recs - it is state code. Anyone want to argue that code is perfect?
Lastly - reflected light is an issue. But given that we are talking .28 vs .19 SHGC and not 1 or .5, how much is that reflected light going to matter?
I certainly know how people further north complain about the SHGC issue on Southern windows and the issues here are similar.
David, we can throw opinions around, or you can pay for an energy model (or learn to do one yourself) that will give you reasonably accurate data to use for decision making. Especially if you want payback options (return on investment is a more fair way to look at it) you might look into BeOpt, a free download from the DOE.
Your building geometry is not clear from your description. Solar shading works as planned at noon, but at other times of day the sun is closer to the horizon, so you may or may not be as well-shaded as you think, at times other than the summer solstice. A SHGC difference of 0.09 will make a difference but if you're going for code-minimum construction it probably won't be too noticeable. You don't mention U-factor anywhere, that is just as important as SHGC; sometimes U-factor changes with different SHGCs, so that needs to be factored into the equation as well.
Sorry the u factor is the same for all the options. .28 or so depending on grids or not. The options to reduce SHGC do not change the u factor.
Does the geometry matter or just the number of windows in each orientation? It is an L shaped house to maximize southern exposure on a lot where the street is to the East and the Backyard is to the West.
A Southern window with a significant overhang blocks the sun for much more than just noon according to the shading software I looked at. I have looked at the June-September numbers and see how they vary. What I have seen was over 8 hours in June. We typically have 4 months of a/c.
I have an energy rater and unfortunately every question takes a week to get an answer. They use Beopt I believe. And despite having a rater, it seems like a second opinion isn't a bad idea.
Suggest you look at this site (if you haven't already):
Sustainable By Design by Christopher Gronbeck. Seattle, Washington.
http://www.susdesign.com/windowheatgain/index.php
The window shading tool is useful, as well as the solar gain numbers for various glazing SHGC per each month for your location.
The critical month for unwanted summertime heat gains is likely August, maybe September for your climate. I'm not very familiar with your climate so don't have specific suggestions, other than there's plenty of ways to shade windows in addition to your overhang. Solar screens, top-down-bottom up insulating cellular shades, ....
Robert, thank you. That is the site I used for overhang planning. I do plan an external motorized shade on one bank which is the bulk of west windows. There is a 100 year old elm that will help considerably.
I hadn't used all the other tools and they were very helpful.
What was interesting to me is that a South window without overhang at all in Raleigh would appear to favor a high SHGC window. The December and January combined exceed the entire a/c season based on kwh of heat. Add in shading and that effect is magnified significantly - like by an order of magnitude. And yes- September is our worst solar gain month - but not our highest electric bill at all as the nights cool enough. Perhaps in heavy southern glazing house, it will be the highest electric bill.
Putting the shading and heat gain calcs together, I get roughly 11 kwh/m2 for all summer and the gain in the winter is more like 200 kwh/m2. By these numbers, surely I should have clear glass, let alone pay more for low SHGC glass. I know code will not allow clear glass even if I could find it.
Am I missing something?
David,
I wish more people would do the calculations for solar heat gain as you have.
Missing something? All I can think of is...
I assume that you have determined that the south-facing windows will get full sun from 9 or 10AM to 2 or 3PM on Dec 21st, or have made adjustments to your solar gain calculations to reflect any shading from other buildings, trees, etc.
Might be difficult to get higher SHGC windows in your location.
If windows on various walls are all the same size, if you get higher SHGC windows for the south side, be sure they are installed on that south wall, not mistakenly installed on other walls.
If you decide to skip the HERO rebate, you might reconsider an active solar hot water system, and use PV instead. Maybe some PV directly heating the hot water, as discussed elsewhere on GBA.
You could compare the rebate to utility bills, and also take into account your preference about daylighting. Lower SHGC glass also has lower VT.