Should I bother with passive solar (Cardinal 180 windows)?
Initially, it seemed like a “no brainer” to go with Cardinal 180 glass on the south side of my home.
I don’t have excessive glazing on the south side of my house (240 sqft of glazing on the south for a 3700 sqft house)
Hoever, Joseph Lstiburek has scared me off from this with his statement about overheating (below).
“Don’t bother with the passive solar. Your house will overheat in the winter. Yes, you heard that right. Even in Chicago. Are you listening Passive House? You should go with very, very low SHGC’s, around 0.2, in your glazing. If this sounds familiar to those of you who are as old as me it should. We were here in the late 1970’s when “mass and glass” took on “super-insulated”. Super-insulated won. And super-insulated won with lousy windows compared to what we have today. What are you folks thinking? Today’s “ultra-efficient” crushes the old “super-insulated” and you want to collect solar energy? Leave that to the PV.” — Joe L
Here are my total glazing #’s:
North 103
East 74
South 240
West 32
Heating Degree Days: 7970
Cooling Degree Days: 798
RESFEN says my savings over a 10-year period are $1,850, not a small amount. But, will I be roasting hot?
There is no obstruction, I have a clear view directly to the south.
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Replies
I too am not worried about summer overheating - but Joe is mentioning winter overheating, even in Chicago! That's what has me rethinking high SHGC windows.
Derek,
First of all: excellent question.
I remain a believer in passive solar design principles. However, our understanding of these principles has changed a lot since the 1980s.
Here is Principle #1: While it still makes sense to put more windows on the south side of your house than the north side -- assuming you are building in a cold climate -- the area of the south-facing windows should simply meet your needs for aesthetics and view. Don't add extra glazing. The logic behind this principle is explained in this article: Study Shows That Expensive Windows Yield Meager Energy Returns.
Second: Joe is correct that many energy models do a poor job of predicting overheating due to excessive south-facing glazing. Whether this is a problem depends to a certain extent on your lifestyle. If someone is home during the day, you can open the windows on cool days in March and October when the sun overheats the house. If everyone is out of the house at a job, you may come home at 5:30 p.m. to an overheated house in March and October.
Some people who live in cold climates love overheating. It reminds them of the tropics. Others balance this load with air conditioning, which is wasteful.
Third: Charlie is right that awnings or exterior shading devices can be used to balance solar gain if you end up making a mistake.
Fourth: The wild card in this analysis is climate change. If the Vermont's climate in 2050 ends up like the climate in North Carolina in 2015, today's designers may be making mistakes.
Joe is exaggerating to make a point. Go with what RESFEN says.
Cooling is overrated. What makes summer uncomfortable is humidity. Solar heat gain doesn't cause humidity. And if you have regrets, you can add awnings or plant trees.
Edit: Oops: you quoted Joe as saying " Yes, you heard that right," except that I didn't!
At least the solar angles in VT will never be comparable to those in NC, eh? :-)
Operable exterior shades can be an effective mitigation tool for any errors of commission regarding solar gain.
Exterior shutters perform better as solar shade but are very difficult in my view to incorporate into a design with a broad expanse of southern windows that are side by side.. Barn doors on tracks aren't a viable option either, as they block adjoining windows when in open position.
I have found in previous homes that interior shades provide a dramatic reduction in solar heat gain. We followed the sun with shades or drapes, opening and closing as needed. Opened windows as soon as evening temp was lower than inside.
That said, we are building a 1,100 sq ft passive solar at 46 degree N in zone 6 on a lake. We intend to use double cell cellular shades mounted against a head piece with edges mounted in tracks to help reduce air leaks. Symphony Shades is an example http://symphonyshades.com/rvalue.html
Moreover, we will actively manage our space by using clerestory windows and "night flushes" to create a stack effect at night by opening lower windows and exhausting heat out clerestories whenever night temp is less than inside.
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Architectural sails are another alternative solar shade that like exterior shutters stop the sun before its heat gets inside the building envelope. http://tenshon.com Naturally take down in winter.
Meanwhile, the bonus and primary interest in shades, in our case, is to reduce heat loss during winter nights when Lake Wobegon freezes and framing in our homes groan and release cracking sounds in protest during polar-like temps.
Even though the all the women here are strong, all the men good looking, and all our children above average, we still don't like getting cold!
You could make yourself some indoor shutters. Use rigid insulation wrapped in your choice of material.
Derek, Regarding windows and solar gain:
If it helps, here is some experience from the Chicago area: (4400 ft2)
1) Our Degree Days are in the 6100-7300 range annually, using 65F as the base temp.
2) Our Heating DDs are in the 5600-6200 range annually, and cooling DD are say 500-1100.
3) We have late 1980s Andersen double pane casement windows. I don't know the shgc or the emissivity.
4) We've lived here for 15 years. Over that time we've reduced our kwhs from 13,500+ to 6400/yr.
5) The energy savings have come from many projects, including sealing and insulating and cfl bulbs but the windows are the original ones. We have no blower door data.
6) I don't recall a single day during heating season when we noticed the house was overheating from solar gain. During mild weather when neither the a/c nor the furnace run, we may overheat but we are able to adjust this with natural ventilation.
7) We use solar grates mounted exterior to the windows to reduce hot weather solar gain. We now use about 530-730 kwh/mo in summer, compared to 500-600 kwh/mo in mild weather. These numbers used to be 1600 kwh/mo and 1150 kwh/mo respectively. In our experience, external window attachments are more effective than internal ones (but there is one named "In'Flector" that may challenge that).
If I were building NEW today I'd rely more on a well sealed and insulated building and go with GBA's recommendation on windows, site orientation, overhangs, et al.
As far as our EXISTING house: I wouldn't replace the windows because they perform like a triple pane design in some cases and have a relatively high shgc in winter and a low shgc in summer. I just don't think I could justify the expense for new windows.
Derek - Your amount of glazing barely qualifies as "passive solar". Half of your glazing is on the south - good. The south glazing is only 6% of the floor area. That puts you on the low end of the range most passive solar home designers in cold climates use, 6-12%. Above 12% in a tight and well insulated house may give some over heating, and over cooling at night and cloudy times. If you like sunny, you could go with a little more south glazing. If that isn't so important, stay at 6%. Martin is right - go with what works for the house. The truth is that a 9-12" wall is cheaper by the square foot than a quality window, so it makes no sense to over-do it. You are not overdoing it with those numbers. Balcomb, a passive solar engineer from the 80's,used to use 10-12% with R-30 walls,with very good results. If you have a first floor slab, you can more easily go with more south glass.You will NOT be roasting hot.
To me, the biggest argument against getting Cardinal 180 glass for some of your windows is finding a window manufacturer who will provide what you want without a huge hassle and/or overcharging.
Who knows of a window supplier that doesn't overcharge for Cardinal 180 glass, and makes it easy to order?
The flip side of Kevin's argument: if you can afford to support the companies that offer 180 glass, I think it's better to support them and help move the market in that direction rather than to support the manufacturers who don't make it easily available, reinforcing their idea that they can run a profitable business without offering it.
My favorite window companies are Loewen (Canadian aluminum clad wood, expensive but triple-pane 180 glass is nearly standard for them) and Fiberframe, a US fiberglass company that seems to offer a similar product to the expensive Canadian fiberglass windows at a better price in the US. Ohio. Caveat: I haven't actually used Fiberframe windows or seen them installed.
Passive solar design is not just south facing windows. Properly designed overhangs and properly placed thermal mass are just as much a part of good design as the (SOUTH + or - 15 degrees) facing glass. The buildings total heat loss has always been a primary component of a properly engineered passive solar building. A passive solar building will have Interior Air Temperatures that fluctuate more than in other building designs and is the reason that I lean towards isolated gain systems. Natural heat flow can distribute warmed air IF openings are properly placed and the primary collecting space is cool in the AM and quite warm in the PM.
I have collected years of hourly energy performance data in passive solar buildings and have 35 years of experience living with passive solar building dynamics. For extreme northern - cold climates, when properly designed, it blows the doors off of 15% efficiency PV panels. I continue to use and specify Passive solar glass for south windows that are properly shaded. I continue to monitor my work and am currently installing Cardinal passive glass U=.24 shgc= .62 (south placement only) and low shgc low e windows on all other orientations. u=.23 shgc =.27 for a project in Northern Montana. As always, insulate where insulation belongs!!
Does your savings of $1850 take into account the expense of the additional SF of glazing you do not need and also the expense to frame and finish for the extra glazing?
Or you save $1850 as drawn no direct construction cost taken into account?