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Passive Solar Windows - High or Low SHGC?
Last Post 26 Nov 2009 12:09 AM by ecobuilder. 17 Replies.
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mapnerd
New Member
Posts:24
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| 03 Nov 2009 01:26 AM |
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What is more important to a passive solar home design - high SHGC windows to allow more solar gain to be absorbed by thermal mass or low SHGC to reduce heat loss? This question is for a cold climate. If we went with high SHGC windows, could we add insulating shades/quilts to reduce heat loss effectively?
Thanks
Mike S. |
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ANGELofDEBT
New Member
Posts:34
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| 03 Nov 2009 07:27 AM |
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for passive solar you want a high SHGC. Curtains or blinds can be used to insulate against heat loss during the evenings |
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BlackHatch
New Member
Posts:30
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| 03 Nov 2009 07:13 PM |
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Good reading.
http://www.wbdg.org/resources/psheating.php
Also do a search on Lawrence Berkeley labs SHGC study.
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vb
New Member
Posts:47
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| 03 Nov 2009 09:35 PM |
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I am still learning myself but my take is high solar heat gain on south facing glass only, (in a properly designed home). And design your overhangs correctly. |
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eric anderson
Basic Member
Posts:103
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| 04 Nov 2009 07:53 AM |
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As a general rule,
you want High solar gain on the south side of the house and low solar gain on the East and west sides to prevent over heating in the summer (unless foliage takes care of direct sunlight from east/west in the summer.
It is possible to get fairly high solar gain in a low E window. It all depends on what the coatings on the windows are and what surface of the windows they are on.
The difference between High solar gain and low solar gain windows can be 3:1 in how much energy gain you get from the windows. .
Also remember when you are calculating solar gains that the GLASS area is only 2/3 -3/4 of the total window so actual gains will need to be calculated from glass area not window area.
Look at Cardinal LoĒ-179™ Glass This is what I have in the south side windows of my house<!-- InstanceEndEditable --><!-- InstanceBeginEditable name="PAGE CONTENT" --><!--Advanced Solar Control Glass:
SHGC 0.25 / UFACTOR 0.26 / TDW 0.35 / LIGHT TRANS 0.40 / All STATS -->
SHGC: 0.70 / U-FACTOR: 0.28 / TDW: 0.61 / LIGHT TRANS: 79% / All stats When the weather turns frigid, Cardinal LoĒ-179™ glass (pronounced low E 179) is the perfect cold remedy. It provides excellent insulating capability by blocking heat loss to the outside and reflecting heat back into the room. What's more, it's the ideal glass for passive solar applications because it allows the winter sun's heat to pass into the home. Regardless of where your home is located, choosing windows that provide you with the highest level of comfort and energy savings year-round is extremely important. And choosing the right glass for your windows is the most important factor in the decision. Go beyond ordinary low-e glass - choose Cardinal LoĒ-179, the ideal choice for passive solar designs.
LoĒ-179 makes it cozy inside
During cold weather, the insulating effect of your windows has a direct impact on how your rooms feel. Typically, 75% of the exposed surface of a window is glass, and the temperature of the room-side of the glass directly affects the air temperature in the room. Therefore, the better insulated the window glass, the warmer your room will be.
Inside Glass and Outside Temperatures
Cardinal LoĒ-179 delivers excellent cold weather performance - its insulation value (U-factor) is a low 0.28, and it lets the winter sun's heat pass into the home. Also, it blocks 76% of the sun's harmful UV rays. While blocking out the cold and UV rays, it lets the daylight stream in - more light than ordinary low-e glass.
Visual Light Transmittance
| Single-pane, clear |
90% |
| Double-pane, clear |
81% |
| Ordinary low-e |
75% |
| LoĒ-179 |
79% | |
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BlackHatch
New Member
Posts:30
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| 04 Nov 2009 02:57 PM |
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It should be noted that using high SHGC windows without having the thermal mass on the inside of the home to absorb the energy is a waste of time. Unless the home, and materials used, are designed with passive solar in mind, don't use the high SHGC windows. |
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mapnerd
New Member
Posts:24
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| 06 Nov 2009 01:00 AM |
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We are planning for concrete floors in the front portion of the house, a central masonry fireplace/chimney, and a mass wall in two of the rooms.
Thanks for the website link above. That is a good Passive 101 site. |
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mapnerd
New Member
Posts:24
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| 06 Nov 2009 01:04 AM |
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Thanks for the feedback everyone. We're committed to passive solar on this project. Here are some of our draft plans and a 3D model - NE Passive Solar Home
Please let me know what you think. Disclaimer - we still have a lot of work to do evaluating each window, the thermal mass elements, the eaves, the roof slope, etc. I've found Google SketchUp and its solar shading capability to be incredibly valuable for helping us see the effects of our design features on the shading/solar gain for our site. Try it, if you haven't. |
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eric anderson
Basic Member
Posts:103
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| 06 Nov 2009 08:30 AM |
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Quick thoughts,
this is looking much better. I have a few small suggestions. 
1. Near the masonary heater, put a large built in spot for holding firewood. you have the chimney suports right there so you can handle the weight. consider a wider then normal door for the door near the masonary heater and make it a tough door becasue it will be easy to smack it carying wood into the house.
I am liking the more centralized plumbing and the utility room location.
My one concern with this setup is the Pantry is directly behind the masonary heater and could overheat. At least insulate this wall if you store food in it.
On the basement windows as far as egress goes, I don't think they will be wide enough. It looks like you have them as two 2 foot wide casements. When you subtract out the frame width it is too narrow to be legal. I think you could substitute a single casement that is 36" wide instead.
On the roof it looks like you are going for large beams. Are you planning on spanning the whole width or do you have an intermedate load bearing point? You might consider using a truss so you have more depth for insulation, say 24". then you can blow in 18" of cellulose and have a very well insulated ceiling with out too much cost.
The last point I see is on the east side of the south wall the glass goes almost up to the roof plane. There is not much room above it for support for the roof. It looks like a 16 ft span in a load bearing wall. How do you plan to support the roof loads? I would want an engineer to make sure that you have that in place. something like a triple 14" LVL would be needed.
http://www.lpcorp.com/Literature/lp_lvl_2650_professional_brochure_tech_guide.pdf
Look at page 5 span tables
You could split this into 2 8' walls of windows with a 1 ft load bearing column in between them.
Amazing how much time it takes to design something and get it right. Makes you realize why Archies charge whtat they charge. Keep up the good work
Eric |
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vb
New Member
Posts:47
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| 06 Nov 2009 09:05 AM |
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Check out this glass- Sungate PPG 500. I think this is what I am going to use and the numbers are similar to the Cardinal glass mentioned earlier. This glass may or may not be easier for you to get. I had to help educate the suppliers I got bids from. A few just do not understand the product that they sell. |
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Dteltech
New Member
Posts:22
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| 09 Nov 2009 05:10 PM |
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The 79% SHGC with a U value of .28 is a passive solar designers dream. Cardinal must really understand the glazing needs for passive design. To control the energy that you are letting into the structure, be sure to provide adequate shading and thermal mass. Natural control of the heat that is created, by trapping sunlight behind glass, is a function of your climate and the amount of space that you are trying to heat. I have found that isolated gain systems allow the main living areas to steady comfort levels, while the passive solar area are allowed to fluctuate between 60 and 80 degrees. These isolated gain rooms make great excercise rooms when cool, a nice place to relax in when you come in from shoveling the snow and a healthy environment for pets and plants. They can also provide a great deal of the heat required by the rest of the building. There are some examples of this type of design at www.mysunspace.com.<!--[if !mso]>
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| Here comes the sun |
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dmaceld
Advanced Member
Posts:860
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| 16 Nov 2009 12:13 AM |
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Your 2448 windows in the bedrooms won't pass code. As suggested use either a single casement 32 or more wide, or a double casement 60" wide. The 30" sashes in a 60" double will exceed the code requirement by only 1 or 2" depending on the hinging arrangement. IIRC, the clear passage area has to be a minimum 21" wide by 30" high. The hinging arrangement will affect the clear open area.
A suggestion based on what I learned building my house. There ain't no such thing as standard window rough opening sizes! Choose your windows and then build the rough opening to fit that window. Prebuild the rough opening and then find a window to fit will give you headaches like you've never had before!
I don't know what the passive house guru's say, but based on my experience with my ICF house don't shortchange the window shading issue. I have good shade in the summer, and pretty good sunlight in the winter, but the fall and spring are a different matter. In those two seasons we have enough solar heat gain coupled with warm outdoor temperatures that our AC load is just as heavy, or heavier, than in the summer. |
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| Building house - what a way to spend retirement! It's done! We're living in it! |
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egouin
Basic Member
Posts:124
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| 17 Nov 2009 07:56 AM |
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Mapnerd - the best advice anyone can give you is... Do your homework! Verify everything to separate personal opinion from true fact. There are some statements above that would benefit from this treatment.
I found this tool beneficial when building my house: http://www.susdesign.com/overhang/. You may find it useful as well.
I used Alpen (now Serious Windows) for my house. http://www.seriouswindows.com/html/alpen-is-serious-glass.html. They can make any window size and shape you desire - just make sure you get the measurements right, or it will be an expensive mistake. Alpen was instrumental in helping me select the right glass and selective coating for each each side of my house. Basically, i told them what i wanted to do, and they helped me select the right product.
I personally found windows the most technically challenging aspect of my house. They are difficult to cost justify. The upcharge to go from "run of the mill" to "super" windows can be steep. With this, you go from ~U.31-.33 for run of the mill to ~U.15-.20 for super windows. Convert these numbers to R values (1/x) and you will see that you pay quite a bit more for a little better insulating value. in my area (Massachusetts), high SHGC windows are NOT allowed by code (they must be low-E). Be sure to check your local code. You may either need to work with the local building inspector, or conveniently hide the rating stickers for the south facing windows.
You also need to consider your location on the earth and the average incident solar radiation that your windows will receive. Once you have this and the U and SHGC values for your windows, you can calculate roughly how many BTUs you are going to get from the windows. In my experience, it is difficult to find a window that will provide a net gain in BTUs during the winter months. Without going into the "super window" category, you will not find a window that gains more BTUs than it loses (when it is cold outside). You might consider/plan on using insulated window treatment (i.e. triple cell blinds). These can also get expensive, but when compared to the cost of increasing a windows insulating value, they are a bargain. I have seen some with R values of ~5.
One other thing to consider is - when - you are going to get BTUs coming in from the South facing glass. It is entirely possible to overheat during the day in the dead of winter. Be sure to plan your overhangs carefully so that you only receive added BTUs when you need/want them.
Now that I have muddied the water, I can tell you that my house is working as planned. On a sunny winter day, the outside temperature can be in the single digits while the inside is a very comfortable 71-72F without the heating system coming on. We have hardwood floors, but took no other action to specifically provide thermal mass. Our exterior walls are ~R47, so whatever heat comes in during the day - stays in for quite a while. Now, if I could just get the family to use the "air lock" mud room I designed into the house rather than the front door or slider, I'd be all set. I quickly learned that the family living in the house can have a huge impact on building performance.
My site is no longer up-to-date, but has the essentials that went into planning and building my house. Feel free to have a look. http://www.gouingreen.com.
Good luck to you,
Ed
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http://www.GouinGreen.com
Superinsulated SIP/Modular House (HERS = 30)
GSHP w/SCW, ERV, Passive Solar, Solar HW |
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eric anderson
Basic Member
Posts:103
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| 17 Nov 2009 01:34 PM |
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ED,
Low E and High solar Gain are 2 different but related things. It is possible to combine a window with High solar gain and that has a low E coating. It just depends which face of which window to goes on. You can also get triplepane, high solar gain windows. They are expensive.
If Mass prohibits high solar gain windows, you may be stuck.
Eric |
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ecobuilder
New Member
Posts:95
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| 22 Nov 2009 12:49 AM |
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How about thermal mass in your window? I have been working with a guy that has invented a thermal mass window. We installed a previous prototype on a house I built in MA earlier this year. I have posed a bunch of videos some explaining how they work and
some showing performance testing through last winter. http://www.youtube.com/user/eebuilder These windows 150sq/ft contained about 3,000 pounds of water. The windows even on a cold winters day have reached temperature over 100F.
The newest version,, used for the team Boston solar decathlon project this fall , contains a gel substance rather than water used for the previous version. The new ones are 2'x7' and contain 1" of thermal mass to store the suns energy along with a triple exertior glazing with a SHGC of .71 and a U-19. The total window has 5 panes of glass and weighs over 250lbs. The windows have been tested for over a year now and they are getting ready for production. They also integrated a motorized exterior shade into the window system to give you excellent control over the unwanted energy. After collecting data for the last year he has been able to calculate how much energy the window is capable of storing, about 100,000 Btu's/Annually/sqft. The window arperture is 13sqft making this window capable of contributing 1.3 Mbtu's a year to your homes heating needs. Another way to look at it is that this window will provide the equivlant of 13 gallons of home heating oil, for free year after year. He just gave me the green light to find people interested inusing it. The company developing this is Hunter Douglas, primarily know for their shades. The inventor is the guy that invented the honeycomb insulated shade back in the 70's. Right now the cost is about $1000 per window including the shades about $75 sqft for not just a window but a heating system as well.
Tom Pittsley
ecobuilder@aol.com
www.eebt.org |
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| "Don't be afraid to go out on a limb. That's where the fruit is." Jackson Brown |
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egouin
Basic Member
Posts:124
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| 22 Nov 2009 07:38 AM |
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Tom - I definitely would have considered these windows if they had they been available last year. Even at $1000 each, I can see the cost/benefit would be more favorable than any other window I have ever found. In fact, with 238 sqft of south facing glass in my house, I expect we would have issues with overheating without a clever mechanism for controlling it. The current windows already heat the 3800sqft house house to 71-72F on a sunny winter day (seemingly regardless of outside temperature).
My first thought is the use of a ground source heat pump and a large water storage tank. Essentially, running the heat pump in air conditioning mode to capture the excess heat (during sunny days) and store it in the storage tank. It can later be extracted by the same pump when needed. This could also be used to heat domestic hot water.
Definitely Interesting.
Ed
www.GouinRreen.com
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http://www.GouinGreen.com
Superinsulated SIP/Modular House (HERS = 30)
GSHP w/SCW, ERV, Passive Solar, Solar HW |
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slenzen
Basic Member
Posts:289
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| 23 Nov 2009 07:47 PM |
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How do the water filled windows prevent heat from escaping and siphoning out from inside to outside? Is there any danger of freezing in an extended cold and possibly cloudy climate like Minnesota? |
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ecobuilder
New Member
Posts:95
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| 26 Nov 2009 12:09 AM |
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The windows work best at tempering the suns energy and then dispersing it when needed. The space directly in front of the windows does get warm during the day but the windows only reach their peak temps at the end of the day (sunset). Then emmit the heat for an extended period during the night. The newest version of the windows have a shading mechanism as part of the window to control the overheating issues that happen in most solar homes. Passive solar works great but controlling it has always been the issue.
The new windows don't have water but a gel substance for mass reducing the risk of freezing and the issues with long term storage of water. I had the water filled windows in this house in MA last winter and the heat was not installed unitl mid Feb. We had one cold spell during Jan. where for a week straight the outside temps never rose above 20F and night time lows well below zero. The coldest the water in the windows got was 48F and never even came close to freezing. As for how they prevent heat from escaping? They do loose some, but not much. The thermal mass portion of the window is not in direct contact with the outer glazing. For the windows we used the exterior glazing was a double pane specialized glass with a U-.29 and a SHGC of .72 then a 2" gap and then the water blocks. The newest version contains triple exterior glazing, a 1" gap and the a 1" thick thermal pane, containing two layers of glass with a gel infill. Additional mass can be added by placing more 1" thick layers mass up to 3". The net result of more layers is more storage for energy and the heat being retained for greater periods. But the mass is the expensive part of the window, Each 1" of mass cost the manufacturer $60/sqft or $840 per window. At their current cost of $1,000 per 2'x7' unit the company is loosing money but is willing to except this to get the product out to the public and build momentum. Once they get the product moving one of two things will happen, the cost of the materials will come down or the cost of the window will go up. If anyone is serious about seeing this product and putting in their home contact me and I will arrange for a meeting with the inventor. At this time he will not allow me to take pictures of the windows and put them out to the public, but what I can tell you is that the window looks like a floor to ceiling fixed casement window 2'x7' and tinted to capture the solar energy. As soon as he gives me the word I will post some pictures.
Tom Pittsley
ecobuilder@aol.com
www.eebt.org |
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