mapnerd
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| 11/03/2009 1:26 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
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| 11/03/2009 7:27 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
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| 11/03/2009 7:13 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
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| 11/03/2009 9:35 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
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| 11/04/2009 7:53 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
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
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| 11/04/2009 2:57 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
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| 11/06/2009 1:00 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
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| 11/06/2009 1:04 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
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| 11/06/2009 8:30 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
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Posts:27
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| 11/06/2009 9:05 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
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| 11/09/2009 5:10 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.
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They lacked the knowledge of houses - Aeschylus |
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dmaceld
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Posts:805
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| 11/16/2009 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
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| 11/17/2009 7:56 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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eric anderson
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| 11/17/2009 1:34 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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