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geo vs. windows - who wins?
Last Post 05 Feb 2010 01:31 PM by toddm. 20 Replies.
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jerkylips
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Posts:203
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| 30 Jan 2010 11:35 PM |
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this may be a bit premature, but after investigating even more, I'm starting to see a interesting scenario possibly emerging..
We all operate within a budget, & I know I won't be able to do everything I want to do. Based on the size, tightness, & r-value of the house we're building, our heat loss should be fairly low. I would love to put in the Serious windows, but I'd also like to do geothermal. Recent numbers I'm getting are lower than expected - roughly $20,000 for the geothermal vs. $10,000 for a natural gas furnace. After the 30% credit, geo is down to about $14,000. So the question is, if the best windows come back $4000 more & I have to choose between the two, which makes more sense - keep the furnace, knowing that I'm going to have extremely low heat loss so the heating costs should be low to begin with, or pay for the system that will heat the house cheaper, knowing that I'm going to lose more of the heat I'm producing?
A riddle, wrapped in an enigma.... |
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toddm
Basic Member
Posts:252
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| 31 Jan 2010 07:51 AM |
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Dunno if you have found HEED, the UCLA energy modeling software, but this question is exactly why it exists. http://www.energy-design-tools.aud.ucla.edu/heed/ There is really no other way to get specific enough to come up with a realistic answer. If memory serves, the software doesn't have a geo option, but you could substitute air source heat pump and adjust the annual cost. (Cut it in half?) |
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Bruce Frey
Basic Member
Posts:338
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| 31 Jan 2010 11:22 AM |
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Also consider the exit (which may or may not important to you). Would you pay more for a house that has brand x or brand y windows? or, would you pay more for a house with geothermal? You will probably have a better chance recovering the geothermal premium on resale than recovering a window upgrade. It may also contribute to a qucker sale.
Let us kow what you decide.
Bruce |
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geodon
New Member
Posts:23
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| 01 Feb 2010 11:23 PM |
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Do the math, what are you going to save with the window upgrade in energy costs? the cost of energy is going to go up no doubt about it, I would put my money on the future costs. installing Low E glass with a minimal U factor of 32 is a good start or base, this is just what I have experienced while doing energy audits and Manual J calcs. Most homes are being built with this standard in our area and has proved to be a good choice to start with. be sure your builder is going to install your components in a wa to ensure that your homes performance is going to be the best it can. sometimes the simple things save you the most. Also air changes in home can kill any savings expectations, so building tight and control ventilation is a key factor. |
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Jesse Thompson
New Member
Posts:81
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| 02 Feb 2010 09:26 AM |
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Advantages of better windows beyond $ energy savings include:
Windows are harder to change out in the future than mechanical equipment.
High-R windows provide comfort improvements beyond energy savings including less condensation on the glass, warmer radiant surface temp, etc.
Better windows = less need for perimeter heating distribution (= lower first cost on mechanical systems).
Resale is a tough one since fashions change over time, right now Geo is on everyone's hot list, it used to be radiant, who knows what the next thing will be. |
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Jesse Thompson
Kaplan Thompson Architects
http://www.kaplanthompson.com/
Portland, ME
Beautiful, Sustainable, Attainable |
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jerkylips
Basic Member
Posts:203
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| 02 Feb 2010 10:12 AM |
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Posted By ground.loop@yahoo.com on 02/01/2010 11:23 PM
Do the math, what are you going to save with the window upgrade in energy costs? the cost of energy is going to go up no doubt about it, I would put my money on the future costs. installing Low E glass with a minimal U factor of 32 is a good start or base, this is just what I have experienced while doing energy audits and Manual J calcs. Most homes are being built with this standard in our area and has proved to be a good choice to start with. be sure your builder is going to install your components in a wa to ensure that your homes performance is going to be the best it can. sometimes the simple things save you the most. Also air changes in home can kill any savings expectations, so building tight and control ventilation is a key factor. Savings? My guess would be pretty significant. You mentioned u-values of .32, which is roughly R=3. If I went with Serious windows or something similar, I can get R-values of 7-9, so 2-3 times teh insulating value. To me, the difference akin to running your furnace but leaving a window or two open. I've always heard that you get the most bang for your buck by conserving vs. producing energy. That said, the wildcard IS the cost of fuel in the future. |
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www.greenss.net
New Member
Posts:5
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| 02 Feb 2010 12:26 PM |
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I just finished an ICF home in Southwest Michigan. We used the North Star triple pane window. Our main focus was on the building envelope. The geo system is looking like it is very close to overkill. We used the smallest waterfurnace synergy 3D system they offer. If I could do it over I would install a Mr.Slim from Mitsubishi. They have a unit that will continue to heat even in tempratures as low as -17 degress. Go to the this link http://www.mehvac.com/ and find a couple different local contractors in your area. Talk to them all and let them know the effert you are putting into your building envelope. If you have a good enough building envelope and the sq. footage is low enough, it can be affordable. This system as far as I know does quilify for the 30% tax credit. Worth looking into. CONSERVING is always more cost effective than PRODUCING. |
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Brian Shier
Green Structure Supply LLC
brian@greenss.net |
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Stephen T
New Member
Posts:19
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| 02 Feb 2010 12:37 PM |
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I agree 100% with what Jesse says about comfort, condensation and mechanical systems
In my experience the simple mechancial system is a big leap of faith. Most of our customers are grossly over mechanized. I know of two here in the Ottawa area ( 8200 HDD) that have installed radiant floor heating systems and after several heating seasons have yet to use them never used them. (They each use small wood stoves).
When they go away for multi-day trips, they both independantly report interior temperatures have never dropped below 55F.
Even though both were very very well versed in the both the fundamentals and nuances of low energy building they both still spent a significant amount of money on an unnecessary machnaical system. After all, baseboard heaters would have satisfied the code requirement of having a heating system.
Mechanical systems exist only hide the imperfections of the envelope. As the envelope becomes more perfect, the mechincal system can get simpler. But as always 'easier said than done'. However as we create more examples of minmal mechanical systems, the 'done' part of 'easier said than done', gets done more often. |
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Stephen Thwaites P.Eng.
Thermotech Fiberglass Fenestraion
Ottawa, Ontario |
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eric anderson
Posts:99
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| 03 Feb 2010 09:13 AM |
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You have a lot of variables to consider. Cost, comfort, reliability are the big ones. You would have to start with the cost of energy for electricity vs whatever source you use. The other thing to consider is what other uses for natural gas will you have? (Like a gas range and gas clothes dryer) Do you have to put in a gas line anyway?
The other real big thing is solar gain. The sun is your friend. A reasonably executed sun tempered design can provide 1/4 -1/3 of the total heat required for a house. Now that is a cheap fuel source.
Stand in front of a single pane window on a cold night. Even when it is 70 in the room you feel cold. Repeat this with a double pane then a low E Triple pane window. Quite a difference in comfort levels.
IF you have a poorly built house with lousy insulation and not too many windows, they don’t matter too much because they are not the biggest problem. If the rest of the house is insulated very well, and or you have a lot of glass and possible solar gain, the windows become a greater priority as they become the largest heat loss or potentially a heat gain for south facing glazing.
If your heat loss is low enough your heating system does not matter because your costs will be low anyway. My heating bill with a mod con propane boiler this year should be around 200$ for heat and hot water. A geothermal system at a cop of 4 and 0.186 $/kwh for electricity would be around 160$. If I took the money I spent to upgrade insulation and windows and spent it on geothermal instead, my bill would be more then what it is now. IF my propane fuel costs double, it will cost me 400$ per year to heat my house. I can live with that. |
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jerkylips
Basic Member
Posts:203
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| 03 Feb 2010 11:40 AM |
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Posted By eric anderson on 02/03/2010 9:13 AM
You have a lot of variables to consider. Cost, comfort, reliability are the big ones. You would have to start with the cost of energy for electricity vs whatever source you use. The other thing to consider is what other uses for natural gas will you have? (Like a gas range and gas clothes dryer) Do you have to put in a gas line anyway?
The other real big thing is solar gain. The sun is your friend. A reasonably executed sun tempered design can provide 1/4 -1/3 of the total heat required for a house. Now that is a cheap fuel source.
Stand in front of a single pane window on a cold night. Even when it is 70 in the room you feel cold. Repeat this with a double pane then a low E Triple pane window. Quite a difference in comfort levels.
IF you have a poorly built house with lousy insulation and not too many windows, they don’t matter too much because they are not the biggest problem. If the rest of the house is insulated very well, and or you have a lot of glass and possible solar gain, the windows become a greater priority as they become the largest heat loss or potentially a heat gain for south facing glazing.
If your heat loss is low enough your heating system does not matter because your costs will be low anyway. My heating bill with a mod con propane boiler this year should be around 200$ for heat and hot water. A geothermal system at a cop of 4 and 0.186 $/kwh for electricity would be around 160$. If I took the money I spent to upgrade insulation and windows and spent it on geothermal instead, my bill would be more then what it is now. IF my propane fuel costs double, it will cost me 400$ per year to heat my house. I can live with that. lots of good stuff to consider, thanks. We are doing a passive solar design, but 1/4-1/3 of our winter heat needs surprises me. If we can really gain that much, it's more than I was expecting, for sure. I did get some further info on windows. Right now I'm looking at the Northstar triple pane windows with argon. They are .19 u-value (for the fixed windows) and .53 SHGC, and almost exactly the same price as the wood frame Andersen windows my builder normally uses. The "tradeoff" is aesthetic - wood vs. vinyl, but we were planning white windows anyway. Just to throw out a couple numbers, a 60x60 picture window was $458 for the Northstar and just under $600 for the Serious. The u-value is significantly better (.11 vs. .19) but the SHGC was not as high (.50 vs. .42). Not sure how to reconcile those numbers, in terms of whether the extra money is worthwhile.. |
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eric anderson
Posts:99
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| 03 Feb 2010 01:25 PM |
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Posted By Stephen T on 01/30/2010 5:58 PM
Back on Jan 25th eguion wrote:
"........In my math, double pane, argon filled, low-e windows - never - gained more BTUs than they lost during the winter months in the Northeast US. Of course, the math works out differently depending on location.
If you want to learn how to do your own calculations read this: http://www.easternct.edu/energyscience/EES%20305/305_PDF/Heat%20Loss.pdf "
I was surprised at eguoin's conclusion that double glazing without low-e was more energy efficient than double glazing with low-e. I suspected he hadn't been able to consider a high solar gain low-e.
So I downloaded the fact sheet linked his post and compared several window choices over the heating season. I used a 5 month heating season (Nov.-Mar.) because this is typical of a better bult and insulated new house. Older and less well insulated homes would have a longer heating season.
According to the linked fact sheet and my math, over this time period there are 4837 HDD in Hartford CT . Over the same 5 month period a south facing window would receive 144,670 Btu's from the sun, again according to my math and the data in the fact sheet.
I ignored window air leakage, because it is relatively small load (loss) for a hinged or a fixed window. I used, perhaps not surprisingly, data from our windows, specifically our casement windows. The results are listed below:
R-30 Wall
- 3,870 Btu/ft^2 (Net Loss)
based on: SHGC 0.0, U 0.033 /R-30.0
Double glazed, low solar gain low-e
+ 8,119 Btu/ft^2 (Net Gain)
based on: SHGC 0.27, U 0.27 /R-3.7
Double glazed, clear glass
+ 27,459 Btu/ft^2 (Net Gain)
based on: SHGC 0.53, U 0.42 /R-2.4
Triple glazed, clear glass
+ 33,911 Btu/ft^2 (Net Gain)
based on: SHGC 0.48, U 0.30 /R-3.3
Double glazed, high solar gain low-e
+ 38,109 Btu/ft^2 (Net Gain)
based on: SHGC 0.51, U 0.30 /R-3.3
Triple glazed, 2 low solar gain low-e
+ 39,992 Btu/ft^2 (Net Gain)
based on: SHGC 0.45, U 0.19 /R-5.3
Other windows would have different characterisitcs and would give different results. However, while the absolute energy flows for each glazing option will likely change by manufacturer, the relative ranking of the glazing options would not likely change for this location. In other words, as an example, the high solar gain low-e will produce the lowest heating bill of any of the double glazed options.
Hope that helps. This is taken from a thread in in the windows and doors forumn and is a great read for you This explaination sums it up quite well as to what the tradeoffs are in windows in my house I am basicaly using
Double glazed, high solar gain low-e
+ 38,109 Btu/ft^2 (Net Gain)
based on: SHGC 0.51, U 0.30 /R-3.3
on the south side and then low solar gain triples on the other 3 sides. Done properly Pasive solar can be a big chunk of the energy bill. The addition of a solar air heater can also give a huge boost. It has all the heat gain of a double pane clear glass with little to none of the heat loss IF you email me at eric.anderson at uconn.edu I will send you a spreadsheet I made for my own house as a guide to calculation your own stuff |
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jerkylips
Basic Member
Posts:203
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| 03 Feb 2010 03:25 PM |
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Looks good, thanks!! I'll shoot you an email.
Your reply brings up yet another question that I have. Some window lines I've seen have significant differences in u-value as the SHGC changes - as the SHGC goes up, the u-value also goes up. With the windows I'm looking at right now (Northstar), the lowest u-values still have pretty high SHGC values. Is there any DISADVANTAGE to having higher solar gain on the west, north, & east side if I'm not sacrificing the u-value? Since the majority of sun is coming through the south side, it seems like the difference could be negligible.. |
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eric anderson
Posts:99
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| 03 Feb 2010 04:12 PM |
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Posted By jerkylips on 02/03/2010 3:25 PM
Looks good, thanks!! I'll shoot you an email.
Your reply brings up yet another question that I have. Some window lines I've seen have significant differences in u-value as the SHGC changes - as the SHGC goes up, the u-value also goes up. With the windows I'm looking at right now (Northstar), the lowest u-values still have pretty high SHGC values. Is there any DISADVANTAGE to having higher solar gain on the west, north, & east side if I'm not sacrificing the u-value? Since the majority of sun is coming through the south side, it seems like the difference could be negligible.. Yes, In the summer, with a high sun angle, the sun has a direct shot at the east windows in the AM and west windows in the evening. It is hard to shade those windows with overhangs and you get a big heat gain when you don't want it (unless you have shading from vegitation etc) Typicaly you want low shgc on the east and west, north does not matter too much. I was in a house a few years ago that was a passive solar design the the builder faced west instead of south because the view was to the west(and the builder was stupid). The place was so hot in the summer it was crazy. They resorted to coating all the west windows with the witewash for greenhouses. Eric |
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toddm
Basic Member
Posts:252
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| 04 Feb 2010 07:31 AM |
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No offense jerkylips, but you shouldn't be guessing about how much radiant heat will be coming through your windows. You can calculate it from the tables in this manual: http://www.nrel.gov/docs/legosti/old/7904.pdf
The surprise in my case was how little solar radiation there is my part of the world in December. So I skipped low e and designed a pocket inside the woodwork above my windows for insulating roman blinds. You can get them with sewn in radiant barriers. SHGC=0.60; U=0.14.
Yes, I'll spend five whole minutes a day raising and lowering them.
To my mind, the sine quo non for windows is tightness, and the way to get it is casements. |
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psammy
New Member
Posts:21
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| 04 Feb 2010 10:51 AM |
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toddm.......is that just a pocket for the blinds to retract up into at the head? are they rather tight fitting on the sides or in a pocket there as well?
that's an interesting solution....your impression on the effectiveness, especially convective air movements negating the insulation of the blinds? thanks.
psammy |
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jerkylips
Basic Member
Posts:203
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| 04 Feb 2010 12:28 PM |
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Posted By toddm on 02/04/2010 7:31 AM
No offense jerkylips, but you shouldn't be guessing about how much radiant heat will be coming through your windows. You can calculate it from the tables in this manual: http://www.nrel.gov/docs/legosti/old/7904.pdf
The surprise in my case was how little solar radiation there is my part of the world in December. So I skipped low e and designed a pocket inside the woodwork above my windows for insulating roman blinds. You can get them with sewn in radiant barriers. SHGC=0.60; U=0.14.
Yes, I'll spend five whole minutes a day raising and lowering them.
To my mind, the sine quo non for windows is tightness, and the way to get it is casements. thanks for posting the link. I did have that document, I think you may have given me the link some time ago (I'm having deja vu). Just to clarify, I did look at the numbers & see pretty quickly that I would see benefit from passive solar. There have been some things that were still undecided - specifically insulation system & the window specs - that's the reason I haven't really been able to get a "hard number" on the savings. The stat of 1/4-1/3 surprised me a bit, but the fact of the matter is if we save anything, it's probably worth it. With the windows I'm considering, all have very low u-values (.11-.19) so it doesn't seem to be an "either-or" (u-value or solar gain). If I was looking at something that was high solar gain at the expense of u-value, I would definitely need to crunch the numbers a little more. |
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eric anderson
Posts:99
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| 04 Feb 2010 01:31 PM |
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""The surprise in my case was how little solar radiation there is my part of the world in December. So I skipped low e and designed a pocket inside the woodwork above my windows for insulating roman blinds. You can get them with sewn in radiant barriers. SHGC=0.60; U=0.14. ""
My only issue with this aproach is that you get alot of condensation and or frost on the windows in the winter and the moisture may damage the millwork. Deffinitly don't use mdf for the trim and stool.
Growiing up we used 1" eps foam covers over in the windows in the winters. After the sun came up there were often puddles from the condensation.
the objective of Low e is to reduce the radiation back out of the window, ie cooling not heat gain from solar. avoiding low E because there is not alot of solar gain is not such a good idea. It is a question of what type of low E you want, not yes or no.
Standing in from of a low e window feels warmer then standing in front of a double pane (at night especialy if there is a clear sky).
It does not realy matter if there is little solar potential in Dec. The heating season is 8 months of the year in much of the country. If you are getting good solar gain in the shoulder seasons it will help overall.
Cheers,
Eric |
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toddm
Basic Member
Posts:252
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| 04 Feb 2010 04:56 PM |
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OK, I am having deja vu, too, jerkylips. We did have this conversation about a concrete floor. Sorry. If you know the heat loss in your house, you can eyeball how much of it will be handled by passive solar, and conversely, whether you might come to regret buying windows optimized for SHGC. Again, this is site and design specific.
Eric, the trouble with low e is that more U means less SHGC even in the relative handful of windows optimized for solar gain. Skipping low e, I'll get 30 percent more radiation through the glass than I would with the same (Andersen) window with low e/high solar gain (SHGC of 0.60 vs 0.47.) With the curtains down, I should be ahead of the game because a physical radiant barrier in the curtain surely will work better than a coating on glass. My passive solar/high mass design is aggressive. I hope to shorten my heating season substantially with thermal mass and then rely on passive solar and occasional wood heat.
The curtains also have a vapor barrier, so condensation won't be a problem given a tight seal. http://www.builditsolar.com/References/Calculators/Window/condensation.html
Psammy, my window wall will have 10 4060 casements set with nailing fins in 4x6 post and beam. I'll have an approximate 6"x8" pocket at the top of each window to hide the blinds. I'll improvise from there. If roman blinds won't fold up into that space, I'll use a roller system, which would be fairly easy to set in tracks. Some curtains come with magnetic strips to hold it tight against the window. Another possibility is small slats that flip out over the curtain. I halfway expect to add frosted glass in interior shutters to take the edge off intense sunlight. If that is the case, I'll add a tab on the inside of the shutter's hinge edge that would press the insulating curtain up against the window frame. |
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toddm
Basic Member
Posts:252
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| 05 Feb 2010 08:28 AM |
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I fell to pondering how to get insulation and light reduction in a single step that complements my homage to Frank Lloyd Wright, and I came up with an answer: shoji screen shutters. The layup from the room out would be rice paper, bubble wrap, one-sided mylar radiant barrier with the shiny side out. (Dunno if bubble wrap adds anything but I need a vapor barrier on the warm side anyway.) I'd have some whole shutter r value issues with the rails and styles, but it should be good for r2, or a U factor of ~0.25 with clear double pane glass. While the room would still be bright with the shutters closed, you could put away the sunglasses anyway. I could add insulating blinds if needed with no worries about sealing.
Comments? Suggestions?
BTW, this calculator illustrates the dark side of insulation -- the law of diminishing returns. http://www.builditsolar.com/References/Calculators/InsulUpgrd/InsulUpgrade.htm Bumping my windows from r2 to r4 would save $250/yr at 12 cents/kwh. Bumping them from r2 to r7 would save $360/yr, and getting the last $110 is a good bit more expensive than getting the first $250, and counterproductive as well if you reduce SHGC in a passive solar design. Your results will definitely vary because of the amount of glass in my design. |
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eric anderson
Posts:99
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| 05 Feb 2010 08:33 AM |
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Todd,
Interesting conversation, you made me think a bit. I think your approach can be effective, if you are willing to put in effort every day. I would still argue that you would be better off with Low E. I will explain my reasoning below.
Windows provide many wonerfull things, natural light, solar heat gain, views of the outside world, and ventilation. Effective use of windows requires tradoffs in all these things.
I also agree that casements are usually a better choice for 2 reasons. First they tend to have much lower infiltration rates, second when you want to open them, they have twice the square footage of ventilation as a double hung window. This is especially useful in kitchens and bathrooms.
http://www.cardinalcorp.com/technology/residentialGuide.htm is cardinals site for glass and has a lot of great info.
First off, the best of the low E coatings, on the correct surface cost about 11% in solar gain in a double pane window vs clear glass. Right off the bat you have 11% less solar gain available to deal with.
Every day as the sun comes up, you raise the shades to let in the sun. Lets say for 10 hours per day. During those 10 hours, your clear glass windows have both radiative and conductive losses that total 70% higher then lowE solar glass alone. Radiation outwards accounts for over ½ the total heat loss of the window. You may find, based on your particular climate that low E high solar gain windows do better overall during the day. Cover them at night to increase the effectiveness if you want. That part makes perfect sense.
The second point is what do you do when you are on vacation or not in the house? Do you leave the shades open or closed and accept lower gain or higher heat loss. Personally I am gone on average 3 days per week in the winter so this would be critical. What do you do on a cloudy winter day when little solar energy is available? Do you open the shades, or use artificial light in the house? If you open the shades, you are losing compared to a low E window all day long. What if you work late and the windows stay exposed radiating to the sky all evening long?
The third point is what to do with all the other windows. You don’t want clear glass on the east and west because you get too much solar gain in the summer so low solar heat gain Low e is the way to go there. The north windows are always energy losers so low E is definitely the way to go there high or low solar heat gain is not to relevant.
The fourth point is that where I live, clear glass double pane windows are not allowed under the building code. Maximum U value is 0.35 I have never found a clear glass double that meets this. If you are building under IRC in the northern sections of the US, clear glass only cuts it in triple pane windows. |
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