Large window area vs. energy efficiency.
Last weekend I was making a first run through ResCheck to estimate how well a proposed house design for mid NH would conform to energy code. I entered the various parts of the walls and ceilings and was pleased at how much better than code the thing was – to that point. Then I started entering the various window areas and watched the margin between my total U*A and that for the code-conforming equivalent building wither away. Actually, I wasn’t really surprised, because prior to that I built a spreadsheet to do largely what ResCheck does anyway, and I had a good feel for how bad windows affect the heat losses. The proposed design has lake and mountain views, so naturally there is a desire to have a fair amount of glass on the two living levels on walls facing that way. Still, I am dismayed by how fast R-3 window space dilutes R-40 walls and R-50 attic.
So, I wonder what others think about window area vs. energy efficiency in houses with “views.” My thought would be to determine carefully where seating will be and concentrate the expanses of windows there, while minimizing them in other areas. My wife would prefer to have the whole side that faces the view be largely glass, and I groan. And what about wall space for pictures, etc. Perhaps I’m looking for convincing argument either to keep the window area down or to just accept that a lot of glass is going to be there. Thoughts?
Replies
Yes, That's a BIG problem. I wrestle with this all the time. It makes a huge difference in the heat loss calcs for a house.
If they are at least facing south, you will get a lot of free heat when the sun is out, but at night you've got a real problem.
If you cover them with blinds or drapes, you get condensation all over the windows at night that deteriorates the wood trim.
Not only are they energy wasters, but if you get large banks of them, they become a real structural problem.
I love big windows, but I really do think you have to minimize the amount of glass in a house where possible.
I'm with BoJangles, but I'm curious about the condensation issue. Do you mean if you use thermal curtains, you get condensation? I haven't heard that... and I haven't yet successfully talked anyone into using thermal curtains yet, so I don't have any experience with them. Or are you referring to some particular window product?-------------------------------------
-=Northeast Radiant Technology=-
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one of the problems is .. do we design our homes for 3 months ( Rhode Island winter , bad months are December, January, February)
or for the other 9 months ? when we really want to bring the outdoors .... in
windows are so important for the liveability of the home..
i remember some of the designs we came up with in the '70's.. no north facing glass, blah, blah , blah
now i try to use the windows that are appropriate for the 9 month season and use other strategies to reduce heat loss
it's is a constant battle... but windows are so importantMike Smith Rhode Island : Design / Build / Repair / Restore
Hi MIke,I understand, but really, here in the northeast the choice you make could cost several hundred dollars a year in heating fuel. So the question is, are the clients willing to pay that much for the view in that way, and is it ethical in the first place? And it's not just three months. Just because it's not sub zero out doesn't mean that R3 isn't consuming fuel.I'm not sure how you'd reduce the loads 'other ways'. As another poster noted, R3 glass offsets an awful lot of superinsulated wall. If you're using the high R thermotecs, then you're a little better off...Windows are important. Windows from floor to ceiling, however, really aren't always necessary, are they? They are impressive, they are nice, and they cost.-------------------------------------
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--"I'm not sure how you'd reduce the loads 'other ways'.Thermal loads on homes can be reduced by 10-20% just by proper landscaping.Conifers on the North and Northwest sides to block chilling Winter winds. Deciduous shade trees on the East and especially the West sides of the home to cool shade in the Summer.Avoid trees of any kind on the South side to allow maximum solar gain in Winter (even a bare tree can intercept 30% of the heating rays of the sun).If you have enough acerage for it, trees and shrubs can be planted in a funnel shape to the Southwest of homes to focus prevailing Summer breezes.
Well, that's fair. But keep this in mind. Every square foot of R3 window, assuming it's perfectly sealed, equals a bit more than 6 square feet of standard R19 wall. And if you seal the house properly, wind load should be fairly insignficant to the structure in terms of thermal efficiency. Outside air film is a fraction of an R value. If it's not driving significant infiltration into the home, I don't think the effect is that great (inifiltration itself usually accounts for about 30% of the load and you can almost totally eliminate it these days). I'm open to other arguements though, if you have any sources.The thing you didn't mention in the first post, that I didn't catch originally either being strictly a heating guy myself, is those windows do affect cooling loads in the summer as well. So this isn't a 9 month/3 month issue. Those windows affect the home negatively year round. Yes, you can shade the house, but you still have an R3 portal to that warm outside air all summer.I do love solar gain.. can't beat free heat. But taking steps to cover your radiator to the outside at night sure makes sense to me, at least.. unless, of course, it rots out the house.. I sure hope that's not true or has more to the story!-------------------------------------
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well, the glass front chalets and deck houses wre not really on my radar
what i was getting at was not going the otherway in making the windows too small... or eliminating them on north walls
if we do other things to the envelope to superinsulate it, then i feel i can justify my use of traditional windows in our designs.. and i will probably stick to the R-2 + ..... i'm not eager to take a product that should last the life of the house and wind up replacing it in 10 years because the mfr. sacrificed longevity to R-value
BTW.... what happened to the Triple -glazed windows we were all pushing in the 80's.. ???
. i remember Hurd had some nice ones... but i never hear Hurd mentioned anymore Mike Smith Rhode Island : Design / Build / Repair / Restore
Fair enough; but have you ever seen a window that really, in good quality, "lasted the life of the home"? Maybe my view is eschewed, since I'm usually involved in rehabs on existing buildings.. don't get called in on ones working fine all that often!.. but I tend to expect windows to have to be replaced eventually.I'm not sure about Thermotec's lifespan. But many people I respect very highly swear by them. They are more than double pane as well.-------------------------------------
-=Northeast Radiant Technology=-
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well... i've worked on some where the windows were 150 years old.. still in good shape.. ( of course they were single glazed )
and i would expect an Anderson Narrowline to last 50 years ( or more )
i'm working on houses i installed windows in 30 yers ago.. still in great shape
.. your other point about summer cooling... roof overhangs keep the sun off the windows except for eastern & western exposuresMike Smith Rhode Island : Design / Build / Repair / Restore
.. your other point about summer cooling... roof overhangs keep the sun off the windows except for eastern & western exposures
I don't think this is considered by many people. In our area, a roof overhang of about three feet on a south facing window bank, will keep the sun out of your living space in the summer and will usually light the whole space in the winter.
This makes a huge difference in the solar heating of many of the homes we build.
I'm not talking about solar gain... overhangs are great, but I'm talking about heat transfer across an R3 surface from hot summer air into a cooling house. Even if you aren't getting direct solar gain, you get heat gain through your windows in the summer at six times the rate of the rest of the wall on a per-square-foot comparison. temperature differentials are smaller, I'm not saying this is always a gigantic effect, but it's significant and this is not a question of "designing for three months". Todays homes are usually climate controlled all year.-------------------------------------
-=Northeast Radiant Technology=-
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not where i live.. more like 5 months climate controlled heating ( Nov.. Dec... Jan... Feb... Mar )
& 2 months climate controlled cooling ( Jul & Aug )
the rest of the time the heat is off and the A/C is off
Mike Smith Rhode Island : Design / Build / Repair / Restore
well, we can nitpick if you want. the point is, it's not a 3 month consideration.-------------------------------------
-=Northeast Radiant Technology=-
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no.. this is not nitpicking.. i've been building superinsulated houses since '75...we are in a "cold climate" as is more than half of the geographic area of the US
fenestration is extremely important in home design....90% of the time we use an Andersen window...... they average a U of about .32 (.30 to .34 ) for total unit
so .. it's safe to use R-3 in my calcs... equally important is air infiltration.. so the window also has to be tight
and it has to perform it's function, which is to let in light, afford views to the outside, make the house look good in terms of design, have a life-cycle low cost in terms of maintenance and replacement...
i can't think of anything in the house that affects design more than windows / doors / roof lines... and you cann have great rooflines , but poor selection of fenestration will really screw up the design
energy conservation is a primary goal... but i give equal weight to liveablity and pleasing design... so my 4-part equation includes:
energy conservation, design esthetics, design liveability, low maintenance.. those are my goals
i agree that glass walls do not fit in with my equation
but reducing the size of windows below what is accepted as pleasing design is also not in the equation....
perhaps you and i agree ?Mike Smith Rhode Island : Design / Build / Repair / Restore
what do they call it, violent agreement? I suspect we do :D I just wanted to be clear, summertime window coverage, even with overhangs and shade, have big effects. And central air conditioning is getting more and more common even in coastal Maine.-------------------------------------
-=Northeast Radiant Technology=-
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THAT"S WHAT I SAID !
hah, hah, hah.... i suspected as much.. violent agreement...
good phrase, has a nice overtone to itMike Smith Rhode Island : Design / Build / Repair / Restore
You must also take into account the diminishing returns on higher and higher insulation values. Each added layer or value of insulation only slows the transfer that has made it through the first layers. So increasing your glazing efficient by a small amount is better performance wise than adding much more to an already well insulated area.The best areas to increase the insulation for performance are the poorly protected areas now, like the foundation and basement floor. On the condensation issue, air flow will dissipate condensation but controlling humidity is the first step in most cases. Air exchangers and other methods of mechanical venting can go a long way in reducing condensation. Garett
thanks for the tipMike Smith Rhode Island : Design / Build / Repair / Restore
If you are looking at non-operable windows, one option - albeit costly - is sandwiching multple sets of double glazed window units in one frame. I have seen this done between cold rooms designed for food processing and the rest of a building. With this aproach you will get a much better insulating value than any single commercially available product.
Another thing to consider with windows and heat loss is the importance of detailing around the edges. Edge losses can be a very significant component of overall heat loss. Some units have thermal bridging which can reduce these losses, but also consider the headers above the window and all of the framing around it - these are all areas where there is no insulated cavity, heat travels straight through the framing. If you want to get really serious about reducing the heating bills, consider exterior insulation over these areas to reduce the thermal bridging.
Preventing radiant losses is the last step, on cold nights you aren't just radiating to the 10F directly outside, but if you have large expanses of glass with good views of the sky you are also getting night sky radiation. Simply put, you are radiating heat from the house out into space which is a lot colder than 10F. If interior condensation with curtains is a problem, the exterior shutters would be a good solution - or what about the fancy windows with blinds built into the gap between the panes of glass? Again - a pricy alternative.
On the cheap side, you should be able to solve the condesation behind curtains issue with a very insulated window and frame, and some air movement, space the curtains back a few inches. I suspect the issue with window frames getting rotted out with the use of curtains has more to do with edge losses around the window and its framing than the window itself if you have a high performace unit...
In our climate, you can't get away with separating the windows from the warmer room temperatures at night or you will get condensation on the windows.
People that have tried to use thermal curtains to save heat have paid the price of having their windows deteriorate from the condensation. Even the solar home builders that I know have given up on this.
Even common blinds, like duettes, cause this problem. I notice this especially when the temperatures get below 10f, which occurs 4 or 5 months of the year around here.
If there is a good solution to this problem, I'd be the first to jump on board!
Hmm. I've heard of builders skipping vapor barriers by using insulation values to control where the dew point occurs and making sure it occurs in breathable space.. it's beyond my humidity knowledge at this point, so this is pure speculation, but I wonder if with a more significant R-value covering, whether the window cavity could be kept cold enough to prevent condensation...?That's really sad to hear though. Go ahead, rain on my parade! sniff...-------------------------------------
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try closing your curtains or blinds on those cold february nights and you will see what he means especially if your curtains lie in the window jamb as opposed to outside. and it doesn't matter how efficient your windows are if it gets cold enough it gets worst.
one thing I have seen used routinely in europe that seems totally missing from north america is shutters - curtains on the inside = condensation/frost - shutters on the outside puts that condensing surface on the other side of the glass -
the shutters in modern contruction in europe seem to resemble roll-up garage door systems and pretty much disappear to the top of the window when not in use -
I believe generally they are aimed as much to security as energy issues, but my limited experience with leaky wooden hindged shutters is that they make a tremendous (sorry I can't quantify it) difference on cold nights - just the reduction of radiation cooling (I'm not sure I'm using the correct vocabulary) from the naked glass raises comfort inside the house -
the issue is brought up periodically, and I've not ever seen an north american source or distibutor for such systems -
looks like someone/we are missing the boat on this one...
You're correct. Steve Baer had an even better plan way back when in New Mexico. Large hinged shutters that, when opened, reflected more sunlight into the house. Gotta plan your landscaping accordingly, not difficult in New Mexico.I've thought about them but haven't bothered, due to our passive heating system. On another forum (for passive heating/cooling) there's been large surprise at the amount of glazing we have, far beyond recommended. It's a penalty, but as Mike says, plan for it. Not simply with burning more whatever. These are houses, not places to store people.BTW, those roll-up external shutters are/were available here. My sister has them. Probably haven't been closed for the past 5 years.IIRC, Emory Lovins, Rocky Mountain Research Center, had some windows awhile back that gave net gain facing north. Inexpensive they weren't.PAHS Designer/Builder- Bury it!
I am thinking about utilizing the roll up shutters. I have done a web search and about the only suppliers I have found are in southern Florida for hurricane protection. I don't recall seeing any R-values, I assume that even the foam filled ones can't be all that much as they are pretty thin. I wonder, however, about egress issues. I assume that if they cover bedroom windows, there has to be some type of emergency release. Anyone have any ideas on this?
how are the shutters operated? electric? I'm thinking that that is the only way that they would get used regularly - throw a switch...
are manual units available? -
how about posting a link?
thanks -
"there's enough for everyone"
I suppose if someone designed it ahead of time in a new construction, they could come up with something similar to a Star Trek door for the windows. Have a button on the inside that when you push it, the pocket-shutter slides across on tracks and covers the window (on the outside). Wouldn't have to be all that thick, maybe some XPS or polyiso with a steel skin. Thicker insulation for colder regions, thicker skin for hurricane regions.
Design it with some form of quick manual release in case of power outtage or fire.
And then there is the decorative element to consider. What is it going to look like from the outside when the shutters are closed? With some clever application of trompe l'oeil you could probably make it look like closed shutters or some such (at least from a distance).
Maybe an upgrade would be to put a light sensor on it so that it automatically closes or opens based on the light outdoors.
And the track system it slides along would need to be designed in such a way that it doesn't get clogged with snow/ice.
jt8
"The flower that follows the sun does so even on cloudy days."--Robert Leighton
Edited 3/1/2007 10:09 am by JohnT8
From a design psychology standpoint, one solution I like is substituting the stereotype wall of windows with a smaller "significant" window placed where you will get the biggest impact (sit close to a window and it doesn't need to be big, for example) and then augment the psychological effect of windows by placing several small, or very small windows as accents around the room. In contemporary designs there can be some pretty cool effects from multiple windows as small as 6"x6". In more traditional design you want to be a little larger and more conventionally placed. But in either case, you get a pleasing effect of letting the outside view (and light) in, but without the gigantic energy waste. As a side benefit, those little windows are simple and cheap and don't need to open, so they don't leak. If you're ambitious you can even buy the insulated glass pieces from the factory and "frame" the window yourself.
As one who is afflicted with seasonal affective disorder, for me there is no substitute for a large expanse of windows. Rooms with small windows are almost claustrophobic. To some extent, you make up for the lack of ambient light with having to run lots of lights, although high efficiency units makes this less of a problem than it used to be. Still, there ain't nothing like "real" light and lots of it...