Do You Really Need That Much Window Area?
Windows are thermal bridges in pretty much every home. The best windows available have an R-value of only about 12. More affordable windows are in the R-3 to R-6 range. Wall R-values are in the 13 to 15 range on the low end and some cold-climate homes have R-40 walls or even higher. Whether you put an R-4 window in an R-13 wall or an R-12 window in an R-40 wall, windows are the weakest link (like the uninsulated attic hatch in a well-insulated attic). So, a good question to ask in the design phase is: Do I really need that much window area?
Windows increase heating and cooling loads
Conduction, convection, and radiation through windows suck heat out of the house in winter and give it easy entry in summer. Cooling loads can increase dramatically when windows take a direct hit from the Sun’s rays. Windows typically add more to the cooling loads than to heating loads because of direct solar gain on sunny days. But how much? The pie chart below shows that the windows contribute 19% to the heating load in a particular 4,000 square foot house in Virginia. That’s not too bad.
The cooling load in this house, however, is dominated by heat gain through the windows (pie chart below). Nearly half of the total peak cooling load comes from the windows for this particular house.
Those two pie charts come from a heating and cooling load calculation. They’re used for sizing heating and cooling equipment, so it’s a peak load. Another way to look at the effect of windows is to look at the annual heating and cooling loads. I don’t have that kind of energy model for this house, but the relationship is similar to the peak loads.
Windows cost more for cooling than they do for heating most of the time. The percentages and actual loads vary for different houses because of the type of windows, window-to-wall area ratios, shading, and climate, but they’re always a weak link in the building enclosure compared to the insulated walls they sit within.
Windows can make a room uncomfortable
For the same reason that windows increase the heating and cooling loads, they also can lead to comfort problems. That’s why I used a single-pane window to illustrate the issue of mean radiant temperature in my article about how naked people need building science. Because they have less thermal resistance, their temperature will be lower in winter and higher in summer than the surrounding walls.
And it’s not just the radiant heat transfer between people and windows. That house in the lead photo, for example, has a lot of window area facing east, south, and west. Even with decent windows, those rooms may overheat so the air temperature also can create comfort problems, too.
Plotting your window strategy
Windows are the Achilles’ heel of the thermal control layer. Getting the best window you can afford is one way to limit the amount of heat that flows through them, but there are other ways, too. Here’s a quick summary:
- Put majority of windows on north and south sides of house. (You might want to limit the north-facing windows in a cold climate.)
- Reduce window area. (How much of a view are you really getting from the bottom of that floor-to-ceiling window?)
- Use overhangs or shading devices on east, south, and west sides.
- Choose windows with a U-factor less than 0.3.
- Choose windows with an SHGC appropriate to the climate:
- < 0.3 for cooling climates
- 0.4 to 0.5 for heating climates
You can’t eliminate the thermal penalty of windows, but you can reduce it greatly by following this guidance.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to the Energy Vanguard newsletter and follow him on LinkedIn.
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I would also add – consider using fixed-pane windows wherever possible. The R/U values given for windows are only for the glazing (glass) areas, not the frames nor the weatherstripped areas (which are huge on double-hung windows). And use casement windows where mandated for emergency egress.
I thought that I’d seen a value of 15% of floor space as a guide to proper window sizing (of course if you have mountain or beach front views…)
A properly designed, sealed, insulated, sited home does not need windows for comfort control or ventilation
Keep in mind that overhangs cannot provide much, if any, shade on the east and west sides–the sun is just too low in the early morning or evenings. For those windows, you probably have to plant trees to get any significant shading.
I am one of these strange people that never open windows, so I try to minimize the use of operable windows too.
As for shading, I will never ever own a house again that does not have the patio on the east side which is naturally shaded in the evening when we like to be outside in the summer. My previous house had a pool and patio on the west side and I just couldn’t get the trees to grow fast enough to shade it.
Yes, windows are good. Tall ceilings and tall windows make a house feel large and give the ability to enjoy the outside for those who live in a beautiful area. City slickers and neighborhood dwellers may prefer small or no windows….
Thank you, Allison, for another thought-provoking article and for reminding us about the elephant in the room, or house. Random thoughts:
(1) Existing mediocre windows in millions of homes will continue to be a liability
(2) Amount, type, and placement of glazing in a home have stopped to be comprehensive performance considerations for a long long time, or maybe they never were? This is not to be confused with “how do I satisfy the current building code with my windows?” And performance encompasses more than just: thermal comfort, visible light transmittance, glare, usability (does it open? easily?), wind resistance, etc.
(3) There is more to SHGC than just latitude and climate.
Having noticed substantial condensation on my two pane windows recently with an internal temperature of the house at 71 degrees and the OD being below the dewpoint of the outdoor pane made me wonder is there a heat gain factor for windows on houses where super high outdoor moisture level occurs occasionally?
thanks again for a thought provoking article.
Do you have any tips on calculating window requirement from a lumin/ft sq aspect?
I recall reading about tools architects use to establish how much glass facing which direction would be needed for good natural light at a given latitude. It was surprisingly little, if they’re properly situated.
I think another thing that effects how much glass people want is how it looks from the Outside; many of those famous architect designed homes feature huge amounts of glass that look great from outside, even though, as you said, how much of a view do you get from those floor to ceiling picture windows when you’re indoors?
As a counter example, check out traditional moorish architecture in hot climates; the openings are quite small, but you get a great view looking out, and folks can’t easily see in (great for privacy), plus it helps keep the interior cool.
The claims I generally find are about 200 lumens per square foot, with up to 500 in direct sunlight. Avoiding direct sunlight, it takes 3 square feet of window to replace a 60W light bulb in the same location. However, a light bulb in the middle of the room is about twice as useful as one on the exterior wall and four times as useful as one on the corner. This is because the light goes out in all directions, and unless you have mirrors all over your walls, the walls will generally transmit or absorb 80-98% of the light that reaches them. Note this means putting a window in the corner of a room makes it less effective for lighting. The general minimum lighting standard is met by 8% of the floor area, or 1600 lumens for every 100 square feet. Some rooms may need more lighting for specific tasks.
Helpful information on the lumen calculations. But there is more to the perception of light than lumens. If we place a light in the center of our ceiling our eye is drawn to the light. Our pupils shrink in response to the glare and the room is perceived as dark with glare. It is similar with windows on the center of walls. Placing sources of light near corners causes light to reflect off the adjacent surfaces. The ceilings and walls are glowing with reflected light that is canceling shadows. We are more comfortable and see better with less light that is more even. This is one of the reasons why Frank Lloyd wrapped glass around the corners and pushed it all the way up to the ceiling. This type of lighting is especially comfortable to those with epilepsy and vision issues. I played with this when designing my office, and find the atmosphere very calming.
In designing a dream house, I would try to meet most window requirements with high windows all around the perimeter walls (maybe some glass to wall pattern that repeats every 6-8 feet with different ratios on the four directions). That way the south could be well shaded, the north would provide even lighting, and east/west could be limited to one window per room to have some natural (but possibly awkward) lighting. Exceptions would be egress requirements and distinct view expectations. If there were a low cost structurally sound way to have a 1 foot high window around the entire perimeter of the house just below the ceiling, I would do it.
I have allot of issues with how windows have been done here in TX. Many houses dating back to even the 50s had huge windows. Our house built in 1960 has 72×72 windows in the living room and dining room and all the bedrooms have 3×5 windows on two walls.
I replaced them asap when we bought the house as they were old aluminum garbage. Now to make things worse this is a brick house which is the norm here. Had it been a sided house I would have made those 3×3 windows. The reason being is that you have a door on two walls then a floor to ceiling window on the other two walls. Now where do you put your beds and furniture….. No matter what you will end up putting a bed or other piece of furniture against a window and that window will be below the top of the furniture. Very poor design that still exists today in new houses.
Also make sure to limit the number of windows facing south in the hot climates a well. We have large overhangs and get no appreciable direct sun into the windows but newer houses have no overhangs. We have blinds that are basically closed year round. The living room ones could just go away as they serve no purpose and most all the rest of the windows in the house could be half the size and make the house more comfortable and more livable since you have wall space without a stupid window.
Don’t get me started on how I hate brick houses in hot climates. I find it interesting where the major heat loads are in those graphs. I’m here in TX (hopefully not forever) and our primary heat load is the ceiling. With the temps in the attic being 140* there is a big heat gain to be had there.
As for that uninsulated attic hatch… Mine is not insulated and in fact I keep it open(with the garage door cracked) as my garage is not climate controlled yet. It will be once I get up there and insulate the attic over it. I need a place to store stuff and keep it out of the heat so it doesn’t get destroyed.
I have many soffit vents but they are extremely restrictive so they just can’t flow enough air especially when it’s 100*-110*+ from 10am until 9pm for months on end.
It is very important to design for the climate and sun. An interesting university study experimented with many different ideas to come up with the most energy efficient home for the Central Florida climate. The results were identical to a 150 year old cracker house.
I always design for the specific climate of each property. It is by far the cheapest and most successful way to increase efficiency and comfort. Shade the hot weather glass. Sun the cold weather glass. Use the roof and direction to do that.
Being in Montana, I can so relate to the chill that one gets when you radiate your body warmth out through large windows towards the snow-covered ground; doesn’t help that two sides of my home office have expansive ceiling to near floor windows. Love the views, hence no drapes in our home and am not concerned about privacy, neighbors can see what we are doing both day and night if they are so inclined.
Our second home in southwest Montana is a very large 1895 era Victorian with 46 massive, single pane, double hung windows with several sets of large bay arrangements. Loads of low southern sunlight to warm and brighten the home during winter and yeah, the occasional frost on the inside of the single panes. The humidity is very low so no harm from the water condensing / frosting.
Exception:
Designing the roof overhang to shade the glass in the summer can make a big difference. But that south facing glass can warm up the house a lot in the winter. And if the cost of heating is greater than cooling, a lot of south facing glass is a plus. (My 460 square foot Southern Appalachia house costs $100 a year to heat and cool with the heat pump).
I believe they should replace low E glass with teaching planners how to shade the glass all summer. This would increase winter heat gain, and reduce summer heat gain.
Many houses showcased in architecture magazines have vast expanses of windows. How do they pass energy codes? Aesthetically pleasing, maybe. But comfortable?
We built our house in 2000 with a passive solar design. Most of the glazing on the south side (12% of floor area) and a 2′ overhang that turns out to be a bit undersized as the sun starts coming in the bottom of the windows in August when there are still plenty of hot days in Western North Carolina. We compensate for our undersized overhang on the South by having summer reflective curtains and in winter we have insulated night curtains to cut down on heat loss.
We followed the Passive design principles with less glass on the east and west and have trees that shade on both of those sides as well. We also incorporated lots of thermal mass in concrete floors and a brick interior wall.
All in all it works pretty well the other day it was 91.4 outside and 76.4 inside at about 2pm with no AC.
If I was building this house no I would reduce the South glazing to about 7% of floor area and raise it up more from the floor where the summer sun starts coming in.
One fact about good windows that I read a few years ago really amazes me: A high quality, triple-glazed, Passive-House rated window in a cold climate will actually GAIN more energy on an annual basis than it loses. In that sense it does something that no wall, no matter how well it is thermally insulated, can do. Walls always LOSE heat, no matter how well built, but good windows can be NET GAINERS of heat in cold climates.
Just realized, overhangs above windows also reduce radiative losses to sky (day or night) in heating season, although the steep angle sky portion of radiative losses to outside may not be large enough to get too excited about.
DON’T get me started on crazy window designs…Too late, all y’all got me started!
A few weeks ago, a Man J for a custom beachfront house crossed my desk. Despite the fact that the beach is to the east, a two story great room features FIVE 14.5′ high by 7.3′ wide windows facing southwest (i.e. inland toward a marsh)…disgustingly conspicuous consumption!
As is often the case, designer is a big name local architect looking for the next magazine cover article, I guess.
Hello,
I am getting hung up on deciding on what would be the best energy modeling software to use for about 30 homes that underwent a Weatherization project in Charleston, SC.
On average, these houses received additional insulation that put the home at 2009 IECC or higher, and air seal that resulted in a 20% reduction in air infiltration. Other typical improvements were suggestions provided by BPI. In some cases, I had to go beyond and replace windows and HVAC equipment and leaky ductwork with high efficiency HVAC systems with a tighter ductwork.
I would like to be able to predict the energy savings for these homeowners once the weatherization project is complete. I really am curious to see the savings of the blower door, total duct leakage, and insulation improvements. Are there any suggestions?
I appreciate any help.
Fortunately I have only moderate sized windows (single pane with interior storms) throughout my house. Meanwhile the failure to have even a small high private window in exterior bathrooms bothers me anytime I must change the bathroom light or deal with an electric outage. While the bathrooms have fans, they don’t work in the admittedly rare (but sometimes lasting up to 12 hours) electric outage. Given vinyl siding and 2×4 framing without exterior insulation (possibly adding 1 inch in the next few years), is it reasonable for me to ask a contractor to put in such windows? If so, what kind of contractor as it appears most window companies only want to replace existing windows? For reference, I live in zone 4a outside Knoxville.
Stacy, A good carpenter will be able to cut into the wall and reframe it for the window you want to install. You will have to tell him what type of window you want so he can come up with a cost estimate. You may want to consider a high wall hopper style window for privacy with light and ventilation. I have a house with aluminum siding and installed a high wall hopper, double-glazed window many years ago. It’s not a difficult job and took less than a day to finish inside and outside details.
Hmmm….Allison, I thought we were holistic, system thinkers, not just focused on energy and thermal comfort.
Like you, I once was hyper-critical of home designers who included 1,200 sq. ft of glass in a 3,000 sq. ft floor plan. But the “gift of Covid” changed me. That’s when I brushed up on human physiology. I actually broke down, read some books and lots of medical research papers, many of which were pure nonsense.
It seems there’s a lot that we thought we knew which turned out to be untrue. As a matter of fact, I’ve almost adopted the philosophy that if the consensus believes something is true, its’s usually a good indication it’s not.
It appears we have forgotten the key role that light (specifically sunlight) plays in our health and wellness. Fundamentally, it triggers the circadian cycles via non vision related ganglion cells in our eyes. And then there is this thing called “vitamin D”, although vitamin D is actually a hormone. But most important, the frequencies of light play a huge role in overall health and energy production.
The “old theory”, which hasn’t died, is our energy (and healing) relies on the chemical bond being broken in AdenosineTriphosphate (ATP) to form Adenosine Diphosphate reaction (ADP). ADP is then recharged by our mitochondria . Now, contemporary research (new biology) has shown that although ATP-ADP are key players, the process doesn’t work the way we were taught. Apparently no one ever asked what triggers the mitochondria to recharge or convert ADP back to ATP. Research has now shown that structured water (exclusionary zone or EZ water) in our bodies is the transport mechanism that provides the electron flow, triggering the ATP-ADP process. (As Dr Bill Rea once told me, “the only difference between a live cell and a dead one is the electrical charge is gone”).
Guess what moves those electrons????? LIGHT. Not artificial light, (likely another misguided energy savings initiative), but sunlight. (If you dig deeper, you will also see the same process used in circulation. (See Rudolph Steiner and Tom Cowan. Tom is a Dookie, but he’s still pretty sharp.)
Studying this subject matter via Dr Gerald Pollock, his mentor Gilbert Ling and others has made me less critical of those engineers who were bad at math, AKA Architects. Although they don’t know it, their fetish for windows might accidentally prove to be supportive of better health and wellness.
I am convinced we have become too civilized and completely overlook the underlying consequences of non holistic thinking. I am not so quick to throw out the bath water until I make sure the baby is still not in the tub. As Mr. Bean teaches, we should design for people and good buildings will follow.
Just my opinion. Others may disagree.
How about virtually no windows?
With the decreasing cost of large LED TV screens, why not keep the R-40 wall with a number of large tv screens mounted on them that would connect to high quality cameras that could switch to any direction or just selected scenes of something better or switched off when not occupied.
“Window” treatments optional.
Those last two comments (Gough / Sorg) cover both ends of the opinion spectrum: Lots of glass vs no glass except for large screens connected to outdoor cameras. There is a class of homes that may test these opinions and be worth studying: row houses. They are very common in urban areas so lots of potential data. They are typically long and narrow, with the long walls being common to neighboring houses and have no windows, of course. My son lives in one and I have often thought how nice it would be to have a few false windows on these walls, consisting of large screen TVs framed with window trim and connected to an outdoor video camera. If you search online, these products are available, some with sophisticated controls that pan the camera as you approach as you would see the view as you walked past the window. If I lived in a row house I would probably try adding a faux window, but I would still go out and walk every day. Fortunately, urban dwellers live in a world where many of our needs can be met by walking a short distance, so they escape from their row-house “caves” pretty often. Any sociologists out there interested in studying this?
I actually proposed this in a different forum, and was met with hosts of ridicule. I still think it has merit; imagine a “picture window” that could give you a view of outside your house, or anywhere else in the world with a streaming camera. A true picture for the inhabitants, not the people passing by. And no heat loss/gain or window treatment costs to shield you from prying eyes.
What would be the most cost effective window shading device(s) to beat the heat?
Tree, Aluminum foil, Exterior/Interior window tint
Exterior/Interior curtains, Exterior/InteriorShutters
blinds, cellular shade, Storm windows, Awning, etc?
Great question. Most effective: exterior, away from the window if at all possible! Don’t let that sunlight fall on the glass in the first place. Overhang or awning on the south side, deciduous trees on the east/southeast and west/southwest.
This online calculator will give you sun’s azimuth and elevation at your location at a given time:
https://gml.noaa.gov/grad/solcalc/azel.html
I agree with Paul about using trees for shading on the east and west sides, but there is another thing that you need to consider. If you buy a fast-growing tree species, it will probably take 10-20 years before you get decent shading. Then in another 10-20 years, you will worry about it falling on your house. If you buy a slower growing tree with a longer expected life, you will probably never see the benefit.
A fast growing maple or poplar will provide some shading after 5 years for the ground floor, assuming you planted a 2″, 8-10ft specimen. Yeah, I know, 5 years is about the average home ownership in this country, and thinking longer term is not.
It’s not just about planting trees. If you’re building, choose your site, and then choose/adjust your design to work with the site.
Maybe some day, planting trees for shading in cooling-dominated climates (a growing category) will be required by building code. If the builders in the 1960’s, 1980’s, the 2000’s, or even the 2010’s had the foresight, most of those home owners would be enjoying their mature tree shading. Never say never.
My father planted two trees on the west side of his house to shade the living room picture window during the afternoon and evenings. This was in the mid-1960’s. He still lives there and has gotten a lot of shade over the years. But now the trees are 50′ tall and the trunks are cracking so we have to worry about them falling on the house.
Another issue is rooftop solar. How do you shade your windows without shading PV’s on the roof, whether it be yours or your neighbor’s? If we keep pushing rooftop solar, will we also have to ban trees?
As for proper site selection, I don’t see how you can design a new subdivision to have all houses properly oriented for the sun. As I sit on my east-facing patio each evening and enjoy the shade and breeze, I get to observe the neighbors behind me with their west-facing patios who have to spend the evenings indoors. We all have large overhangs on our patios, but the west-facing ones just don’t work in the evening.
All good points, Roy. I just don’t think trees are a problem, they are part of the solution. Windows, and their orientation, size, number, glazing type, fixed shading devices, as well as operable ones are a big part of it. Solar heat gain is mostly desirable in some climates and mostly unwelcome in others.
When designing a subdivision, I would think about solar access first (for PV and for passive gain), and that would be relatively easy to do. Then I’d add site features to help with east/west solar gain (if mostly unwelcome). The individual home designs would also need to play ball. For example, run most streets as close as possible to being parallel to east/west line, and if the houses are close enough (which in a lot of new subdivisions it can be as little as 10-15ft) most would shade one another from east and from west. Then you tinker with window placement, horizontal and vertical shading devices etc.
Finally, in the context of this particular blog, the answer to the question “Do you really need that much window area?” is, in my opinion, “it depends”, because “that much” is a moving target if we are considering performance (thermal comfort, energy, daylight, etc) based on climate.
Finally, I don’t see solar PV as being “pushed”. It’s there, available, cheaper than ever, just reach out and grab it. Every kW on your roof makes a difference.
Newer version of the sun calculator: https://gml.noaa.gov/grad/solcalc/
Tree: It would take years for it to grow tall enough, unless you spend a lot of money planting a large one. But if you choose one that loses its leaves, you might get sun in the winter when it would help. If you carefully plant it in the right direction to block hot summer sun, it will do a very good job of cooling the entire window area. But the possible root damage to the house or underground utilities needs to be considered.
Aluminum foil: Cheap, but much less effective and attractive. It would block more heat on the outside, but the weather could blow it off.
Window Tinting: Might void your window warranty because it can cause the glass to crack. It doesn’t block all of the heat, just a percentage. It scratches easily.
Curtains: I have never heard of exterior curtains. I doubt they would stand up to the weather. Inside, they would. And if they are sun blocking can do a better job than tinting. But anything you place on the inside has the disadvantage of letting more heat in the house.
Shutters: Can block much more sun heat than most of the other options. They look better and can be more weather resistant. They would work best if they can be opened and closed, and if installed outside.
Blinds: If you choose sun blocking, insulative blinds and have them carefully installed to fit the opening as tightly as possible, they can perform better than most of these options.
Storm windows: Help keep out the cold, but don’t do much to keep out the heat. They might cost more than most of the other options.
Awning: Similar to shutters.
I feel dumb for my reply about aluminum foil. I think you were referring to reflective film. There are a few considerations:
1. Unless it has holes, it is also a vapor barrier. You would need to be sure this wouldn’t cause problems with the rest of your insulation vapor barrier systems.
2. Don’t install it where you spend more heating than cooling, or it will increase your energy costs. (Also true for light colored or reflective roofing)
3. I have seen recommendations that there be a ventilated air space on the hot side of the reflective barrier. That might not be easy to do. That air space can get very hot in some circumstances. Make sure that is considered.
I was referring to aluminum foil in windows.
https://www.fema.gov/blog/6-ways-stay-cool-extreme-heat
Insulate your house.
You can keep your house cooler by insulating it and covering your windows with drapes or shades. Use window reflectors such as aluminum foil-covered cardboard to reflect heat back outside.
Throwing out ideas
Exterior curtains – mostly for patios
Outdoor window shades i probably what I meant.
How about an exterior awning to shade the window made from a solar panel? You could make the support brackets adjustable to gain heat in the winter if that is wanted. Install an exterior electric outlet above the window and plug the micro inverter into it. It’s the only sunshade that will pay for itself, and if you install it yourself, the payback could be pretty quick.
Shade trees are a great idea at first… Then when you realize all the costs involved with having trees over the long run they will end up costing you far more than the energy they are saving you. When you look at just the tree trimming costs alone you will never recoup that in energy savings. Then the house damage that occurs from falling branches etc. And then when the tree gets old enough or it dies or is damaged in a storm that tree can cost thousands to have removed.
We have many large oak trees and the do provide shade in the morning and the late evening but during the hottest part of the day they are useless as they don’t shade the house much at all. I would never have large trees near a house ever again as the liability they cause isn’t worth the miniscule energy savings.
As for shading windows there are external shade screens that are pretty popular here in TX and other places out west where it gets hot and stays hot for months on end IE 100*-110* for 7-9 hours a day. You won’t see that stuff as much in the *south* IE GA and other more northern states as it doesn’t get very hot there and not for very long the few days a year it does get hot.
I agree with Robert’s points on trees being more of a liability than a benefit and he didn’t even mention dealing with leaves in the fall.
Two things:
1. Why do HVAC persons put the supply register in front of the window where it; creates condensation, thus slowly destroying the glass to frame connection and imparts a good deal of the heating and or cooling to the outside world?
2. How can this product have an r-value of 15 if it is made of Aluminum, have people no shame?
https://www.amazon.com/SmartATTIC-Attic-Insulation-Cover-Aluminum/dp/B0896Y8BXB/ref=mp_s_a_1_17?crid=3EPM741CJPJZF&keywords=attic+stairs+insulation+cover&qid=1692544062&sprefix=attic+stair%2Caps%2C114&sr=8-17&ufe=app_do%3Aamzn1.fos.18630bbb-fcbb-42f8-9767-857e17e03685
Respectfully submitted.
If installed perfectly, this product provides an air barrier and reflective barrier. Most reflective insulation is known for the R value scam. If you include proper spacers, this might get you R8, but the material R value is no higher than 2 from the double bubbles. I’m not saying the product itself is a scam as it may help prevent extremely hot attics from transferring too much heat into your house, but the marketers are acting up again.
As for question 1, it was allegedly needed to prevent mildew and excess condensation when everyone had crummy single pane windows, but I agree it is wasteful even as some are in my 1992 double wide with storm windows.
On the contrary Mark K. Moving air across a cold glazed surface will reduce the risk of condensation. Moreover, since surface temperatures substantially influence thermal comfort, warming the surface can somewhat offset the radiant heat transfer from the body. The same is true for summer,. Although poor performing windows and an improperly installed cooling system could accidentally produce condensation, I don’t recommend it.
Mark K, Danny is correct that the air flow will help prevent condensation, but the point of putting heat supplies bellow windows was originally basic comfort. Even good windows will have a higher heat loss than walls and so the cool surface will create a downdraft that will be uncomfortable. The heat source below the window counter-acts the cool, denser air dropping to the floor and should reduce the draft problem. As far as condensation goes, the glass surface will be the coldest thing in the room, and if it gets below the indoor air dew point, you will have condensation. Multiple glass surfaces (storm windows, double or triple glazing) will raise the the inside surface temperature and reduce the chance of condensation, unless you raise the indoor humidity too high. Having a heat source below the windows raises the glass temperature and reduces the chance of condensation. So heat below windows improves comfort by reducing drafts AND the likelihood of condensation on the glass. Super-insulated walls, air-tight construction and triple-glazing (i.e. passive house methods) make these issues go away, and supply air from a central interior location works fine.
BTW, Mark, you are correct that the R-15 claim for the attic door insulated cover is bogus. Even a cover made of 2″ of polyisocyanurate is not R-15, and that is about the best you can get.