Now is a good time to consider removing window screens to increase solar gain of windows throughout the winter. According to the values given here, removing a standard bug screen over the whole window will increase the solar gain by 32%, so if the screens are over just half the window, the increase is 16%. Standing in direct sunlight, you can feel the difference!

Removing the screens also offers an excellent time to clean the windows, further increasing solar gain. I clean the screens before reinstalling in the spring. In many parts of the world, winters tend to be a little dark and dreary, and removing the screens brightens up the house and the mood, and provides a clearer view outdoors.

Good point, I'm taking mine out tomorrow! Thanks.

Unfortunately out in my neck of the woods (Phoenix), everyone I know wants to DECREASE solar heat gain during winter. These builders installed some insanely high SHG windows in thousands of homes and people are experiencing overheating in Phoenix homes. I shot some thermal readings today and on the interior portion by the window I was getting 112F readings. The current outdoor temperature is 76F. That 36 degree increase is making homes overheat right now. I know people who still have their A/C running right now because their home is getting too hot and these A/C units will run all winter long. These dumb builders did this to thousands of homes and think about all the wasted energy for the next 20+ years because of poor choices on SHG for the windows.

The aluminum spacer in the double pane window was reading 125F. Yikes! I am going to try and find out what the specs are for the SHGC on these windows, I would bet they are in the 0.40 range. Only thing we can do out here in Phoenix is try and shade the windows with some thermal shades, while not perfect, it does help. Two identical windows adjacent to each other, one had a solar shade and I got a 108F reading between the window and shade. The other had a 92F reading. It's a crude test but it shows that the shades are trapping the heat but it still probably dissipates to the interior.

Windows with SHGC of > 0.30 should NOT be allowed to be sold or installed in Phoenix homes.

In the winter, in Denver, we also like to increase the solar gain from windows. I've noticed that there is quite a bit of reflection off of the windows which wastes light and heat that could be going through the windows. Anti-reflection film is quite expensive and some of the films prevent the sunlight from coming through. Is there an easy way to reduce the amount of sun energy that is reflected? Is there an inexpensive product that could be applied to the windows to reduce reflection?

I've installed a prototype Trombe wall and by opening the windows more that allow heat into our home from the Trombe wall, the glass is more perpendicular to the sun. I'm trying to also have the Trombe wall sheeting be more perpendicular.

Trying to stay warm in Denver
www.NegawattChallenge.org

Posted By Denver Dave on 23 Nov 2012 12:46 AM
In the winter, in Denver, we also like to increase the solar gain from windows. I've noticed that there is quite a bit of reflection off of the windows which wastes light and heat that could be going through the windows. Anti-reflection film is quite expensive and some of the films prevent the sunlight from coming through. Is there an easy way to reduce the amount of sun energy that is reflected? Is there an inexpensive product that could be applied to the windows to reduce reflection?

I've installed a prototype Trombe wall and by opening the windows more that allow heat into our home from the Trombe wall, the glass is more perpendicular to the sun. I'm trying to also have the Trombe wall sheeting be more perpendicular.

Trying to stay warm in Denver
www.NegawattChallenge.org

It is unclear what type of windows that you have. Windows without coatings reflect about 4% per surface in the visible part of the electromagnetic spectrum, so 8% for a single pane window, 16% for a double pane, and 24% for a triple pane window, and a similar amount of total solar energy. (Actual values are a little less due to absorption in the glass.) These values are small enough that it is not worth trying to use anti-reflection films to reduce the reflected energy. If they are purposefully coated, then the reflected energy can be much higher. Double-pane, low-e windows that are low solar gain reflect roughly 40% of the incident solar energy, while low-e high solar gain windows reflect roughly 20% of the incident solar energy. For more details, see Fig. 12-1 of http://www.cardinalcorp.com/wp-content/uploads/pdf/residential-brochure.pdf.

I am not familiar with practical ways to increase the solar gain of windows (clear or low-e) by aftermarket coating schemes to reduce the window reflectivity. Obviously external shading can have a big effect on solar gain, but I assume that you have already addressed that. Increasing the albedo of the ground outside the window can increase the solar gain, as you may have noticed with snow-covered ground. Not so practical to change the albedo, though. Maybe a snow-making machine, and the kids could enjoy the benefits too.

Hi Lee - thank you for the reply

I live in a 1922 vintage bungalow home, in Denver, Colorado, with double brick, no insulation walls where the bricks are close enough together that insulation is thought to be impractical. The two south facing kitchen windows are glass slide up and down with wood frames and I have taken the wood frame outside screens that pivot up and wrapped them with window shrink to fit plastic. Inside the windows I have removable plexiglass panes which I typically remove when the sun is shining and inside of that a blanket sandwiched between two plastic tarps. A little better, but similar to other windows in the home. You can see a picture of the outside prototype Trombe wall where the pivoted up (much wider opened now) wrapped up screens are visible if you scroll down a little from my Negawatt Challenge page: http://negawattchallenge.org/challenger/bean.html

On the outside south facing wall, rather than paint it black for winter and then white for summer, I hung the same solar screes that shaded the house in summer against the wall and covered the screens with vinyl sheeting. After a few go a rounds, the prototype Trombe wall sustains itself against at least some wind and the south wall is fairly protected.

The numbers that you gave for reflection seem to agree with other numbers I've found on the internet. However, I'm not sure how to reconcile the reflection percentages with my own experience of standing outside in the reflected sun off of the vinyl sheeting and also glass windows and being able to feel quite intense heat hitting me.

It is still fairly cool today in Denver at noon at 45% in the shade. The Trombe wall is dumping 85 degree air into the house with the two kitchen windows as far open as I can get them. One of the few things that I remember from high school physics is that the reflection is less the closer to perpendicular the light source is. So, with a hollow tube I demonstrated perpendicular and opened the pivot windows wider to be at 90 degrees to the sun. This is November 23 so I may have to close the windows more as we move to December 21 with lower sun and also with colder outside temperatures.

Cleaning the windows seems to be quite important to reduce reflection or at least glare and seems to help get more heat.

Our target daytime temperature for our home is 60 degrees inside before we automatically turn on the heat and 45 degrees inside at night which we've never reached - hit 47 once on a very cold 20 to 30 degree below zero night. So our furnace does not turn on overnight. If we are chilly we use sealed circulated oil portable heaters under our desks and a gas fireplace sometimes in the evening in addition to sometimes temporarily turning up the furnace (programmable thermostat automatically turns the furnace down after a while). With a house with no insulation in the walls and 50 old windows and doors to the outside, I feel pretty good that we tied for the best for the therms part of the www.NegaWattChallenge.org last winter (see page for data - I'm the Bean challenger)

With a 60 degree house - heat sources are very apparent inside our home and we are thankful for any extra we can get.

It is now 1 PM and the neighbor's house will start shading the Trombe wall. So as I start filtering down the kitchen window opening, the 24 - 4 ft x 2 ft Mylar mirrors in the back (East) yard will start reflecting sun to the house - lots of light - not so much heat inside, but lots of heat against the outside wall. Pictures here at the bottom of the page: http://negawattchallenge.org/challenger/bean.html

Denver Dave

Denver Dave-

Let us define an angle of incidence as the angle between the perpendicular to the surface and the actual incoming light ray, so zero would be perpendicular. The reflectivity of glass does not change much between an angle of zero and about 50 degrees, as shown in the Fig. labeled Air -> Glass at http://physics-animations.com/Physics/English/rays_txt.htm. Therefore, you do not need to work hard to get the light exactly perpendicular to the window. I don't know what the reflectivity of the vinyl sheeting is.

Reflection losses are high enough that they would be nice to get rid of when you want passive solar heating. However, I do not know a good way to get rid of the reflection losses within the realm of things practical for a homeowner.

You have quite the challenge trying to heat a house in Denver with no insulation in the walls.

Posted By Denver Dave on 23 Nov 2012 03:09 PM

Our target daytime temperature for our home is 60 degrees inside before we automatically turn on the heat and 45 degrees inside at night which we've never reached - hit 47 once on a very cold 20 to 30 degree below zero night.

Denver Dave

If I read that correctly, are you saying that you let it get down to 47F inside of your home during winter?

If so, how on earth do you take a shower during winter?

The average for most homeowners is around 65F before the heat would kick on. Letting it get down in the 50's and lower would make for a very uncomfortable home, especially for the young, elderly and sick. One would have to walk around wearing outdoor clothing but while inside.

Did you purchase the home knowing that it was completely devoid of insulation?

Thank you for the replies.

Yes, once the first morning temperature in our house was 47 degrees and we've never gotten below 45 which is where the programmable thermostat is set for over night. However, this was a rare occasion and we are quite comfortable overnight. We have a 1 story bungalow home where we started sleeping in the basement overnight one hot summer night when the neighbor's dog was barking and we discovered we liked it. The temperature in the basement stays a comfortable for sleeping 60.

The programmable thermostat turns the heat on at 7 AM to 58 at the thermostat in a cold corner which means 60 or so in the 1st floor. If we are chilly, we manually bump the temperature up and the thermostat automatically turns it back down to 58 at 10 AM when we may be out. We often bump the temperature up again or turn on the gas fire place if we need it, but on an on request basis. The programmable thermostat tries again to turn the heat down to 45 at 8 PM and 10 PM with it not being unusual for us to ask for heat from furnace from 8 PM to 10 PM.

While our target temp is 60 or so (more like a goal), we have a couple of friends with targets in the 50's and we do dress warm when we go to their homes. Strangely, our coldest feeling month is early fall before I have all the storm windows up, we haven't found our long underware and our bodies have not adjusted. We've been splurging a little with the gas fireplace this month, we'll see how this effects our heat bill. A circulating oil heater in the bath room runs an hour a day as we take showers and was measured last year with a Kill - a - watt meter to only cost about $6 for the season - well worth it.

- - - - - -
Thanks for the information about the reflection at angles below 50. This will give me more flexibility for how open the outside storm windows are.

Today, I took the outside storm windows that pivot up off completely off and slid them down in the Trombe wall for storage as a test. Not sure about the overall effect on overall temperature gain coming in, but sun hitting the kitchen is definitely more intense.

- - - - --

Thank you for letting me post in this forum, I didn't realize the forum was for builders. People like me don't spend much money, however, several people in our www.NegawattChallenge.org have spent quite a bit by my standards. One spent 60 K on a 52 panel solar electric and 4 others have smaller systems, 4 have hybrid cars with 2 of those being plug in hybrid and one challenger just put in a geothermal system. I'm not speaking against any of these approaches, but seems to me that certain basic home design, remodeling and weatherizing may pay of competitively. For example our west sloping roof presents a low face to the wind and our larger south facing wall get's quite warm on sunny calm winter days which I attempting to better utilize.

Thanks for the info.

ha, no wonder i found the screens in utility room when i bought my last house in the fall. good idea.

Selected pictures from Fran Rew's Topic Talk Walk at the Steven's home. This was Sunday, December 10 - a chilly day with very little sun - notice how bright the home is.

Annually, the home produces more electricity than it uses.

From February 27, 2012 to December 8, 2012, no therms were used. The gas furnace came on that night. Annual therms for the past 12 months was 70. Steve's goal is zero gas used, but hasn't quite made it yet. Our home used 77 therms in just December of last year.

Water is heated by a GE Heat Pump Water Heater which I thought would be useful in the summer to also help cool the house.

Entry ways are like air locks with a sealed small room at each entrance.

How Stevens got to R 100 insulation on one wall by adding more Polyiso insulation with an additional wall outside his wall.
http://www.meetup.com/TTWalks/photos/12127652/186998252/

Two level solar porch inside of Solar Trombe wall
http://www.meetup.com/TTWalks/photos/12127652/186998722/

http://www.meetup.com/TTWalks/photos/12127652/187004232/

Entrance to top level of two level solar porch
http://www.meetup.com/TTWalks/photos/12127652/187004632/

One of the air-lock doors
http://www.meetup.com/TTWalks/photo...187000122/

Trombe wall outside of the first floor solar porch
http://www.meetup.com/TTWalks/photos/12127652/187000852/

Polyiso insulation
http://www.meetup.com/TTWalks/photos/12127652/187003412/

22" Solar tubes
http://www.meetup.com/TTWalks/photos/12127652/187007122/

Denver Dave-

When you want to introduce a new topic, you might look into starting a new thread with a new title. You post might get more appropriate feedback using that approach.