Solar gain

GroundUp

Happy Monday folks, I'm stuck inside today as it's -18F outside at the moment and I've been thinking about solar gain and wondering whether it's measurable. Now, my house is a 1400 sq ft manufactured/modular thing parked on top of a full walkout CMU basement. The lower level is 12" CMU with about 2" of closed cell foam between the studs inside, the upper level is just regular old double-wide construction with 2x6 at 16" OC with fiberglass batts between and 7/16" sheathing outside with Tyvek and vinyl siding. Windows are vinyl double hung sliders but pretty cheap ones from the big box, they leak and condense. I have done a heat load calc on the house several times over the years and just using what I know about the construction and guessing on the window performance, it always comes out to about 40k at -20F design. My walkout is on the South side which has about 200 sq ft of CMU wall exposed plus obviously the whole 9x50 upper level. There is 60 sq ft of glass on the lower level and 40 sq ft upstairs. Sunny days, I have a phenomenal amount of solar gain on the South side of the house and ultimately it cuts back heavily on BTU used. Today, for example, my air handler has run for a total of 27 minutes in 4.5 hours. Using flow rate and delta T through the coil I've come up with a total of 37,500 BTU during that period meaning ~8333 BTU per hour. Temp has averaged -20F which is design temp, so obviously my solar gain today (being calm and sunny) has been giving me 75+% of the required BTU. This spring, I plan to be adding an attached garage with living space above which will be on the South side and the gable of the addition will be facing South. I'd like to fill the gable end with glass as this will be our "party room" type area for entertaining and, well, I think it looks cool. I am trying to figure out how to calculate the solar gain from this glass to size the heating and cooling system accordingly, and if I can figure that out maybe I can better understand what's going on with the existing house and its ridiculous gain. Maybe it's the glass, maybe the dark brown siding and CMU, maybe the dark asphalt roof, maybe all of the above? Just wanting to learn how to better understand this if someone is willing to teach me.

hot_rod

Those “glass wall” rooms work both ways. Once the sun is gone it is a low R value walk. Some insulated shading helps keep that heat in, and also lower the cooling loads 
Cooling load calculators usually have inputs for glass exposure and shading if you are looking for some numbers

Larry Weingarten

Hi, Do you want the glass wall to be for view or just for light? If just light, there are ways to make fairly high R value translucent walls. Bob is right! I've watched solar homes as they grow up and nearly always, shading and insulating materials are added to those big south facing glass walls.

Yours, Larry

Jamie Hall

Direct solar power -- measured on a plane at 90 degrees to the direction to the sun -- is 1 kilowatt or 3400 BTUh per square metre, close enough so it doesn't matter. Most ordinary glass with transmit around 90 percent of that, though the amount drops off rather quickly if it's more than say 45 degrees off that plane. High e and shaded glass can be much less transmissivity. Double pane windows also have less transmissivity, particularly if they are coated to have better insulation quality.

The problem with passive solar space heating isn't collecting the sun. Your south windows will collect a lot of it. The problem is having it shine on something inside which will store the heat and give it back when you need it. Trombe walls (look it up) are an extreme way to do it -- but with good architecture and some ingenuity you can store your heat and have the view at the same time!

GroundUp

Sorry, I accidentally left some information out. The plan with the glass wall is both for view and solar gain, light is not of any concern. I will have thermal shades on every pane for low light situations and sweltering summer days as I do on all the current South facing glass. Window exposure will be regulated accordingly as it is now. The difference is quite drastic with the shades drawn versus open, but I still have a lot of gain even with them drawn which leads me to believe there's more to it than just the glass. Maybe I'm crazy?

I am not looking to store any heat, but it wouldn't hurt my feelings to save a few bucks on those sunny days

Jamie Hall

What's happening with the shades is they are absorbing the energy -- and then promptly re-radiating it into the space. Either back though the window at a different wavelength, or into your room. Mostly back out through the glass, but it depends a lot on the shade material.

Some very successful partial passive solar designs I've been involved in allow the sun in -- and store the heat in a high mass floor.

With an essentially all glass full height south wall and some care in construction it would be possible to provide essentially 100% of the heat that space you mention needs, even over a few cloudy days, with no sacrifice of the view at all. I've seen it done, a number of times.

lkstdl

...but I still have a lot of gain even with them drawn which leads me to believe there's more to it than just the glass

Are you just calculating your solar gain based on the presumed heat loss and air handler run time? Its certainly possible that your heating load is just not as high as you calculated. There may also be other internal heat gains that could be offsetting some of the load. You could try a direct measurement of day vs night, corrected for outdoor temperature.

If the shades are inside the building, they will reflect some of the sun's energy back outside through the window, but much of it will be trapped inside the building envelope. If you are trying to reduce summer overheating, you could use exterior shutters, fixed overhangs (since winter sun is much lower in the sky than summer sun), or thoughtfully placed deciduous trees.