This is your annual fall reminder that by removing your window screens, you can achieve a dramatic increase in solar heat gain coefficient (SHGC) through your windows for free.  I notice on a State of Hawaii web site, they give a default value of 0.7 for the SHGC of a generic window screen.  Therefore, if you remove a full window screen, the SHGC for the window will be increased by about 1/0.7 = 1.43, or a 43% increase in solar heat gain.  If the window screen covers half the window, then the SHGC will increase by about 21%.  

Removing the screens provides an opportunity to wash the outside of the windows, for first floor windows anyway, further increasing the heat gain.  In addition, with the shorter winter days and, in some locations, more cloudy weather, removing the screens brightens the interior of the house and the mood.  Of course, if you need window ventilation all winter and you have bugs through the winter, better leave the screens in place. 

I removed by screens earlier this week and washed the windows, and you can feel the gain. 

I just read your post, and was surprised at how much screens block heat/light. Thanks for the tip, I'll use it. The tip should be a no-brainer, but it wasn't. (^^)

My screens are on the inside - so I assume there is little or no heat difference with them on or off.

The shading effect caused by screens is the same whether they are inside or outside. So if you want a higher effective SHGC and associated passive solar heat gain into the building, remove the screens.

On the other hand, if the screens are on the inside, they will heat up and provide some convective heat gain inside the building. So while you won't get as much sunlight into the building, the lost heat gain won't be as bad had the screens been on the outside.

I think I'm missing something. I can see how removing exterior insect screens will increase the SHGC. Scant as they may be, they block a fraction of incident sunlight. But, when I read: "The shading effect caused by screens is the same whether they are inside or outside. So if you want a higher effective SHGC and associated passive solar heat gain into the building, remove the screens.", I wonder how the room gets warmer with the INTERNAL screen removed. The insect screen reflects little. When the sunlight hits the screen, the infrared portion either passes or is absorbed by the screen. Either way it seems the room internals are being heated. I'd say, leave your internal screens up if you like or store them; either way heating from the sunshine won't materially change.

Posted By wdongbt on 04 Jul 2014 03:37 PM
I think I'm missing something. I can see how removing exterior insect screens will increase the SHGC. Scant as they may be, they block a fraction of incident sunlight. But, when I read: "The shading effect caused by screens is the same whether they are inside or outside. So if you want a higher effective SHGC and associated passive solar heat gain into the building, remove the screens.", I wonder how the room gets warmer with the INTERNAL screen removed. The insect screen reflects little. When the sunlight hits the screen, the infrared portion either passes or is absorbed by the screen. Either way it seems the room internals are being heated. I'd say, leave your internal screens up if you like or store them; either way heating from the sunshine won't materially change.

sailawayrb made the statement that you are referring to, so I will let him answer. I think that what you are saying is similar to what sailawayrb is saying, except that he is more careful with his wording.

However, I will point out that you have oversimplified the light-scattering process. For both the visible and infrared portions of the solar spectrum, the light may pass through the holes in the screen mostly unaffected (ignoring diffraction effects), it may reflect off the screen material, or it may be absorbed by the screen material. The light reflected off the screen material may reflect into the room, or it may reflect away from the house. Aluminum screens are likely more reflective than plastic screens (except maybe for white colored plastic screens), and aluminum is more reflective in the infrared than the visible. You have neglected the solar energy reflected away from the house in your statements.