Adjusting the blinds is a constant admonition from energy companies, blinds manufacturers, air conditioner manufacturers, bloggers, news outlets, and parents.
The following is something we can agree on right off the bat. It is indisputable that if you leave the blinds open in the winter time, the sunlight can come in and will warm up the house. Not every wavelength of sunlight that comes into the house has the effect of warming it up though. It is the light at the red end of the light spectrum that warms us and our homes, specifically the infrared (beyond red) light waves. This basically leads to a greenhouse effect in your home where infrared light gets into your house becomes convective heat and that heat stays inside the house (greenhouse) because the glass, walls and ceiling keep it there. This is a great thing on a cold winter day.
Electromagnetic Spectrum - Britannica.com
In the summer the same sunlight enters your house for many more (potential) hours per day through your windows and will warm up the house using that same greenhouse effect. Since we know that our house becomes warmer in the winter because we let the sunlight in, it makes a certain amount of sense to suggest that if we close the blinds we will keep the sunlight-generated heat out of the house in the summer. Right?
But if we think about it a little we'll discover that it isn't true.
Experience tells me that no matter what I do with the blinds, my home becomes hot in the afternoon. Why?
Let's start with a pretty solid fact. If the blinds were outside your windows (like shutters on houses used to be) and were reflective, they would keep most of the heat from the sun's light from entering your home. Some heat is always going to be transferred from the outside walls from the sun to the interior of your home depending on the materials the wall is made of and the amount (if any) of insulation in the walls.
If instead of "outdoor blinds" or shutters you used aluminum foil on the glass as a way to intercept sunlight's heat you would be pretty successful at keeping the heat from the sun's light out of your home but that would be a visual hazard and an annoyance to your neighbors. A reflective film placed on the glass should be good enough and there are companies that sell that product for windows. Think the window darkening that's used on your car's windows.
Stopping the sun outside your home keeps the heat from the sun's light outside your home. This is a fairly safe statement, generally.
Unfortunately, the situation that most of us are stuck with is using blinds that are hung inside our home 0.5 to 4 inches away from the glass. The problem is that once the sunlight goes through the glass, the wavelengths that cause heat in your house cannot easily get back out. Visible light is reflected back out, so your white blinds may do a fine job of reflecting light back outside, but it is not the right kind of light. As shown in the graphic below the short-wavelength infrared light stays in your house and becomes convective heat between your blinds and the window. The blinds are warmed up and radiate the heat in all directions.
Greenhouse effect - Comsol.com
Remember also that glass is an insulator and if it is double-paned or triple-paned glass it is an even better insulator than a greenhouse! Heat has a difficult time getting through it. Once the infrared light strikes a surface in the home (ie., the blinds) it becomes convective heat. That heat will rise behind the blinds to the ceiling and circulate with the air in the room. Very little of the heat that enters through the window can escape back through the window and since it is probably hotter outside (which is why you are worried about keeping the heat out) heat is not likely to travel that direction anyway, even if it could get through the double-paned window. Heat travels to cool areas not to hot areas so it is not natural to make the assumption (which we do) that the heat will be reflected back outside to a hotter environment when thermal entropy suggests that heat always seeks cooler areas. The conductive heat from outside near your window is heating up your window because it is seeking a cooler place (ie., your air conditioned or cooler home). Heat is already fighting to get in your house, how likely is it to get out?
So as we can see, there are a number of things which argue against closing blinds to keep the heat from the sun's light out of our homes.
- Infrared energy will not easily go back out the way it came in
- Visible light is reflected back out but that kind of light does not heat up the house anyway
- Glass windows do a very good job at insulating
- A dark house is a gloomy house
Yes the sun is making your home warmer in the winter and in the summer but there is nothing you can do short of stopping the sun's light outside your windows.
So don't live in a dark cave for nothing, let the sun shine.