My wife and I have a debate about the need for window blinds in
keeping a room cool during the summer. All windows are double pane
insulated. One of us thinks that it's important to keep the window
blinds closed all day. The other thinks that the losses are relatively
insignificant until the sunlight starts shining directly into the
room, and then at that point the blinds should be closed to reflect
the light back out of the house.
A brief comment would be welcome.

Of course, the amount of heat entering an undraped window is much
higher when it has direct sunlight on it.  But blinds can help prevent
heat from entering the house even when there's no direct sunlight,
particularly when it's hotter outside than inside.

Here, for example, is the advice of Energy Savings Now: 

Tricks & Tips
"Windows have the lowest insulation value of any element of the
building envelope. They also permit radiant energy to enter a space,
as opposed to conduction through the walls and roof. Thermal radiation
accounts for a large portion of the energy in natural sunlight.
Closing curtains or blinds will reduce the radiant contribution of the
light and also reduce the conduction of thermal energy through the
window. An air space is created between the blinds and the window
that, since air is an outstanding insulator, increases the resistance
to heat flow. Double pane windows take advantage of this same premise
by trapping an air pocket between the panes. This enhances thermal
performance. In the heating season the opposite applies. Radiant
energy entering through the windows will augment the space heating
system."
"Double paned windows with blinds are ideal for all seasons. The
insulation is needed in all seasons to retard the heat flow out in the
winter, and vice versa in the summer. The blinds can either block or
enhance the flow of solar radiation."
http://energy.saving.nu/tips/envelope.shtml

In other words, if it's hotter outside than inside, keeping blinds
closed can help reduce air movement that helps transfer heat inside
the house.  If there's little difference in ambient temperature, it
won't make a lot of difference.

Blinds also have the advantage of protecting furniture:

http://www.pro-tint.net/residentialTinting.htm
*Even without direct sunlight*, ultraviolet can fade valuable
furnishings because unprotected clear glass allows harmful and
damaging ultraviolet to enter your home. [emphasis added]
http://www.pro-tint.net/residentialTinting.htm

Of course, light is a form of energy. Any light coming in will be
absorbed and turned into heat, even if it's not direct light.

Also:

Summertime Tips for an Energy Efficient Home 
"Close drapes, blinds, etc. during summer months to prevent sunlight
from entering the rooms. Allowing direct or *indirect sunlight* into a
room will increase the temperature. Block out the light radiation as
much as possible." [emphasis added]
http://www.energizeamerica.org/Residential/hottopics-tips.htm

I hope this settles your disagreement.

Sincerely,

Mvguy-ga






Google search term: "energy conservation" blinds
://www.google.com/search?hl=es&ie=UTF-8&oe=utf-8&q=%22energy+conservation%22+blinds&btnG=B%C3%BAsqueda+en+Google

Hi Boomering,

I certainly hesitate to become involved in a marital dispute of any
kind.  But, living in the h-o-t south as I do, your question intrigued
me and, even though mvguy-ga has answered it, I decided to have a
quick look for myself.

Interestingly enough, I found a source here:
http://energy.saving.nu/solarenergy/passive.shtml that seems to say
the opposite.  That is, it gives a very comprehensive, detailed, and
well-illustrated, albeit not too terribly technical explanation of the
interactions between light, windows, and shades or blinds, and
basically says that light entering a window at an angle of 30 degrees
or less is mostly reflected. This would of course tend to support
whichever of you is of a mind to wait until sunlight is more directly
entering the window before closing the blinds.

Furthermore, it says that internal blinds or shades "are generally the
least effective shading measures because they try to block sunlight
that has already entered the room," although it does then go on to say
that "simple white roller blinds keep the house cooler than more
expensive louvered blinds, which do not provide a solid surface and
allow trapped heat to migrate between the blinds into the house."

Some other sites also support that viewpoint when they recommend that
west or south facing windows be shaded during the day, as the angle of
light entering the windows from these directions is likely to be
greater than 30 degrees during much of the day.  I'd add east as well
if you live in the south, since even early morning sun can
significantly heat up a room.

Here are a few of these other sites:
     http://www.acustomwindowtreatment.com/cellular-shades/fire-resistant-shades/energy_savings.htm
     http://www.greenbuilder.com/general/articles/AAS.windows.html
     http://www.care2.com/channels/solutions/home/82

And, in addition, double paning by itself is apparently not as
important as some other considerations, such as whether or not they
are of the correct low-emissivity (low-e) type.  Here are a few other
links for you to check out regarding that subject:
     http://www.eere.energy.gov/consumerinfo/refbriefs/eb2.html
     http://www.greenbuilder.com/general/articles/AAS.windows.html
     http://www.lib.niu.edu/ipo/ic970818.html    
     http://www.acustomwindowtreatment.com/cellular-shades/fire-resistant-shades/energy_savings.htm

Finally, here's an interesting report on where the future might be
headed in terms of providing daylight while reducing solar heat gain:
http://216.239.51.104/search?q=cache:-AL7p0sbSjsJ:www.arc.gov.au/pdf/cool_sunlight.pdf+solar+heat+gain+windows+daylight+sunlight&hl=en&lr=lang_en&ie=UTF-8

I don't know if I've made things better or worse, but just wanted to
pass the information along for what it's worth.  Good luck in staying
cool - and settling your debate!

--Byrd