Blinds open, closed or doesnt’ matter
geoffhazel
| Posted in Energy, Heating & Insulation on
West exposure, large 3-bay window, each bay with a metal mini-blind (indoors, of course). Blind color is a light brown/tan. When the summer sun swings around to the west, I wonder if I can keep the house a bit cooler if the blinds are open or closed? If they are closed, they warm up by the sun and radiate the heat into the space nearby. If they are open, the sun streams in and strikes the walls, cabinet, and floor in the kitchen and THEY warm up.
I think I’m better off if the blinds are closed, at least they are reflecting SOME of the energy back out the window.
But I’m also thinking that once that sunlight gets through the glass, the heat energy is IN the house and the blinds open or closed doesn’t matter too much.
What say ye?
Replies
I say closed. That way the blinds will send some of the radiant energy back out through the glass (I think--I am not a heating/cooling expert and only know that there are three laws of thermodynamics, but can't recite them word for word!) and if the blinds get heated up, they have little mass to hold much heat. Let the sun heat all the mass in the room up and then you have more of a load to get rid of. That's my story anyway!
closed, esp. if you are running ac
With the newer window films on the market I'd be on them like flies on stink. You have no idea just how much heat and cold they control.
Oh, closed if you don't mind the look.
Edited 6/2/2009 10:45 pm ET by RobWes
Have a small townhouse which faces southwest. Have newer Anderson sliding doors with a film. What a difference from the old sliders. Can keep the vertical blinds open or semi-open during the afternoon and the living room is comfortable whether the A/C is on or off, depending on the outside temperature.
Geoff Hazel,
It may be too late for you, but other readers should pay attention to the lesson behind your tale: Learn the solar-heat gain coefficient (SHGC) of any window BEFORE you install it in a west wall. Aim low, people. What a difference a good window will make. Near my home, I occasionally visit a relatively new bank building, and during the afternoon the foyer is unbearably hot, even on cool days. The AC can't keep up. Stupid architect and stupid builder put in high-SHGC glass.
While I certainly share your sentiments on this issue, having been there and done that, there are other scenarios that are as likey as incompetence. Not that incompetence can be ruled out by any means.
The engineer may have assumed a low SHGC glass, or the archy spec'd a low SHGC glass and someone "value engineered" (I hate that term because it neither adds value nor resembles engineering) the project after the fact, or some bright soul said "wow, 200 sf/ton, that can't be right"...
Yes, way too late for me. These are 1986 vintage double panes that replaced the vintage single pane 1960 windows. We have plans to replace them once again, and I'll keep a close eye on what goes in when (and if) we ever have the $$.
An double pane, untinted/uncoated window will pass approximately 85% of the incident solar radiation. All things being equal, the same window with light colored blinds (closed, and adjacent to but not touching the glass) will pass approximately 60%.
Where are you getting those stats? Everything I've learned says that the difference will be much, much less, due to the properties of long-wave and short-wave rays.
I'm no expert, though.
This is general design data, empirically derived from the last 40 years of HVAC design or so. I use these basic numbers when no actual data is available, but of course, use actual tested performance when available. The generic terms "light" and "dark" when refering to the color blinds is subjective.
You got a reference or source material?
k
ASHRAE Fundamentals book ... or any reputable heat gain reference manual will have it if your Fundamentals is not handy.
60 is a 25% reduction from 80, so I'd say that's significant. You got a page number in your ASHRAE manual? Or a chart you can post for window coverings (inside the glazing)?
Not, doubting anybody- just looking for hard stats.
Yes it is significant ... changing to low SHGC glass can reduce it from 60% to say 20% ... comparatively speaking, the 25% savings is small compared to what it could be. West facing glass, sqft for sqft can be a VERY significant problem. 25% reduction of a BIG problem still leaves you with a big problem. Better than nothing ... absolutely!
People spend big bucks on dealing w/ solar problems AFTER the sun has passed through the window ... 4, 6, 10+ dollars a square foot ... to block the light/view that they spent a lot of money on windows to create in the first place. Windows is a pet peave of mine .... Design it better, choose the glass carefully, too. You end up with a better solution ... more comfortable, energy efficient, and easier to control. It boggles my mind how this is ignored. People don't give a rip about orientation, glass, etc. when building, but when the problem manifests itself later on ... man ... they complain like they were tied w/ a new rope!
Don't forget things like landscaping, too. It can help prevent the sun from hitting the house/windows.
Not to keep beating a dead horse, but, I guess y'all pulled that 85% to 60% statistic out of yer butt. (And I realize you didn't originally post the stat, C1, but you kind of stepped in to verify it.)
Sources? References? Anyone?
If you're going to throw numbers around, please say where you're getting them. Especially if it's from your own keister.
If it's not from your keister, please, pretty please, what's your source?
k
p.s. Sorry for the challenging words, but I keep asking nicely, and all I get is more opinion. Could be completeley correct opinion, but opinion should not masquerade as statistical fact. See Piffin's response- he qualifies it as his own anecdotal experience, says it does help, and leaves it at that. Proper.
I'll look for a source handy to me when I get a chance. Sorry.
A point to be made is that specific conditions can easily change such values. I always use the rule of accuracy vs. precision. You can get a precise number for a specific thing, but that really doesn't do much good if we are talking generalities. Not sure if this is writing out clearly ... it's late. White blinds will differ from dark blinds. The type of blinds can also create variations. Curtains or shades can do the same.
So even if I find an ASHRAE table ... it's numbers will be somewhat general or be a range of values that in itself is general. Great for accurate calcs ... just not precise calcs ... but in this business, in general, accuracy is more than enough, for weather and climate can vary +/-25% and more.
Your inquiry, however is still respected ... let's see if I can find some figures. Maybe someone else will jump in as well ... my references are a bit scattered, so not always at my fingertips like I like them to be. ... like Ahnold ... I'll be back ...
No. No reference. I have an excellent source of my material, however, its not anything you can look up.
I learned it from a man who had been doing this for 30 years at the time when he taught me - 20 years ago.
I have a collection of notes and data without sources that I have been relying on for that period. It works, I know it. Call it "empirical" data if you will.
Fair enough.
k
Standard ASHRAE heat gain values, I think. While having the blinds closed helps, they would be MUCH better if on the outside of the window. The difference IS small ... relatively ... 80%+ down to 60%+ is relatively small.
Spray the back with silver paint and blind the neighbors.
Keeping the blinds closed when sunlight is striking the windows will keep the room significantly cooler, in most cases. Less so if you have glass with some sort of an energy-reflective coating.
Closed is definately better; it will reflect at least some of the heat back out ... but you are right. Once the sun has passed through the glass, the damage is largely done.
MartinHalloday is very right. People don't usually think very well when it comes to window placement, design, and glass choices. You can (and often should) get different glass choices for different orientations.
First line of defense is to avoid west glass area and protect south glass w/ an eave to shade in the summer. NOW choose the glass type(s) based on the orientation and the need. Focus on low-e glass w/ summer sun control values for the west glass. Use tinting & mirror finishes to augment this.
There are lots of design options available to avoid the summer sun overheating the rooms. People need to use them more .... IMHO. Contractors, designers, and home owners need to begin to understand options when designing new and replacing existing. They can make a huge difference in both comfort and energy costs by putting some forethought into the windows.
One thing I'd like to caution people about when using reflective glass is to be aware of where those windows are reflecting the sun to! I worked at a house where the vinyl siding was actually melted in places from the sun hitting it after being reflected (and concentrated) by some windows! Opened my eyes!
I often thought it would be cool to build a facade that was parabolic and sheath it in mirrors or mirror glass and have a fire pit or torch or similar at the focal point. Be kind of unfortunate for anyone standing in the way though, and since the Earth moves, this focal point would be changing. Thought also of doing similar on a seashore so sound of waves was focused on a patio where the guests relaxed.
Edited 6/7/2009 12:15 pm ET by Danno
On the University of Minnesota campus is a (relatively) new alumni building that is an oddly-sited cube of gold-colored reflective glass. It is fondly referred to as "The Death Star".
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Ours are white and it definitely helps to close them on hot days
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