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Best window material for passive solar heating


Joined: Dec 05, 2010
Posts: 2
Hello all,
I'm currently adding a bedroom to the side of my house, and the whole south facing wall has been left open with the intention of capturing as much winter sun as possible and storing it in the concrete floor. Does the type of glass make much difference to the overall effect? Would triple glazing help or hinder? I'm expecting to use thick curtains at night to stop heat radiating back out. I haven't looked at prices yet, but I'll be going to the glass man this week.

My main concern is that I already have a full height window in the living room, and it seems to do very little to heat the floor, even on bright sunny days. How much of a difference is tile choice likely to make? I would guess that thick, unglazed (expensive) would be best, but they make it much more difficult to clean

Any help welcome


Joined: Dec 01, 2010
Posts: 158
Location: Abilene, KS
Yes, the window glass does make a difference.  We bought double paned windows that have some of the bells and whistles, and I have a hard time keeping a houseplant alive. There's not much solar gain at all, either.
The cheap patio door we had let in tons of sun, great for plants and warmed up the room fast in the mornings.  When we had to replace it, we bought a better one, and now it's the same as the windows.
The flip side is that the better windows block rays that sunbleach everything, and they are a lot better at blocking cold air.  Ditto with holding down the heat in the summer.  You might want to consider a pelmet type window treatment.  That will stop the air from washing down the cold window at night (between the glass and shade).

IMO, regular glazed tile is fine.  Since you'll be putting it over concrete, it's going to take an awfully long time to 'charge', or warm up, but both tile and concrete will hold heat.  Do you know if there is any kind of insulation under the concrete slab?  Just a side note, even room temperature tile is still cold on the bare feet.

Be kinder than necessary, for everyone you meet is fighting some kind of battle.

Joined: Dec 04, 2010
Posts: 79
As I recall from my college class on solar energy, the best type of glass for solar gain is a low-iron glass.  This would look almost clear when looking on the edge.  Most glass has a medium to high iron content, which can be observed by looking at the edge of the glass, whcih will be green in tint.

Joined: Dec 01, 2010
Posts: 158
Location: Abilene, KS
I forgot to mention that you can get tons of info about passive solar at
Ardilla Esch

Joined: Feb 05, 2010
Posts: 183
Location: Northern New Mexico, Zone 5b
Each layer of glass and low-e coating will decrease the solar gain, but will also increase heat lost through the window.  Some people use windows without low-e coatings for solar gain.  The better window companies will use specific coatings for windows intended for solar gain vs thermal retention.

There are trade-offs for all the options.  For most climates I would go double-glazed windows either with no low-e coating or with a minimal low-e coating.

Joined: Nov 30, 2010
Posts: 53
aye its the coatings and films and perhaps the gas fill to a extent that determines the solar gain.  they spatter the inside of the glass with a metallic substance that reflects radiating heat. high solar gain windows do not coat with this. and also the interior membrane film may of be of a different make up
Joel Hollingsworth

Joined: Jul 01, 2009
Posts: 2103
Location: Oakland, CA
Years ago, I read an article on multilayer birefringent films.  These are wavelength-specific, so, theoretically, they could be tuned to allow in visible frequencies and near infrared with maximum efficiency, and then to reflect back the far infrared that warm objects tend to emit.

This is a type of plastic, though, so it would degrade in UV, and so might require a titanium dioxide coating on the outer surface of the window in order to have a reasonable service life.

I'm not sure how this technology has done on the market, since last I heard about it.

"the qualities of these bacteria, like the heat of the sun, electricity, or the qualities of metals, are part of the storehouse of knowledge of all men.  They are manifestations of the laws of nature, free to all men and reserved exclusively to none." SCOTUS, Funk Bros. Seed Co. v. Kale Inoculant Co.

Joined: Nov 30, 2010
Posts: 53
thats what i am thinking a sort of polarized film (and one or two uncoated film) so that high angle (summer) sun would be filtered but lower (winter) sun will pass thru. not sure if its possible perhaps a thick film so one could use huge windows where large overhangs would not be aesthetic. perhaps an adjustable shutterfor summer use replacable with a insulated shutter for winter night and storm use. although need to find a way to remotely close especally on second floor as no one wants to open window losing heat to save heat.... i seem to be brain storming instead of responding .. sorry. thanks i'll look up the wave length film

Joined: Dec 04, 2010
Posts: 79
There is a product used in greenhouses to automaticly open venting windows when the temperature exceeds a certain value.  Perhaps you could adapt that to operate your upper shutters.  This functions automaticly, no controls required.

Joined: Dec 05, 2010
Posts: 2
Thanks for all the replies, and sorry for not getting back to you all sooner. The product Joel mentions sounds ideal. Unfortunately, we don't live in an ideal world, and the window manufacturers all gave me blank stares when I went round asking about "low iron glass" and such things. Here, there's two options: Normal glass, or reflective glass. Hence, I'm going with normal, but I guess my passive heating dreams are out the window, so to say.

Joined: Dec 01, 2010
Posts: 158
Location: Abilene, KS
I feel your pain there, Marlin.  I planned on a lot of passive solar in this house and get very little because of the 'better' windows and doors.  I don't know where you're located, but here in Kansas, we only have about 6 weeks of the year that we have the windows open, the rest of the time it's a/c or heating.  The a/c is more of a drain on energy for us, and that's where the 'better' windows really perform.
On the builditsolar link above, there are tons of projects for the DIY'er, and I bet there's something that would look good and be just right for you.  For myself, I finally got to the point of "First burn wood.  Then figure out ways to burn less wood."

But I still have the lingering dream of passive solar at it's best.
ronie dee

Joined: Mar 04, 2009
Posts: 594
Location: Cosby MO
I bought the Sunlite PremiumII  in 1975 (mentioned in the article) and it is still in good shape 35 years later. Plants seem to love it too.

[Thumbnail for books_003.jpg]

Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.

Joined: Jul 07, 2010
Posts: 508
can't offer any help with the glass question but this guy has developed a pretty neat way to make a portable solar heater..he keeps getting it simpler and simpler and all of them seem to work..perhaps there would be somewhere this might work for you.  certainly not a big investment 
I have some cans I am going to but in a box,paint black and put in a window of an unused room to see if it does anything..hopefully it won't crack the glass of the window!

Joined: May 12, 2010
Posts: 52
Hi Marlin,

I am thinking about the cement as part of your problem in the living room.

Of course the windows make a difference.

I am thinking cement is not as good a storage material as cob (clay and sand) is because it wants to be wet.
It will take the moisture from the ground and evaporate this moisture into the room creating evaporative cooling.
It might work better (if you have not already laid the cement) to put a pond liner or moisture barrier under the cement??
Perhaps painting the cement with a moisture proof deck paint would work

If you have a chance, maybe you could play around with this before it is too late.

Anyway, I am probably too late for you, Marlin, but maybe someone else can think about this before it is too late??

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