Need window advice for spa/greenhouse building

desertmarcyFebruary 14, 2010

I am planning a house addition which will be used as combination exercise pool/spa room and sub-tropical/tropical greenhouse. I have questions about windows for it. Since I live in hot, sunny Tucson, Arizona, keeping it from getting too hot in summer is important, but I plan to use the pool year round and so I would like the heat/sun to come in during the winter. I was planning triple pane windows because I wanted the increased sound proofing, but from another thread, I have read that might not be the best option. Still confused about that. But the other question is, the building asks do I want low-e in the windows? I understand where the low-e coating is applied (outside or inside the pane of glass) depends on whether you want to keep heat in or out and that there is also variation in the % light transmission. For growing plants, is there a detriment in keeping some of the UV light out? And is there any option out there that allows for both: more heat/light getting inside during winter and keep it out in summer, or is it only one way or the other? Thank you for your expertise!

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Windows on Washington Ltd

Triple pane provides no STC (Sound Transmission Class, a measure of soundproofing) advantage over double pane.

What you are asking to happen, keep summer heat down, and allow for winter passive solar heating, is effectively a contraction in technologies when you consider the glass alone.

A low SHGC window will cut down on the summer heat but will also cut down and nearly eliminate the passive solar aspect in the winter.

Putting the low-e on surface 2 (inside surface of the outside pane) will cut down on solar heat gain in the summer.

I you are counting on the winter sun to heat it, you may want to opt for a clear glass window with some solar shades to control the summer heat.

    Bookmark   February 15, 2010 at 11:35PM
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Marcy, If I may, make a couple of suggestions. My personal recommendation for you would be to use a fiberglass window. Fiberglass is impervious to moisture, chemicals, salts, etc. for your pool environment. It does not conduct heat or cold (thermal performance) and is not affected by extreme heat & cold either. Vinyl expands and softens (melts) at high temps, and contracts rapidly as it cools. This will cause it to become brittle with age. Aluminum conducts heat & cold, wood requires Paint & maintenance. So I would recommend fiberglass inside and out for your design. Most underground storage tanks, chemical equipment, and boats are made from fiberglass because of its strength and because it performs so well. Vinyl is the most cost competitive product sold today, however, I personally would stay far away from vinyl if your intent is longevity of your windows. Aluminum windows are still popular in your part of the country, however, even thermally broken aluminum will sweat in that environment.

Let me qualify "Fiberglass Window" by saying an INSULATED (cavities foam filled for sound reduction if sound reduction is important) fiberglass window frame. Also Be very careful of what some consider to be or call "fiberglass" and what truly is fiberglass. For instance, certain companies have products that use the word "Fiber" in their name, but it is merely a composite of wood, vinyl, and fiberglass strands for reinforcement. Thats NOT fiberglass. True fiberglass is thermally set during the manufacturing process, and once cured (in about 20 Minutes) its shape cannot be changed by re-heating or melting.

In the interest of full disclosure, I work for a distributor of winodw products, and thus have gained a VAST education in the subject of Manufactured goods & thermal performance. Thus, I wont recommend a specific product, but I will tell you that spending a little more for a fiberglass frame in your area of the country will give you many more years longevity, & they can even be painted (even dark colors) without voiding the warranty. If you think about it, Fiberglass windows & Glass (key components are derived from silica sand) are both made of the same raw materials. Therefore, what small amount of expansion and contraction they do have from hot and cold temps is minimal, but they will move at the same basic rate. This prevents separation of frame & glass, and degradation of the product over time.

Tri Pane windows are expensive, and will improve your thermal performance and your STC/OITC rating, but not enough to justify the expense unless you are in a severely cold climate. If noise & thermal control are your primary concerns, I suggest you ask any window seller to SHOW you their STC/OITC ratings, their thermal (NFRC) ratings, and a Corner cut sample of the product you are interested in. Much can be learned about quality and serviceability by these 3 things.

I do however recommend you use at least a doubled layered (LowE2 - 1 Layered and 1 sputtered) coating on your windows. Double coat low-e reflects heat gain in the summer and retains heat in the winter. It provides some solar heat gain during the winter months as well. This is difficult to explain, but the mental picture is this: The secong coat is sputtered on and done so at an angle. The reason for this is because the angle of the sun relative to the earth is higher in the summer and lower in the sky in the winter. Therefore, in the summer it reflects the suns rays and the winter it allows more of those rays to pass. Since the area is enclosed, without some sun protection, you simply wont be able to cool the area when competing with the power of the sun. I suggest you use solar sun shades for summer time as suggested above, this way you can manually control summer vs winter heat gain, and even on an hour by hour basis. Besides, they will also provide you privacy for a night swim if desired. As for your plants, Low-e coatings still allow a generous portion of the suns spectrum through to nourish your plants. My house is covered in vegetation that performs very well behind these coatings.

I hope my knowledge helps you better understand how these things work. Best of luck! JH

    Bookmark   February 18, 2010 at 1:32PM
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JH, thanks for your thoughtful reply. My contractor was actually going to buy the glass and install/build the frames himself. We are looking at possibly using a wood composite product called TimberSIL. It is a fairly new "green" product, real wood but impregnated with glass. It is rot, termite, and fire resistant and looks like the ideal wood product without nasty chemicals. You can read about it on their website or

Any recommendations for solar sun shade companies? That does sound like the way to go.

    Bookmark   February 20, 2010 at 4:46PM
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