Part IX: Tools, Calculators, and TestingChapter 15. Design Tools and Data
Material and Supplier Guide (Glazing, Absorbers, Insulation)
What to ask for when you buy glazing, absorber metal, and foam, without a single preferred store.
- Difficulty
- Beginner
- Cost
- Free
Buy the data sheet, not the nickname. "Solar glass," "absorber paint," and "the good foam" are not specifications. A glazing has a U-factor and a solar heat gain coefficient. An absorber has a metal, a thickness, and a coating with an absorptance and, if it claims to be selective, an emittance. An insulation has an R-value per inch and a list of places the code lets you put it. Any seller who will hand you those numbers is a possible seller. This page names no store.
The glazing comparison is what the numbers mean once you have them. Where foam goes in a wall is the insulation guide. Supplier lists on other sites, including the links below, are their lists. Use them as a lead and still ask for the sheet.
Difficulty
- Level
- Beginner
- Time
- The length of a counter conversation, if you know the questions
- Skills
- Leaving without a product that has no numbers
Cost estimate
Free
The questions are free. The materials are not, and a cheap sheet with no U-factor is often the expensive one after you have built the collector around it. Tempered glass costs more than annealed and is the one you want where a break would hit someone.
Materials
| Item | Qty | Notes |
|---|---|---|
| A data sheet for the glazing | 1 | U-factor, solar heat gain, transmittance, tempered or not, and which face of a plastic sheet is the UV face. |
| A data sheet for the coating or the metal | 1 | Absorptance, emittance if it is selective, and the temperature the coating is rated to see. |
| A data sheet for the insulation | 1 | R per inch, facing, moisture, and the fire or code limits on leaving it exposed. |
Glazing
Ask whether the U-factor is the center of the glass or the whole window including the frame. A center-of-glass number flatters a small pane in a fat frame. Ask for the solar heat gain coefficient, not for the words "low-e." Low-e is a family of coatings. South glass you want as a heater usually wants a high solar heat gain. Glass you want as a summer window may want a low one. Visible light and solar transmittance are different. A pane can look clear and still reject a lot of heat, or the reverse.
Twinwall polycarbonate has a UV-protected face that goes toward the sun. The sheet needs room to expand, and the edges need the closure the sheet maker describes so the flutes do not fill with dirt and bugs. Film is a short-life cover. Annealed glass is the wrong choice in a door, overhead, or anywhere people stand under or beside a break. The glazing guide walks those tradeoffs. Your job at the counter is to come home with the sheet that states which product you were sold.
Absorbers
Copper and aluminum both make absorbers. Thickness buys stiffness and a little stored heat. A selective coating absorbs most of the sun and emits less infrared than flat black, which matters when the plate runs hot. If the seller cannot tell you an absorptance and an emittance, you have black paint, and black paint is an honest product if you call it that. Ask what temperature the coating survives. A collector with no flow, in full sun, stagnates well above boiling. A paint that powders at that temperature is a repaint and a mess on the glazing.
Potable water does not touch an unknown paint, an unknown flux, or an absorber you cannot identify. The hot-water choice guide is the boundary. Air collectors are more forgiving of the coating because the air is not your drinking water, and they are not forgiving of a coating that fills the room with fumes the first time it gets hot. Ask, and heat a scrap outdoors before you fill a box with it.
Insulation
R per inch is the comparison, at a temperature near the one you will use. Some foams change as they get cold or hot. Facings are vapor and fire details. Exposed foam is a code and a fire question. A product that is fine behind drywall may be forbidden as the inside face of a shop. Below-grade insulation is a wetter job than an attic batt. Mineral wool, rigid foam, and a scrap of whatever was on sale are not interchangeable because they are all "insulation."
Do not put a vapor-closed layer on the cold side of a wall because the foam was cheap. The insulation guide and a local builder who knows your climate outrank a general page. This book will not draw a wall section for every county.
Ask for U-factor and solar heat gain on glazing
Center-of-glass or whole unit. Tempered or not. UV face on plastics. Low-e is not the answer by itself.
Ask for absorptance, emittance, and temperature
If there is no emittance, call it paint. Keep unknown coatings out of potable water.
Ask where the insulation may be left exposed
R per inch, facing, moisture, fire. A batt from the sale bin still has a data sheet or it is a guess.
Leave if the sheet does not exist
No store is preferred. A seller who will not produce numbers is the one to skip.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/References/Calculators/
- https://www.builditsolar.com/References/Source/TMYWeather/TMYWeather.htm
- https://www.builditsolar.com/References/Source/RadOnColProgram/RadOnCollector.htm
- https://www.builditsolar.com/References/SketchUp/SketchUpEx.htm
- https://www.builditsolar.com/References/Suppliers/SolarHeating.htm
- https://www.builditsolar.com/References/HowMuch/HowMuch.htm
- https://www.builditsolar.com/References/BestRefsRS.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.