Part V: Passive Solar Homes and SunspacesChapter 9. Sunspaces, Greenhouses, and Solar Rooms
Glazing Materials Compared: Twinwall, Glass, Film
Transmittance, insulation, weight, and lifespan of the covers people actually buy.
- Difficulty
- Beginner
- Cost
- Free
The cover on a sunspace or a greenhouse has to let sun in, slow heat leaving, survive weather, and fail in a way that does not injure people. No common material wins all four. Film is cheap and temporary. Twinwall plastic is light and a modest insulator. Glass is clear and heavy and, if you buy the right kind, long lived.
Ask for numbers. Transmittance is how much light gets through. U-factor is how fast heat leaves. Solar heat gain coefficient is how much of the sun becomes heat inside. A coating sold as low-e can be a high-gain coating or a sun-blocking coating. The overhang page makes the same point for house windows. Read the data sheet for the product in your hand.
Difficulty
- Level
- Beginner
- Time
- An hour with three data sheets
- Skills
- Comparing U-factor and transmittance instead of brand names
Cost estimate
Free
This page only compares. Film is the cheap cover and the one you will replace. Twinwall and tempered glass cost more up front and usually less over a decade. Get local prices. Overhead glazing is not a place to save with the wrong glass.
| Cover | What you are trading |
|---|---|
| Single plastic film | High transmittance, almost no insulation, a few seasons of sun |
| Double film, inflated | Better than one layer, a small blower to keep, still a film lifespan |
| Twinwall polycarbonate | Lighter than glass, some insulation, less clarity, a UV-coated face that must go out |
| Glass | Best clarity and the longest life, the most weight, and a safety-glazing rule |
Materials
| Item | Qty | Notes |
|---|---|---|
| Data sheets | One per product you might buy | Transmittance, U-factor, fire notes, and whether the sheet is rated overhead or only vertical. |
| A sentence about the job | 1 | Heating porch, greenhouse, or a window in a lived-in room. The room changes the acceptable risk. |
Film
A single layer of greenhouse film transmits a lot of light and insulates about like a tent. Heat leaves almost as fast as it arrives once the sun is gone. Ultraviolet light makes cheap film brittle. A lifespan of a few seasons is normal, and a windy site shortens it. Double film inflated with a small blower adds an air gap and a real improvement in U-factor, and it adds a fan you notice when it stops. Film is a reasonable cover for a seasonal growing frame. It is a poor night wall for a room attached to a bedroom.
Film rattles, it condenses, and it is not a security layer. Do not use it as a stand-in for safety glazing in a door.
Twinwall
Twinwall polycarbonate is two or more skins with ribs between them. Thicker sheets insulate better and transmit a bit less. A U-factor around half a BTU per hour per square foot per degree is a fair mental picture for a common twinwall, and the sheet you buy may be better or worse. Use its number in the greenhouse water sketch.
The UV-protected face goes toward the sun. The other face chalks and cracks if you flip the sheet. Cut edges need the maker's closure so the ribs do not fill with dirt, algae, and bugs. Twinwall is light enough for a wood frame that glass would overload, which is not permission to skip the structure. It expands more than glass. Holes and gaskets have to allow that, or the sheet buckles and leaks.
Impact resistance is better than annealed glass. It is still plastic. A fire rating, if you need one near a house wall, is a product question and a code question. Do not assume a greenhouse sheet is allowed as the wall of a living space.
Glass
Glass stays clear, does not chalk, and can last as long as the building if the installation drains. It is heavy. The frame, the stops, and the people carrying the pane have to be ready for that. Annealed glass breaks into large pieces. Tempered glass breaks into small cubes and is the usual safety glazing beside doors, near floors, and in many overhead cases. Laminated glass stays in the opening when it cracks, which is why people use it overhead. Match the product to the location, and do not save money with an annealed lite in a door.
Double glass with a high solar heat gain coefficient is the house-window choice for a south heating wall. A sun-blocking coating will look modern and then fail to heat the room. For plants, you also care about the light spectrum the crop needs. A dark coating that is fine on a west bedroom can be the wrong roof for a greenhouse.
Condensation between failed seals of an insulated glass unit is the end of that unit. Drain the rabbet. Do not set the edge of the glass in standing water.
Name the room
A seasonal frame can be film. A greenhouse that must hold heat wants twinwall or glass with a known U-factor. A lived-in wall follows the window code, not the greenhouse catalog.
Collect transmittance, U-factor, and weight
Ignore a label that only says high performance. Write the three numbers down.
Check the failure and the face
Safety glazing where people and overhead loads require it. UV face out on polycarbonate. Expansion gaps on plastic. Drain the glass rabbet.
Shade the summer anyway
A better U-factor does not stop June sun. Overhangs and exterior shade still apply.
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/Projects/Sunspace/sunspaces.htm
- https://www.builditsolar.com/Projects/Sunspace/LowThermalMassSunspace.htm
- https://www.builditsolar.com/Projects/Sunspace/LTMSSGuide/LTMSSGuide.htm
- https://www.builditsolar.com/Projects/Sunspace/SolarRooms/SolarRooms.htm
- https://www.builditsolar.com/Experimental/GlazingTransmittance/GlazingTransmittance.htm
- https://www.builditsolar.com/Projects/SpaceHeating/Glazing.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.