I’m planning to build a small greenhouse with old windows. Is there anything I should be aware of? Do I need double glazed windows?

 John M. — San Jose, California

Safety is the main issue. If you build a greenhouse with old windows, never use them for the roof. A window can break if something falls on it, or from snow loading or wind shear or, in your area, because of an earthquake. Falling glass can be very dangerous, possibly blinding or even killing someone.

The same is true for glass doors. Make sure that any door and, ideally, any window that operates by swinging upward, has safety glass which will shatter into small, blunt pieces on impact, not into sharp wedges that can cut you. (Home safety glass is similar to windshield glass.) Use Lexan, windows with safety glass or code-approved skylights for the roof.

Also, building a greenhouse with old windows can be a lot of work. It’s often hard to find enough old windows of similar size and style to build with, and many old windows need to be scraped down and puttied and repainted.

Your windows do not have to be double-glazed. Double glazing gives you about three times as much resistance to heat loss (R-value) as single glazing. But that’s still not enough to make a significant difference in performance in most climates, because single-glazed windows have almost no resistance to heat loss. (Also, older, discarded double-glazed windows have often have broken seals which allow the insulating gas between the panes of glass to escape.)

To keep a greenhouse with single or double-glazed windows heated (which is probably not necessary in your climate), you would need to throw an insulated blanket over it at night and/or use a lot of energy running a space heater. Single glazed windows will work fine  for a small garden greenhouse.

Building a greenhouse with old windows can be very rewarding. For one thing, it will save you quite a bit of money, if you have the time and patience to fix up the windows. Done well, it will give you a charming result that you can’t duplicate with shiny new windows. And of course you’ll be reusing material, which is good for you and the planet.

~PSW

I’m confused about the difference between passive solar heating and active solar heating. Don’t they both use fans, making them both “active?”  Which system is best? 

 Becky S. — Portland, Maine

Strictly speaking, passive solar heating systems are not supposed to have any working parts other than mechanical airway baffles that do not need external energy to operate — that’s why they are called “passive.” By contrast, most active solar heating systems use electric pumps (needing external energy) to move fluid that’s been heated by the sun through pipes in the building. However, many “mostly” passive solar heating systems incorporate electric fans which, of course, need external energy; thus the confusion between the two systems.

Learning how each system works will help you understand the difference between them, so let’s consider a some typical designs.

The simplest passive solar heating systems allow sunlight to pass through a building’s windows, glass doors or skylights during the day, warming a “thermal mass,” a heat-retaining material such as a masonry chimney or a concrete or tile floor. At night, the warmed thermal mass releases (radiates) its stored heat back into the room.

Variants on this design include “passive solar greenhouses” — with their own built-in thermal mass — attached to the outside of a building.

Heat that’s collected outside the building must be transferred inside. This is usually accomplished by passing air over the thermal mass to pick up its heat, then moving the warm air into the building. Since warm air rises, it can be moved by natural convection if the greenhouse is on the lower story of the building.

But natural convection is not always the most efficient means of moving air, so many passive solar heating systems incorporate small fans to push the warm air along and, sometimes, to return the cool air from the building back to the greenhouse for reheating.

 Active Solar Heating

Active systems are designed from the outset to capture heat in one place and move it to another. A typical design consists of panels on a roof through which a fluid circulates. The sun warms the fluid, which is then moved by an electric pump into the building where it can be used for direct space heating via radiators, or indirect heating through radiant floors or walls. (The warm fluid heats the floor or wall, which in turn radiates its heat into the living space.)

The fluid can be water, so long as antifreeze is added to keep it from freezing in the panels on cold nights or some winter days. Other fluids can store heat more efficiently than water, but can be more expensive and possibly release toxins if there is a spill.

Many factors determine which design will work best for a particular building. However, as a rule of thumb, choose the design with the fewest working parts to reduce maintenance costs and the chances of a breakdown. Also, avoid complex piping systems and toxic materials whenever possible. Finally, a multi-use design such as a passive solar greenhouse which helps heat your house and serves as a three-season room where you can grow plants, dry clothes in winter without a dryer and so on is desirable.

Generally, passive solar heating is the best choice for houses and small buildings.

~PSW

Relevant Reading:

Buy books and help Ecotecture! If you liked this article and want to learn more, we invite you to buy books through the links below — we earn a small commission on each purchase you make, without raising your cost one cent. We’ll use that commission to expand our efforts to empower you to solve environmental problems.

 The Passive Solar House (The complete guide), James Kachadorian
The Solar House: Passive Heating and Cooling, Dan Chiras
Passive Solar Architecture, David A. Bainbridge

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Attic Hatch Insulation?

I am thinking of gluing rigid foam polyisocyanurate insulation to the back of our upstairs attic hatch access panels. Any feedback or suggestions?

Bob K.

Polyisocyanurate insulation is eco friendly

First, let’s consider the environmental aspects of the material. Polyisocyanurate (called “polyiso”) is a rigid foam insulation panel that usually has at least one heat-reflective foil face. Although polyiso’s very name sounds as anti-environmental as “clear cutting,” and as toxic as “carcinogen,” it is in fact rather benign.

Virtually all polyiso foam has some recycled content, mostly discarded plastic bottles, and the recycled content of the aluminum foil facing varies between 80 and 100 percent. While the foam was once expanded by blowing ozone-depleting CFC’s through resin, it is now produced with zero-ozone-depleting blowing agents.

With higher R-values (insulating values) than any other readily-available insulation, polyiso’s superior insulating qualities far outweigh its environmental negatives. Both the Environmental Protection Agency and the U.S. Green Building Council, a non-profit industry group that promotes sustainable building practices, have recognized polyiso products as beneficial to the environment.

Polyisocyanurate insulation R value

A 1.5 inch thick polyiso panel is rated R-9 —that’s almost double the rating per inch of typical attic insulations. Ideally, your attic hatch insulation will match roughly the R-value of the rest of the attic insulation, so the “almost double” value provides a handy guide to the thickness of your polyisocyanurate insulation.

For example, if you have standard 6.5 inch-thick fiberglass insulation in your attic (R-19), you would want to install 3 inch-thick polyiso (+ /- R-18) on your attic hatch. Polyiso is available in half-inch-increment thicknesses, so for the example above you could either buy a three-inch thick panel or stack two layers of 1.5-inch thick foam, depending on the panel’s price and availability in your area.

Planning the Installation

Which brings me to polyisocyanurate insulation’s main drawbacks: price and flexibility. Typically available in 4×8 sheets, polyiso is at least twice as expensive as standard insulation, so you don’t want to use it for your (usually small) attic hatch and throw the rest of the sheet away. (That’s an environmental “no-no” anyway.) Try to find a use for the polyiso scraps, or perhaps go in on one sheet with a neighbor who also needs attic hatch insulation.

Polyiso is easy to cut with a hand saw (wear a dust mask), and because of its closed-cell foam structure, it will maintain its structural integrity in a seldom-used opening such as an attic hatch — that is, you shouldn’t have to put a wood frame around it to keep it from disintegrating at the edges. (Wrapping the edges with some reflective metallic tape might be a nice touch, however.)

The foils side should always face toward the living space. You can use contact cement to attach the panel to your hatch. (Be careful to keep the cement away from the polyiso core which might react with it.)

~PSW

Hi Philip,
Our home has a crawl space underneath it with deteriorating fiberglass insulation that needs to be replaced. Can we use a “green” insulation in the crawl space?
— Geoff & Barbara, Oakland, California

If your fiberglass insulation is deteriorating, make sure you have adequate crawlspace ventilation. Mold and bugs do not eat fiberglass, so the deterioration is likely caused by moisture accumulation that has collapsed the air spaces between the fibers — your insulation is probably getting wet. If that’s the case, your new insulation will soon deteriorate as well.

Building codes require that you have a certain number of square feet of ventilation for every square foot of crawlspace. Find out what the code ratio is for your area by calling your local building department.

Then measure your basement vents and compare them to the square footage of your crawl space. You should have at least as much ventilation as is required by code, and ideally more — especially if you live in a moist climate.

Cutting holes into an existing house to add ventilation can be expensive. One easy trick is to replace your crawlspace hatch door with a screened door. (Double screen layers work best —one based on _-inch wire mesh to keep animals out, and one regular window screen to keep bugs out .) Building and hanging a new hatch door is simple and cheap compared to cutting in vents, and a typical hatch door is as big as four to six typical foundation vents.

Also, it’s a good idea to place a heavy layer of black plastic on the floor of your crawlspace to reduce moisture and make it easier to crawl .

One “green” insulation that would work in a crawlspace is made from remnants of the blue jean manufacturing process — parts of the cloth that can’t be used to make clothes. Although the cotton from which the product is made is water-and-pesticide-intensive, the insulation itself is made from a “post-industrial” scrap that would wind up as landfill if it weren’t used as insulation. Also, it’s 100 percent nontoxic — you can put your toddler in a pair of blue jeans.

Blue jean insulation is available in batts, so it is appropriate for installation in crawl spaces. (Cellulose “powder” insulation won’t stay in place, and although foam insulation will stick to the subfloor and joists, it is very expensive.) To find out more, google “green insulation.”

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