Design - Maximizing Passive Solar Gain (Cont'd some more) - Windows, Thermal Mass and Surface Colors

This is the third of four posts on passive solar gain.  The first post was an overview of the subject and the second post zeroed in on the shape and orientation of the building, room arrangement within the building and a protected entry for the building. Here we are continuing to use Mazria's book on passive solar and our project as a bases for a look at windows, heat storage and surface colors. And it goes without saying that we are talking about passive solar dwellings that are well-insulated, if not super-insulated.

Windows
A previous post several months ago explored the ins and outs (no pun intended)

of energy efficient windows and doors.  In the process, I discussed the advantages of......

• Casement, hopper or awning, whereby closure is against an airtight seal -- as opposed to sliding windows
• Fiberglass frames
• Double glass
• Low-E coating
• Argon filled

However, our discussion of windows here goes beyond energy efficient glass, frames and ventilating style.

One of the recommendations in the second post on passive solar was for an east-west orientation for the building so as to maximize the amount of south-facing glass. But how much glass? Mazria offers ranges for the ratio of glazing to floor area (assuming that the floor is insulated thermal mass, i.e., capable of storing and radiating heat) that is needed to maintain average indoor temperatures in the upper 60 degree range in various climates.  For example, his chart shows 0.19-0.29 square feet of glass for each square feet of floor in the not-so-cold north to 0.27-0.42 sq ft in the cold-cold north.  For temperate climates, the glazing should range from 0.11-0.17 sq ft per square foot of floor in warmer climes to 0.16-0.25 in cooler climes.  Any fall-off from the recommended ratios means less solar gain and more supplemental heat.

Double and triple glazing and low-E coatings reduce solar gain to some extent but the loss is more than offset by a reduction in heat loss back out through the glass at night and on cloudy days. Gain can also be diminished by shade on the glass from the surrounding wall when windows are recessed, as is common with many energy-efficient structures.  There are three options for the problem: accept the loss of radiation as the lesser to two evils over thinner walls, move the glass closer to the outside plane of the wall and suffer more heat loss from wind washing or bevel the outside wall away from the glass to let the sun in.

Translucent glass is especially useful for direct gain passive solar.  It diffuses solar energy over a wide area, which helps when the energy would otherwise overheat low-thermal-mass structures like framed walls instead of finding its way to structures like masonry and soil that can absorb it.  And, compared to raw sunshine, the diffusion makes for a much brighter environment without unpleasant glare and helps to prevent color changes in furniture and fabrics.  These characteristics make translucent glass especially suitable for clerestory windows where a view through clear glass may not be critical.

In order to preserve heat gained during sunny days, it may be necessary to use thermal shades to cover the windows at night and on cloudy days.  The literature is replete with designs for thermal shades -- homemade and store-bought.  The best designs cover the inside of the window and seal against the sash on all four sides so as to prevent convective heat loss.

Heat Storage
Except for the work of Hiat and Stephens (for details, click on the "Featured Post" in the left

column), references to thermal mass for storing heat are typically focused on mass inside passive solar buildings.  For direct solar gain systems, concrete is king -- floor, walls and sometimes roofs. When it is in the envelope of the building, it is insulated on the outside surface, including under the floor. Research has shown that the concrete need not be thicker than 4" because daytime solar heat only penetrates to this depth before heat is withdrawn by falling nighttime temperatures.  

For indirect solar gain systems using thermal mass between the windows and the living space, the most common material is again concrete but thickness matters.  The thicker the wall the less temperature fluctuation within the living space.  The other, less popular, option is the "water wall", typically steel drums or other containers filled with water. For all practical purposes, the thermal performance for a given thickness is the same for concrete and water even though each behaves differently with regard to absorption and radiation of heat.

Hiat's umbrella in conjunction with maximum earth contact

With regard to thermal mass, the AGS system Hiat and Stephens co-fathered combines uninsulated earth contact with internal thermal mass like concrete.  They recommend a below-grade "umbrella" extending outward from the building to waterproof and insulate a volume of thermal mass (earth) much larger than the interior mass of traditional passive solar installations. The large thermal mass means that temperature swings in the living space are modulated to the extent that, instead of being measured in hours, remain relatively constant year-round.

Surface Colors
Our discussion of surface colors here is limited to direct solar gain systems since they predominate.  In general, dark colors are okay wherever the sun doesn't shine, which may be counter-intuitive because they would absorb more radiation -- a good thing.  However, they often overheat because absorption is faster than penetration and storage.  This is absolutely true for surfaces containing minimal thermal mass, like frame-and-gypsum-board surfaces, but it is true for concrete as well. Therefore, all surfaces receiving direct sunlight through transparent glass should be light in color so as to diffuse and scatter solar energy for distributed absorption by mass throughout the structure.  The possible exception might be a medium-shade for a masonry floor.

Surface colors become less critical when sunlight enters through translucent glazing. The glass itself diffuses the energy making overheating difficult even for dark colors.

Our Project 
In our warm temperate climate, the ratio of glass to floor area to maximize passive solar is rather modest. Mazria's chart seems to indicate that our 400 heating degree-days/mo calls for 390 sq ft of glass.  We will actually have 420 sq ft which was determined more by meeting the glass-to-floor-area code requirements than any intentionality about passive solar requirements.  

Our walls will be right at 17" thick and, since the glass will be recessed 10- 11" in from the exterior plane of the wall, the wall will shade the periphery of the glass.  But the deep-set windows will be ideal for minimizing wind washing.  Since we will depend on the AGS system as our primary heat source, we can easily tolerate a minor loss of wintertime solar gain. (For an explanation of "wind washing", go to the previous post on windows.)

Heat storage?  Thermal mass?  Our design is all about heat storage in concrete but more so in soil:

• 900 sq ft of earth sheltered west and north concrete walls, largely uninsulated, which means that the contiguous earth is part of the mass
• 2800 sq ft of concrete floor having no insulation under it, which means again that the earth is part of the mass 
• 4,500 sq ft of insulated earth under the umbrella extending 16-20' outward from the living space of the house in all directions

So much thermal mass is essential to the AGS system.  It will store heat from passive solar gain in winter only secondarily.  But each BTU stored and radiated from solar gain means one less BTU from the AGS system. 

The long and tall 2 x 4 and gypsum board wall between the living space and the north earth contact concrete wall would seem at first glance to isolate the concrete from solar gain through the windows.  However, for its entire length, the stick-built wall will have continuous openings at its top and bottom to allow air to reach the wall via natural convection. Having said that, it is important to reiterate that the heat from the windows that does reach the concrete wall is welcome but less consequential than the heat emanating from the soil behind and below the wall that was deposited there by the AGS system during the summer.  Consequently, the openings in the framed wall are there more to move cool room air to the concrete wall for warming than to move warm air from the windows to the wall.

The solar gain through the transparent windows on the first story will find its way directly into the floor as is typical with most passive solar systems. Some of it will be reflected/diffused and find its way through the high-low wall openings in the framed wall to reach the concrete wall.  

Second floor layout showing long stick-built wall
between the clerestories and the concrete wall
(click on the image to enlarge it)

The clerestories will comprise more than half of our south-facing glass.  Not only will they be facing the long 2 x 4 wall but they will be 15' above the concrete floor or backed by the second story wood floor -- all low mass scenarios.  For the sun's rays through them to reach the thermal mass in the floor and the back wall, they will have to be diffused by a preponderance of translucent glass and light colors on low-mass surfaces.