To my way of thinking building a structure with SIPs and not integrating passive solar technologies is like having a car, never turning on the engine, and pushing it around to wherever you have to go. I am well aware that building design preferences – especially home design preferences – are fraught with imbedded psychological drivers and demons that have painfully little to do with the world of the rational. At the same time, their owners will go on for hours about all the features that the building does have, and therefore how smart they were to think of them and include them. Their building obviously has such value that it speaks volumes about their good taste and brilliance. Most of these features that building owners brag about are strictly cosmetic and have very little to do with the idea that the building might actually perform for them; keep them warm or cool appropriately with minimum energy expenditure, or minimize the time and money required for their maintenance and longevity. But such reality benchmarks are pushed aside by the rationalizations of the building owner who only has a mind for some kind of fantasy that projects who he might be or thinks himself to be.
All of the above to say that buildings should....
SIPs can create such a terrific thermal envelope that it seems a shame to not go all the way and aim for a final product that is not only an energy miser, but also perhaps even a net energy producer. We all know that SIP buildings require less energy to heat and cool than a conventional code-minimum building, but exactly how much less? Well, it depends…
In terms of thermal efficiency, I divide buildings into six types:
1. Conventional Code-minimum
3. Sun tempered
4. Full passive solar
5. Active solar
6. Hybrid combinations – usually of 4 & 5 above
Even though the builders of our first homes intended for them to minimize conductive losses and resist infiltration, they readily admitted that they were not very good at either of these. Part of the explanation for this may be that though they were heating with wood, and it would have been nice to minimize the effort to provide enough fuel for heating, this wood was primarily used for cooking; the fireplace was both the stove and the furnace. Much of the time the cooking activity could overheat the space, so a little leakage might prove welcome. We have clearly evolved to a point where our homes have improved enormously in the envelope loss department. However, as we all know, unless a stick structure is upgraded with an expensive remedial package -- including a rigid insulation wrap, an exterior air barrier, and an interior vapor barrier – it can’t hold a candle to the thermal performance of a SIP envelope. Field tests (see Sips.org for Oak Ridge National Laboratory report) show SIPs outperforming equal-R stick construction by about 25%, my own experience is that SIPs do much better than that, closer to 50%. This is because the stick default building is not constructed with equal R-value, and certainly not equal infiltration resistance. So this brings us to our Type 1, or Code minimum house.
SIPs may also be used to construct a home with the minimum R-values that the regional code prescribes. In my area this yields a home with envelope losses of about 100,000 Btu per gross square foot per heating season. This translates to 3,000 gallons of oil per year for a 3,000 square foot home.
Type 2, the Super-insulated home, will do much better. With a more normal SIP construction of 6 ½” walls and a minimum of a 10 ¼” roof (R-40), we usually figure this as cutting your heating bills in half. As we increase the R-values from here the curve flattens out showing diminishing returns. This reflects that significant elements of the envelope are not changing, such as your windows or floor construction. Note that this building will still drift down below freezing if there is a power failure or the mechanical system fails to operate for any reason.
At this point we have been looking at only the demand side of the equation. Aside from small interior heat gains from people and cooking, etc., the mechanical equipment will make up the difference between envelope losses and the required setpoint. When we look more closely at the supply side of the equation, after improving the overall efficiency of the mechanical system, the next thing to come to our attention is the gains we may harvest from the sun. If we can rearrange our plan to allow us to reconfigure our glazing such that the majority of our glass is toward the sunny south and a minimum is to the east, west, and north, then we have achieved our Type 3, or Sun-tempered building. This building will experience temperature swings, sometimes uncomfortably large, but will further reduce the overall firing of the mechanical system. Savings may be anywhere from an additional 5 to 15 percent. This building will also go down to below freezing should the mechanical system fail to operate.
Type 4, the Full Passive Solar building adds the crucial unsung hero of building thermodynamics: Thermal mass. If we are looking to create a building that will run as fully on the sun as possible, the amount of solar energy we have to collect in a day must be at least equal to the total envelope loss for that entire day. Well, while the envelope loss scenario “operates” for the full 24 hours, the solar gain mode only exists for about 6 hours on a typical January day. Therefore, while we are in the solar collecting mode, we have to collect 4 times the amount we need to heat the building during the 6 hour collection time in order to have enough energy to drive the building for the whole 24 hours. Where does this extra heat go at the time of collection and how to we draw it back into the space after the sun goes down?
This not only serves the function of storing and later releasing the heat needed to warm the building during the dark hours, but also keeps the building from overheating during the daylight collection hours. The Passive Solar designer has to calculate the supplemental energy required to offset the envelope losses, translate that into energy gained by glazing and, most importantly, integrate the appropriate amount of thermal mass into the scheme to balance the solar gain.
This is not the time or space to go into the complete strategies and methodologies for good passive solar design, but it should be sufficient to list the variables that must be known or calculated in order to both accurately size a supplementary mechanical system and predict the solar performance of the building. The local climate must be known. This includes degree-days, prevailing winds, seasonal deviation from the norm at your latitude due to altitude, ocean tempering or other geological factors. Also necessary are latitude, window areas on all elevations, reflectivity and emissivity of finishes, thermal capacitance of construction materials and their volume, and programmatic user patterns.
The beauty of a good SIP envelope is this. Just as the size of the mechanical system is reduced to be as exact a match as possible for the small losses calculated, the “solar furnace” and its accompanying amount of thermal mass may also be reduced to be the engineered match of the envelope loss. This balance and juggling act requires the designer to move through several iterations as he or she works, testing the predicted performance with rough calculations as they go. One can see here that computer software is a tremendous boon to the solar designer.
The correlation is clear, when the envelope losses are high the amount of glazing and thermal mass go up. When the envelope losses are low, the amount of glazing and thermal mass required to do the job is reduced. This has great impact on both the cost and “style” of the building, as you can well imagine. A bonus for a proper passive solar structure is that it will “free cycle,” that is, without benefit of mechanical systems, above freezing. Broken pipe disasters are no longer a possibility.
Type 5, Active Solar, utilizes solar gain devices other than the windows of the structure to collect the sunshine, and through pumps, fans, motorized dampers and valves causes the collected energy to be sent to some remote storage device and then drawn on and distributed when needed. These systems can be expensive additions to the building. Passive solar utilizes the materials you were already buying to do the job! Just as with conventional heating systems, if the power goes down, the building’s temperature can plummet down to below freezing. (You can see my prejudice for type 4 coming through here!)
Type 6, Hybrid combinations may make a lot of sense where other programmatic requirements preclude a full passive solution. This could be where glazing orientation is driven by other factors, or we are retrofitting an existing building.
In all cases, the proper procedure is to minimize envelope losses first! This has also been shown to be by far the most cost-effective approach. This is why SIPs are such a natural starting point for the design of a good energy efficient building. Whether your supplementary heating system is a conventional fossil fuel system or the sun, it really makes the most sense to first focus on cutting the loads. Get them way down for both first cost savings and operating cost savings which will be money in the bank for the lucky owner of this SIP home.