There have been many threads at the virtual conference table concerning the choice between trusses and SIPs for roofs, HVAC systems for SIPs and the use of an engineer and/or architect. In my thinking these are all related and it is important for the prospective DIY homeowner to think about the final value of his project being a function of the process utilized to bring it into being.
Even if we choose not to think of the home as an integrated system, Mother Nature (the laws of physics) will have her way. If the home is conceived of and built in the conventional way with individual components glommed onto a conventional structure we may characterize it as non-integrated, but Mother Nature will experience
The first step is to perform a true heat loss calculation for the proposed building. We don’t use ResCheck or similar free on-line services because they neglect a critical part of the calculation, or at least make it “unavailable” for tweaking and correcting. I am speaking here of the infiltration variable. The irony with this business is that the more insulation you use (R-value) the less important it becomes. That is, the higher the R-values of the walls and roof, the smaller fraction they represent of the entire heat loss.
Below is the standard spreadsheet we use in our office on the construction documents for A) demonstrating Code Compliance, and B) for sizing the HVAC system. It is also very helpful in deciding, for example, whether or not to bump up the SIP walls from 4 ½” to 6 ½” or upgrade the windows.
The table on the left side shows a “code minimum” solution. The idea being that if we propose a building with more glass than the allowable 15% of gross wall area (New York State Energy Code requirement) we still have a compliant scheme as the total BTUH loss is still less than the total of the Code Minimum scheme. We can see that that is in fact the case here; in spite of the glazing accounting for 22.4% of the gross wall area, the total for our building is well under the allowable – 37 ½% under!
We know that SIPs excel in air infiltration resistance. The question is how well do we actually do? The only way to know the infiltration value for sure is to perform a blower door test on the completed envelope, as the results are entirely dependent on field performance. Many buildings have tested way down to .05 (No, the decimal point is in the correct position!) ACH or lower! ASHRAE recommends a value of .35; any lower is decidedly unhealthful. We use a slightly more conservative value, .04, by specifying an exhaust fan, heat exchanger, or some similar ventilation unit that will bring the building up to .04 ACH. SIP buildings do so well as a general rule, that blower door tests are not required for them in order to obtain an “EnergyStar” certificate.
We also deal with other aspects of heating strategies, such as system design, component efficiencies, distribution sizing, layout and location. It is poor practice to place any part of the HVAC system outside of the conditioned envelope. In the schematic design phase we are all ready looking at return air duct size and location, and of course, the location of the remainder of the duct tree and the actual mechanical equipment. All this, so we can keep the whole system within the conditioned envelope, which is not only good for operating efficiency, but minimizes wear and tear on the equipment and facilitates servicing.
This is one reason we find that SIP-walls-with-a-truss-roof doesn’t cut the mustard. The vented (cold) attic space thus produced is invariably utilized for locating the distribution and return ducts, if not all the mechanical equipment. We prefer to design with SIP roofs that either form “cathedral” ceilings or provide conditioned attic spaces that are appropriate for mechanical systems or storage of items that need protection from mold and mildew.
This kind of integrated “whole house” system approach is what a good architect should be utilizing when designing a home for you. As important as the SIP envelope is, it isn’t the whole ball of wax. We are looking to achieve a building where the performance exceeds “the sum of the parts” and are mindful of siting micro-climate issues, orientation, solar trajectories for both the heating and cooling seasons, and pertinent building details. We want the correct roof overhangs, avoidance of thermal bridging and water piping on outside walls, appropriate roof pitch for solar panels (thermal or PV), and – if possible – full passive solar design with its associated thermal storage.
A good architect should be able to deliver all these performance features, accommodate the owner’s specific program with grace and panache, and wrap up the whole thing in an attractive package that isn’t clunky or awkward in any way. If all this is achieved, the market value increase of the project usually more than offsets the initial cost of using an architect, but certainly the lower operating and maintenance costs experienced also accrue as savings to the owner for the life of the building.
What I am doing here is really making more of a case for the Whole House Integrated Design (Whid?!) process than for hiring a licensed professional. While anyone with enough research and trial runs can actually perform a successful tonsillectomy on one’s own child, the sheer economics of spending enough time to do the preparation, for one operation of one child just doesn’t work out! (If you were going to do all the kids in the neighborhood, it may then be good business sense.) Most DIY homeowners are going to do this only once. If you do elect to go through with this, you will understand and appreciate every little detail of your home so much more and also be perhaps better equipped to perform emergency repairs than a phoned-in professional. However, the time investment cost of dealing with the learning curve is usually not offset by the realized savings. This is why tonsillectomies and homes are executed by licensed professionals.
The best homes are the product of a considered process where all the variables are juggled and resolved so that the final product is much more than an optimally engineered people container. It is a place where you want to be; a building that shelters efficiently, but still allows you to engage your environment.