Page 84 - Zero Net Energy Case Study Homes-Volume 1

CASE STUDY NO. 4
MERITAGE PRODUCTION HOUSES
(Below) A worker installs open-call SPF insulation on a residential construction site.
Low Energy Design Strategies
General Design Considerations
The working principle for the design of the ZNE houses at Sierra Crest was that they should look and function no differently than any other house. The differences would be in the construction techniques and the energy technologies used in the houses, which would be basically invis- ible to the owners and their neighbors. Therefore, the basic design strategies would have to be higher insulation levels and air-tightness of the envelope, EnergyStar appliances, high-efficiency LED light fixtures, electric heat pumps for heating and cooling, heat pump water heaters and, of course, a baseline solar PV system.
Building Envelope – Insulation and Windows
The houses were built with advanced framing3, a materials-efficient method of standard plat- form framing that minimizes the thermal bridging effect. The method of insulation selected was an open-cell spray polyurethane foam (SPF) insulation applied between the exterior framing members, combined with a 1” layer of interlocked expanded polystyrene (EPS) boards over the outside of the framed exterior surfaces. The water-blown spray foam provides a high insulating R-value (R = 4 per inch) and increases the air tightness of the exterior envelope by filling in gaps and holes in the construction4.
The specific type of SPF product chosen, manufactured by Icynene®, is chemically different from other SPF products on the market that are made with some chemicals of concern5. The SPF used in the Meritage Homes was carefully evaluated and selected because of its environmental neutral properties and absence of hazardous chemicals after installation6. The higher R-values possible with the use of this SPF and the large reduction in sources of air leakage were consid- ered a good value in return for the added complexity of installing the material.
Although the advanced framing is less susceptible to thermal bridging at the wood framing ele- ments at the exterior surfaces, there remains a significant area where this extra heat transfer can occur. The 1” layer of EPS sheathing, with an R-value of R=4, successfully eliminates all such thermal bridging at the exposed framing members.
3 See “Advanced House Framing”, United States Department of Energy, https://www.energy. gov/energysaver/energy-efficient-home-design/advanced-house-framing
4 As noted in Case Study No. 1, Corona del Mar New Houses, the open-cell spray foam is permeable and does not have the full air-sealing characteristics of the closed-cell product. Also, the R-value of the open-cell insulation if only about half of that of the closed-cell product. However, for the mild California climate, the open cell product is decidedly more cost effective in terms of energy-efficiency.
5 “Health Concerns about Spray Polyurethane Foam”, United States Environmental Protection Agency, https://www.epa.gov/saferchoice/health-concerns-about-spray-polyurethane-foam
6 It contains no PBDE (flame retardant) and uses carbon dioxide to expand the foam, there- fore having the Global Warming Potential (GWP) of 1 (the lowest number on the index scale). While there is a level of toxicity during installation, after the SPF has cured for 24-72 hours,
no measurable outgassing occurs and no toxic chemicals are released if the material burns. The installation of the SPF requires special protection for the workers and the job site must be vacated during the curing period. The workers must be protected by a Tyvek® suit and wear full respiratory masks to protect against inhaling any toxic chemicals or airborne foam particles during expansion.
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Zero Net Energy Case Study Homes: Volume 1