All of the houses in the HUD-sponsored technical study were built in the 1920s and ’30s, and were in need of complete interior gut renovations.
The six DER houses received continuous exterior insulation as part of the plan to achieve a deeper level of energy reduction. To save money, the plan was to apply the foam over the existing siding, but after the first house treated this way was tested, the decision was made to strip the remaining homes down to the sheathing to achieve better air-sealing numbers.
A HERS score is a measure of a home’s energy efficiency based primarily on the results of diagnostic/performance tests using standards established by the Residential Energy Services Network (RESNET). A lower score indicates greater energy efficiency. According to the DOE, a standard new home that exactly meets the energy code scores 100 on the index. The homes in the HUD-sponsored technical study described in this article score well below that.
Four of the DER houses got dense-pack cellulose cavity insulation plus exterior insulation for a combined R-28. The other two DER houses received an additional staggered studwall on the inside, creating a double wall that boosted total wall insulation to R-42.
House-by-house performance data shows that, compared with the Energy Star (ES) houses, the additional steps taken in the deep energy retrofit (DER) houses significantly reduced energy usage and projected operating cost.
In the DER homes, existing slabs were removed so that new plumbing, a passive radon-mitigation system, and rigid foam insulation could be installed. The interior foundation wall foam was applied prior to the slab pour, making it easier to keep the below-slab foam ìpan,î as shown here, continuous with and connected to the wall foam.
Most foundations in the DER houses needed waterproofing anyway, creating an opportunity to add foam to all of the foundation exteriors. Most received a double layer of rigid foam.
In the most efficient DER homes, wall-mounted mini-splits were used in the first- and second-floor hallways. Bath fans were installed nearby to pull conditioned air from the hallways into the bedrooms, but comfort issues led to relocation of air outlets from the continuously running ERV.
The largest additional cost in the DER upgrades was for rigid foam insulation at the foundations and slabs and on the exterior walls (siding costs are excluded because siding had to be replaced anyway). At current fuel prices, the total upgrade cost pays back at a rate of about $1,000 per year. The authors consider the improvements to occupant comfort and building longevity to be worth the investment.