1.6  Alten Design Technical Assistance

• Hoak Residence

Figure 37. Hoak residence in Longwood, Florida.

This three-story, 4,250 square foot home was completed in February 2001 by Mr. David Hoak and Sharpless Construction in Longwood, Florida near Orlando.  (Please see Figure 37.)  FSEC assisted the owner and builder by recommending a package of features that produced an exceptionally energy efficient design at a reasonable cost. Because the building envelope design and mechanical equipment selection work together as a system, the home can be cooled with a much smaller air conditioner than is needed by most homes of this size in this climate.

Envelope Features:

Figure 38.  Home energy components affect the size requirements of the cooling system.

$ High Performance Windows:  Roughly 25% of the annual cooling load in a typical Central Florida home is introduced through the windows. Recent advances in window technology allow this load to be greatly reduced.  The windows in this residence are particularly useful in Florida because they have a very low Solar Heat Gain Coefficient (SHGC) to reduce direct solar gains, and a relatively high Visible Transmittance (VT) for natural daylighting.

Figure 38 (right) shows the impact of the window upgrade over a typical, minimum code, single-pane clear glass installation. The upgraded windows reduced the peak cooling load by nearly one ton (10.6 kBTU/hour). Other improvements to the building envelope and duct system reduced the cooling load enough that the entire home could be cooled with a 2½ ton air conditioner.

Figure 39.  Roof deck insulation creates semi-conditioned space for ductwork.

$ Unvented Attic:  Most Florida homes have vented attics with batt or blown insulation applied just above the ceiling. This exposes the air conditioning ductwork to very high temperatures and magnifies duct leakage problems. Sealing the attic envelope and insulating at the roof deck, as shown in Figure 39, provided a semi-conditioned space for the ductwork.  This reduced conductive heat gain and minimized the detrimental impact of duct leakage.

$     Expanding Foam Insulation:  A layer of expanding foam insulation was applied to the underside of the roof deck to create an unvented, semi-conditioned attic (R-22). The same insulation was applied to all above-grade walls (R-11). While the insulation R-values were standard, the foam created a nearly airtight seal and greatly reduced outside air infiltration.

$     Continuous Air Barrier:  Infiltration of Florida's hot and humid outside air can have a big impact on energy use, building durability, and occupant health.  The continuous air barrier, placed toward the outside of the building envelope, reduces this infiltration.  Indoor air quality concerns were addressed by installing an energy recovery ventilator to introduce outside air.

The air barrier consists of a tightly taped housewrap installed over the exterior sheathing on all above-grade frame walls, and extruded polyurethane foam boards glued to the interior of the below-grade block walls.  Expanding foam insulation provided an extra measure of airtightness at all above-grade exterior surfaces including the roof deck.  Special care was taken to seal wall details such as corners, floor interfaces, and the roof junction.  Blower door performance tests verified the home's level of airtightness (ACH50 = 2.0).