Seven Ways to Improve Your Net Zero Energy Building Envelope
There are a number of things you can do when designing and constructing a building to improve building envelope energy efficiency. Here are seven of them.
1. Create a tight building envelope.
It is important to create a tight building envelope around all six sides of a building (all four walls, the basement, and the roof). Depending on the climate the air tightness standard is between 1.5 and 2.0 Air Changes per Hour (ACH).
2. Tightly seal the exterior shell.
First, make sure that the exterior sheathing does not have any unintended gaps. Use an Energy Star Thermal Bypass Inspection Checklist that can help identify areas that need sealing. Next, tighten the joints with mechanical devices rather than by hand. Then, seal the exterior sheathing and the drywall ceiling before installing the drywall. You can accomplish the exterior seal using glue, caulk, foam, or other air sealing products. Don’t skip the testing step. Use a smoke stick while the blower door runs. After sealing the discovered leaks, make sure to test again using the smoke stick. Obsessing about testing and re-testing will provide rewards in a tighter seal.
3. Update the framing.
Use modern framing techniques that save wood and allow space for insulation. Double 2×4″ walls with off-set studs set at 24″ and set 5″ apart will help form a wall cavity that is a full foot thick. This will help keep out the cold in northern climates. It also helps to use raised-heel trusses for insulation in the ceiling. If you’re building in a northern climate, use dense fiberglass or cellulose to an R-50 rating in the walls and blow loose insulation to R-60 for the ceiling.
4. Thermal bridging to nowhere.
Where possible, identify and eliminate thermal bridges because trying to fix it with gels or foams is not as cost-effective as avoiding them in the building’s design. Use the Thermal Bridging Checklist to help design and build with minimum reduction in the building’s R-value. It’s especially important to design decks and patios that are separate from the building so there is no heat loss between the building and the deck/patio.
5. It’s clear as glass.
The make-up of the windows and their placement is critical to a net zero building. It’s important to have 50% of the windows on the south side of the building which should house the common living area. Also, strive for a 6% window-to-floor ratio for maximum solar gain. Whether or not it is cost-effective to have 50% of the windows on the south side for any given location is a decision to address during the project’s design phase. In any event, choose triple-pane windows with a U-value of .20. Of course, depending on the climate at the building site, double panes may suffice if the U-value is also .20. Bear in mind that sliders and double-hung windows both have leak issues. Finally, use fewer large windows over more small windows. Large windows have a higher glass to frame ratio. The frame allows more heat transfer than the window glass.
6. Use that sun well!
Using a solar heating system for hot water and heat is an obvious design decision. Yet, it’s just as important to use the largest roof overhangs to make the most out of the winter sun and avoid the summer sun.
7. Moisture management.
If the building is airtight, it is especially important to keep indoor humidity to less than 30% or less to avoid condensation. Using a high performance vapor barrier will ensure moisture does not get into the wall assembly. Using either an HRV (heat recovery ventilation) or ERV (energy recovery ventilation) system will help keep the air moving to prevent mold and assist with moisture management.
You can read more about these Net Zero Energy ideas in the ZeroHomes.org article entitled “Twelve Steps to Affordable Zero Energy Home Construction” which was the inspiration for this post. You can read more about advanced framing ideas in the Department of Energy’s Office of Building Technology Fact Sheet on Advanced Wall Framing, intended to help builders construct more efficiently, with less material, and to save energy.