They used the Passive House Planning Package spreadsheet (PHPP) to model our house based on the plans, details, and the type of windows and doors we planned to use.
Their recommendations included:
- Bring the basement into the thermally conditioned envelope. At the time, I was unsure what to do with the basement. Passive-houses and super energy efficient houses don’t typically have basements because they suck heat from the house. But I don’t know how we would have built a house on our site without a basement. So they made the following recommendations.
- Add more insulation under the slab. We added another 4 inches for a total of 6″ of XPF or R30.
- Insulate the foundation walls. We added 2″ of EPS and 9″ of dense pack cellulose to bring our basement walls up to the same R value as the rest of the exterior walls (R42).
- At the time I was considering insulated concrete forms (ICFs), but once we decided to insulate the foundation walls to the same level as the other walls, it didn’t make sense for the extra expense of the ICFs which would not even bring us up to the level of insulation we needed.
- Reposition the radon exhaust pipe to vent outside the foundation wall. It is usually recommended to run the vent pipe up an interior wall to promote upward air drafts for passive ventilation of any possible radon gasses. But this essentially brings outside cold air down an interior wall right through the core of the house. So we positioned the vent pipe outside the foundation wall and stubbed it off. If we detect elevated levels of radon gas in the basement, we can add a vent stack and fan to actively ventilate the gasses.
- Use open-web trusses to facilitate easier installation of ventilation system.
- Exterior overhangs for the first floor south-facing windows to minimize overheating in the summer.
- Simplify double wall system and just use dense cellulose. We had considered a number of options including using 3″ of spray foam and 9″ of cellulose, as well as rigid insulation on the exterior of the sheathing. We decided that the extra effort of sealing the walls as well as our use of an exterior rain screen detail would limit the possibility of moisture condensing inside the wall.
- Remove wood burning appliance (wood stove). They insisted a wood stove would cause more problems than solve in a super tight, super insulated home. (not to mention it is a budget saver) So we opted out of the wood stove for now. Once we live in the house for a while we’ll see what we think. (we can always add one later, but don’t tell the energy guys)
We accepted all their recommendations.
The Final Energy Analysis
I updated the plans and they updated the PHPP model.
They determined that our yearly heating demand would be 12,416 kBTUs and our maximum heating load would be 12.2 kBTU/hr. At these levels a single mini-split air source heat pump (ASHP) on the first floor will supply all the heat that is required. The coldest of days (below 5°F) may require supplemental heat which will be supplied by electric resistance base board or panels, approximately 15 lineal feet throughout the house.
The mini-split ASHP are very very efficient in converting energy to heat. They consist of a small unit in the house, connected to a small unit on the outside of the house. Anyone that has traveled to Europe or eaten in a small New York City restaurant will be familiar with these systems. They are steadily gaining a foothold in the US.
Based on our estimated energy usage, we will require a 6kW (approx.) solar electric (PV) grid-tied system to get to net zero. We would need less if we decided to use a solar hot water system, but these are expensive to install and maintain over time in our climate, too many moving parts. PV is solid state and has proven to be maintenance free for many years.
We also may require a larger solar array depending on the actual amount of solar energy available at our site. We get plenty of sun, but there are tall trees and low hills to our east and west. We plan to live in the house a year or two before adding solar PV to the house. This will allow us to verify our actual energy usage (and budget).