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Moretown’s Second Passive House is Underway

House can be heated with the equivalent of fifteen 100W light bulbs

By Indigo Ruth-Davis

Head north leaving Montpelier, Vermont, hang a right, and before you know it, you’ll be lost on the dirt roads that crisscross the foothills of the Green Mountains. On the side of one of these hills, a crew of workers is installing triple pane, high-solar-heat-gain, R-9 windows on a super- insulated timber frame house. These windows will provide more heat to this house than its entire heating system. Active heating will be necessary only on the coldest winter days. On those cold days the house will be heated with the equivalent of fifteen 100w light bulbs.

This is the Timber Frame Passive House Cottage. The setting is rural, the style, well, DIY-rustic, and its design is certified passive. While the tools to build a house that performs this well are basically the same as a conventional building, the design process requires a new set of tools.

The secret to a Certified Passive House Consultant is The Tool Box, which consists of:

  • The Passive House Planning Package, an energy modeling program
  • WUFI-ORNL, a Hygrothermal analysis program
  • THERM, a thermal bridge analysis program
  • The Solar pathfinder, a shading calculation tool

The most important of these tools is the Passive House Planning Package (PHPP). It was developed by the Passive House Institute, (in Germany) in the 1990s — to simplify the energy balance calculations that are necessary to meet the Passive House standard. Areas for each building component, R-values, window performance values, shading conditions, and information about the mechanical systems are entered into the program. The PHPP combines this with climate data and calculates the energy use of the building’s design.

The solar pathfinder is used for shading analysis.

The solar pathfinder is used for shading analysis.

At the cottage, clients Greg and Barb Whitchurch knew they wanted to build a small, well insulated house for their parents. When Greg showed me Barb’s sketch of what they had in mind, I said “that’s a Passive House.” This project didn’t have a professional architect, so getting there required intense collaboration among the homeowner, the builder Chris Miksic of Montpelier Construction and me, the certified Passive House consultant .

The first step is to complete a rough PHPP. Every building’s geometry, size, orientation and climate have different implications for its energy balance. This is the heart of performance-based design. In our case the Passive House design criteria of 4.75 kBTU/sq. ft./yr. called for 16 inches of dense-pack cellulose for R-56 walls and 22 inches of dense-pack cellulose for an R-77 roof. This is based on the maximum allowable air tightness in a Passive House of .6 ACH@50 pascals. The initial PHPP takes a few days to complete.

The next step is to analyze the site’s shading conditions. Harvesting heat from the sun is serious business in a Passive House, so accuracy in the shading analysis is all-important. For complex shading conditions such as mountainous locations this is best done with the Solar Pathfinder. The Solar Pathfinder consists of a dome that projects shading objects at the building site onto a sun- path chart. Shading percentages are tallied and then entered into the PHPP for radiation reduction factors.

Before the assemblies are finalized they should be analyzed for moisture and mold risk. Hygrothermal analysis is more important in high-performance building than in conventional building because of higher temperature differentials within the wall assemblies. I use WUFI-ORNL for this analysis because it’s free and is suitable for most non-commercial construction.

In our project the roof design required special attention. The plan called for 22 inches of cellulose under a flat unvented membrane roof. On a flat roof, condensation risk is usually mitigated by adding vapor closed foam insulation to the outside of the assembly to a thickness determined in the building code for each climate zone, or by back venting below the roof deck. When I modeled the proposed unvented roof, WUFI-ORNL indeed predicted that the assembly would not be able to dry out and therefore the moisture content in the building materials would slowly rise over the seven year period that I analyzed. I compared this assembly to a back vented assembly. The results were dramatic. With only 1 ACH in the back venting plane, the roof assembly showed a significant dry-out over the seven year period. Based on these results I advised my client to add a ventilation plane to the roof assembly.

The final step in the design is to look for potential thermal bridges and either eliminate them, or account for them in the PHPP. Thermal bridges that can’t be eliminated need to be modeled in a two-dimensional heat transfer program such as THERM, which is free from the Lawrence Berkley National Lab.

Our biggest thermal bridge concern was our window connection mullions. To determine if they were a thermal bridge I first drew them in THERM with the window frame geometry. Then THERM runs a heat transfer simulation with and then without the mullion. The conductance of just the mullion is determined by subtracting the results with the mullion from those without the mullion. This number is entered into the PHPP. The mullion thermal bridge ended up throwing off our energy balance and was a condensation risk. We used THERM again to determine that an exterior trim piece that incorporates an EPS foam plug significantly improves this thermal bridge.

I enjoy being able to bring the precision of the PHPP, WUFI and THERM to the design process. Informed decisions instead of best guesses become the basis for a Net Zero-ready design fit for the 21st Century.

Indigo Ruth-Davis is a Passive House Institute US Certified Passive House Consultant and builder. He is a partner at Montpelier Construction, one of central Vermont’s leading building performance companies.

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