Mud Lake House Update: The Mechanical Systems

Our Recent Design-Build Using Passive House Design Principles

As we saw it, the post series was a chance for our community to get a designer’s- and builder’s-eye view of the exciting world of Passive House design, and catch a glimpse into what makes TDS Custom Construction unique when it comes to passive house construction. After all, we were one of the first companies in Wisconsin to have a Passive House certified designer in-house. Now, four years later, not only are we fielding inquiries about more possible Passive House builds in the area, but we also have some exciting updates on the Mud Lake home—and, now that the owners have moved in and lived in the house for a few years, a view from the other side of the project.

Follow along for the continuation of this blog series—where we’ll first update you on the mechanical systems (heat and ventilation) that we used for the home, then share some interesting data points on the home’s energy performance.

Using Passive House Principles for the House on Mud Lake

Passive houses use an integrated, conservation-based approach to building, to drastically reduce the amount of energy used. Using a combination of super-insulation and superior air-tightness, along with passive solar strategies, high-performance windows and doors and continuous ventilation with energy recovery, the building strategies and materials work together toward the goal of deep reductions in energy use.

The result? Vastly reduced utility bills, of course—not to mention more comfortable living and better indoor air quality: passive houses eliminate the drafts, cold spots, and temperature variation common to conventional homes, featuring instead a constant flow of filtered, fresh air, free from allergens, pollutants, and CO2 levels.

Beyond that, passive houses are a tangible way to consider the planet. When passive house principles are used in conjunction with solar panels, they can often achieve “Net-Zero energy use”—reducing the energy use of the house to zero.

Building a passive house relies on a number of different elements. For our series on the Mud Lake home, we focused on a few specific features, having to do with the foundation system, the wall system, and the roof system. Some important passive house elements we discussed in previous posts that we used for the Mud Lake house were:

• Frost-protected shallow foundation with slab-on-grade and R-38 foam insulation.
• Walls with R-27 cellulose cavity insulation and R-24 continuous Roxul insulation.
• Roof with R-90 cellulose insulation.
• Windows and doors by Kneer-Sud (U-value 0.11, SHGC 0.5)

Besides the elements above, the home’s mechanical systems are another important element of passive house principles. There are two main areas to consider when it comes to the mechanical systems in passive house design—heating and cooling, and ventilation—both of which work a bit differently from conventional buildings.

Using a heat pump for heating and cooling

Given their excellent air sealing and super insulation, passive houses have a very low heating load compared to typical homes, so using a conventional HVAC system is overkill. 

For passive house construction our preferred solution is a ductless heat pump. Heat pumps are high performing and energy efficient, so they go a long way toward the goal of energy reduction that makes passive house design so appealing. Beyond that, the heat pump works as both a heater and an air conditioner, with some dehumidification aspects as well, so it’s a particularly efficient and elegant option from that angle, too.

How does a heat pump work?

Essentially, heat pumps take heat from the outdoors or other sources and, using refrigeration technology, use that energy to heat or cool the home. For the Mud Lake home, we went with a Mitsubishi hyper-heat ductless mini-split heat pump. Mini-split systems are made up of a small outdoor unit and one or more indoor units, and the hyper-heat model uses an enhanced compressor system to deliver heat even when outdoor temperatures are as low as -13° F.

Ventilation systems for passive home design

The other major component of the HVAC system is the energy recovery ventilator (ERV). While the heat pump covers heating, cooling, and to some extent, dehumidification, another important aspect of the mechanical system to consider when it comes to passive home construction is the ventilation system.

Passive homes require a different approach to ventilation than conventional homes do. Given that passive homes are so much more tightly sealed than conventional homes, more intentional ventilation is required. A lot of  attention in passive house construction is focused on creating an airtight seal—using both high quality doors and windows, plus great air sealing achieved through an air barrier like the Stego Wrap below slab and the ZIP sheathing on walls and ceiling we used for this house. Because of this commitment to creating an airtight seal, homes designed using passive house principles require a really good ventilation system. The purpose of the ERV is to take fresh air from the outside and bring into the home. The heat from outgoing air is transferred to the cool/cold incoming air, so it’s not dumping cool/cold air on people, while reducing the amount of energy needed to heat the incoming air.

Better quality air

There is a definite upside to this approach to ventilation in passive house construction—the air quality is considerably better. In conventional houses, a lot of air comes in through cracks and crevices in the home due to poor sealing. But in passive house construction, we can more easily control the quality of the air entering the home using an ERV, because almost all the air that filters in or out of the house does so through the ventilator unit. The result is that the air is healthier for the humans inside. ERVs use excellent air filters, which means that allergens are filtered out. In this case, we used a Zehnder ERV.  Zehnder offers a range of options in the way of ERVs.

Blower Door Test

As you may have guessed by now, the secret to efficiency and quality in both the heating and ventilation systems of a passive house hinges on the effectiveness of the airtight building envelope. The more airtight the house is, the lower the heating load (and therefore energy usage) will be. This is where the blower door test comes in.

A blower door test is a diagnostic tool that measures air infiltration through walls, roofs, and windows of the building, and in doing so provides an accurate read of a home’s airtightness. The test is done by mounting a blower door to the frame of one of the home’s exterior doors, using a fan to blow air out of the house in order to depressurize the inside of the house, and setting up a manometer to measure air pressure and air flow. Once the house has been depressurized, air leakage through the walls and other potential leakage points can be more easily detected and then sealed up. This should be done before drywall is installed so you have a chance to fix the leaks.

Stay Tuned: Solar Power and Energy Savings

Wondering how all this translates into energy use, cost, and the lived experience of inhabiting the home? We were too—and fortunately, the owners obliged, sharing some data about overall energy use in the home, following the installation of solar panels in March 2020. Stay tuned as we discuss the home’s overall performance since completion, in the final blog post in the Mud Lake series, coming soon.

Interested in Building Your Own Net-Zero Energy Home?

Get in touch with the Passive House Experts at TDS Custom Construction today.