The design of this home was, in part, a test of a design process—of a philosophy of design—known as performance-based design. The central idea is that performance analysis (energy, daylight, and thermal comfort analysis, among others) has a central role to play in informing design decisions at every step of the process.
Things have to come through the walls, roof, and floors — even in an airtight house. How do we detail these elements in a way that (1) keep bulk water out, (2) meets our ambitious (Passive House level) air-tightness goals, and (3) avoids risk of condensation? Here are my details.
How should you design and detail a wood stove in an airtight Passive House? Here are some of the considerations, and the approach we used on this project.
First off: should you even have a wood stove in a Passive House? This is rightly a matter of some debate. It’s certainly not necessary, as our temperature data have shown: this particular house has proven it can stay habitable even through the coldest of cold snaps—maintaining ~55 degF even in -20 degF outdoor temperatures without mechanical heating.
And, a wood stove in a Passive House comes with downsides and risks:
Below is the slideshow I presented on April 20, 2017 to the Iowa Building Enclosure Council, and to a sustainable construction class from Hawkeye Community College.
The version below is slightly edited to make it easier to follow online. (I tend to avoid text in my presentations, which can make them difficult to follow without accompaniment.)
I incorporated energy and daylight analysis as part of the conceptual design for the Iowa Nest Residence. This meant that early design discussions encompassed both traditional topics like floor plan layout, siting, and aesthetics, as well as performance criteria. The addition of analysis added a trivial amount of time, but proved incredibly valuable. Here’s how I did it — and what I will do differently next time.
Iowa, like much of the American Midwest, has a treacherous combination of deep, cold winters and hot, humid summers. Could we overcome this with good passive design? Could shading, natural ventilation, earth berming, and the like obviate the need for air conditioning?
Here’s how I went about answering this question. This analysis was done in early design so that the answer could inform basic design moves.
We knew we wanted a Net Zero Energy house — but was this a feasible goal? What would it take to deliver? Here’s how I investigated this question before we even had a design in hand.
In my post on passive strategies and simple box modeling, I showed how sensitivity analysis can be used to identify the most important passive strategies while still in the pre-design phase. In this post, I’ll show in depth how I did that analysis.
For reference, here is the end product: graphs showing the potential impact of a number of individual strategies.
Once we evaluated which passive design strategies were most important when it came to energy use and thermal comfort, we asked the question: which will give us the most bang for our buck? Rather than engage in a detailed (time-consuming) cost analysis at this early stage, we did a simple overlay on our existing sensitivity analysis — which provided some key insight for remarkably little effort.
Before I ever put pen to paper for this project, I wanted to answer a deceptively simple question: What passive design measures are most important? What basic strategies did the design need to employ to be successful? —to maintain human comfort with a minimum of added energy?
It’s important to answer this question before design begins in earnest for two reasons:
- With a limited budget, it’s important that we invest in the right things; and
- If we know the most important strategies before we start designing, we can incorporate them into the DNA of the design—often more effectively and at lower cost than if they were afterthoughts. For instance, if thermal mass or natural ventilation is important, these things will begin to dictate the construction or interior layout in ways that are difficult to “add on” later.