Air and Moisture Management

Water Resistive Barrier (WRB)

The WRB is not only a code requirement but it is something you just don’t want to screw around with. Try to purchase the best system you can afford, ideally a ‘smart barrier’ that is resistant to bulk water but at the same time open to vapor diffusion so that the wall can dry to the exterior. Some of the best products are Solitex Adhero or Mento 1000 by Pro Clima, Majvest by Siga, or anything equivalent. If you want to cheap out then just use 15# asphalt felt paper (cellulose mixed with asphalt, which is an industrial waste product) instead of virgin petro-based Tyvek or some other generic knock-off house wrap.

Rainscreens are not optional

There seems to be some debate in terms of whether or not a rainscreen is necessary because some siding manufacturers will still warranty their product even without it. For example, I believe you are “allowed” to install James Hardie fiber cement siding directly over Zip sheathing without voiding either warranty. I don’t care what they will or won’t warranty because warranty claims are kind of a mythical creature and the manufacturers will take that bet all day long. Who has ever made a warranty claim twenty years later? Who determines if it was a product defect, an installation defect or a design flaw? The only thing we know for sure is that the building scientists recommend a rain screen and the bigger you can make it the better it will perform. They also say it needs to be more than ¼” so that means any sort of textured plastic wrap with its “patented three-dimensional design” isn’t going to work as well as a readily available 1x3 furring strip made of, you guessed it, untreated wood (aka cellulose). Also, for cedar shakes I don’t think a plastic dimpled rainscreen is going to work any better than 2 layers of 15# roofing felt that buckles naturally and has worked well for over a century (but not as well as a full 3/4” rainscreen). The other thing we know is the cost of any patented proprietary product is always going to be either more expensive or exactly the same as wood - not because it really costs more but because they know that is what you are comparing it to and they know their market better than we do. In other words, don’t believe the hype and just stick with wood.

Open-Built

The interior of the wall should be “open-built” which means there is an open chase for running wiring and other utilities that is more easily accessible to accommodate future work. Even if you have to remove finishes for light remodeling, it is comforting to know the exterior wall assembly will remain tight. Having an interior chase also helps prevent accidental violations of the air barrier by even the most conscientious subcontractors.

Wrapping Up

Finishes

The nice thing about the plant-based wall assembly is you can put whatever finish you want on both the inside and outside. The furring strips at each stud are great for panels or horizontal siding but not so great for vertical siding or shakes. In those cases, you would have to add a second layer of horizontal furring. It sounds like a pain but if you think about it, you are talking about a handful of hours of work and a couple hundred dollars of materials to build something that should last for over 100 years or more. Suck it up! Or stick with horizontal siding.

But what materials should we use for the finishes? The basic rule is to stick with low embodied energy materials like FSC wood or wood composite rather than metal, cement or plastic. Don’t be afraid of wood strand or fiber boards like LP Smart Siding or Miratec because you’ll have this incredible rainscreen to help keep the siding dry should it ever get wet on the backside. Metal siding is cool and everything but I would rather make buildings that are easily adaptable to changes over time. Good luck cutting and patching corrugated metal siding. Wood filler, caulk, and paint have been in use for centuries compared to metal siding, which is clearly a fashionable trend at the moment. Don’t get me wrong, as a designer, I like fashion trends and will definitely use metal siding in certain applications.

Speaking of fashionable trends, is Yakisugi (aka Shou Sugi Ban or Charred Cypress) fashionable or a tried and true ancient and sustainable technique? We will have to cover that in a future post.

Details

So, in this final installment, we want to share the final results of many years of internal debate and discussion about what we believe to be the best wall assembly for new construction projects. Below are the three basic wall types starting with the highest performance wall on the left and a lower-performing but still better than code and equally carbon positive assembly on the right. All three walls use the 2x8 stud as the structure and cellulose insulation with the differences simply being the various optional layers for air/moisture barriers and substrates for finishes that can be added or subtracted depending on budget.

Final thoughts

Consider panelizing the studs in a shop but everything else can be done on-site fairly easily. Panelization companies need to take care to ensure proper fire ratings such as at exterior walls close to any lot lines and double top plates where separating tenants horizontally. Bensonwood out of Walpole, NH effectively has its own version of this wall using engineered lumber and a sandwich of plywood that works well for factory-built projects. Maybe we should just go with Bensonwood when it makes sense and there is little doubt that factory-built homes are the future (of course we’ve literally been saying that for over 100 years!). But having worked with Bensonwood in the past, we know that it takes an incredible amount of upfront planning and time to get into their queue. Large scale multi-family projects, upscale homes, and subdivisions full of single-family homes en masse make sense but individual new homes and renovations can be done much more easily and less expensively and with nearly the same performance as a factory-built home as long as you know what you are doing. Just please don't fiddle with the core wall assembly during the design process so the architects can focus more on the outward appearance and aesthetic details of your building.

The STREAM Wall - Part 3 of 3