Editor’s note: This is one in a series of blogs detailing the construction of a net-zero energy house in Point Roberts, Washington, by an owner-builder with relatively little building experience. A list of Matt Bath’s GBA articles can be found at the bottom of this page. You’ll find Matt Bath’s full blog, Saving Sustainably, here. If you want to follow project costs, you can keep an eye on a budget worksheet here.
Not too many people have heard of an ERV (energy-recovery ventilator) or the very similar HRV (heat-recovery ventilator). These machines are relatively new to residential buildings so you would be hard pressed to find one in anything other than a highly efficient house. This is unfortunate because they make a house healthier, more efficient, and more comfortable for the occupants at a very low operating cost (although there is a decent upfront cost).
Houses lose heat in the winter (and gain heat in the summer) mainly through conduction and infiltration. Conduction is the biggest culprit, and fighting it is the reason I designed the house with a ton of insulation in the walls, attic, and under the foundation, and installed windows with very low U-factors.
Infiltration is right behind conduction, though, and it can be responsible for as much as 35% of total heat loss. Fighting infiltration is the reason I will be using a ton of caulk and spray foam to seal any penetration in the walls, attic, or foundation. I also designed the house with both a continuous outer and inner air barrier, and will be running a blower door test as soon as the outer air barrier is complete so I can seal up any flaws I find before covering it.
The vast majority of houses have a major problem with infiltration, and I have even seen plenty of new construction with massive breaches in the air barrier. Most building codes now require blower door testing, but in my area enforcement is nonexistent. The result is that most home buyers pay the price in heating or cooling costs over the lifetime of the house with zero consequences for the builder. The flip side to this is that these average houses are very well ventilated. Air goes in and out through cracks and crevices all day and night.
In a net-zero home with a well designed air barrier, this certainly is not the case. Very little air escapes the house, so it is very important to ensure the house is well ventilated. Many builders don’t like the idea of building a tight house where air can’t escape because, they insist, the house won’t get any fresh air.
Don’t you agree that it makes a little more sense to build the house tight and then ventilate the house properly so that all of the ventilation air is run through a high-quality filter before it comes in? This is where an ERV comes in. It takes stale air from the bathrooms and kitchen and vents it outside, while drawing in fresh air from the outside (after running it through an air filter).
In winter, as the two streams of air pass each other, heat from the outgoing air is passed to the incoming air in a heat exchanger (an ERV exchanges not only heat but also moisture). My ERV (the UltimateAir 200DX RecoupAerator) has a sensible heat recovery efficiency of up to 84%. It also will exchange up to 75% of its moisture and filter out 95% of air particles larger than 1.8 microns. For some perspective, a human hair is 100 microns thick!
The result is a healthier house with comfortable humidity all through the year. When you combine it with excellent insulation, you also get a house that doesn’t need a whole lot of added heat to keep it staying at just the right temperature.
Planning the ductwork
Turns are formed in round, metal duct by using adjustable elbows. The elbows have four asymmetrical rings that swivel so that you can form any bend you desire between 0 and 90 degrees. It is a gloriously intelligent yet simple design. Although they have the ability to make those tight turns, I kept mine to a maximum of 45 degrees with just a few exceptions, and used two of them together anytime I needed to make a 90-degree turn. This will mean smoother airflow inside the duct with less turbulence and will improve the system’s efficiency.
Sealing the ducts can be accomplished either with a UL-rated aluminum duct tape (this is very different than what most people know as duct tape) or mastic, which has the consistency of mayonnaise and hardens like a cast. I mostly used the tape, but there were a couple of areas (where I thought the duct needed strengthening) where I used mastic .
Sealing the ducts is not as important as it would be if they were installed in an unconditioned space. If you have duct leaks in an unconditioned attic, you are allowing air you just heated to escape into the attic. That said, sealing is still important when you install ducts in a conditioned space, because it reduces turbulence and ensures that the exhaust air doesn’t leak back into the house.
Assembling the ducts
I began the installation of the ductwork at the ends of the branches and worked my way back to the ERV. At the end of each branch, I measured the length of straight duct that I needed before the first fitting and added a half an inch. Duct terminations stand out a half-inch ahead of the studs just like electrical boxes. This way, they will end up flush with the 1/2-inch drywall. I used scrap pieces of 1/2-inch plywood and some scrap wire to hold the duct the right distance from the wall.
Straight sections of round metal duct come in rolled sheets that must be snapped together in a zipper-like fashion. They are much easier to cut while they are still flat, however. I hooked the end of my tape measure around the end of the duct and then held a marker at the right distance as I pulled both ends of the tape at the same time around the metal. This is a great way to make a straight line on a curved surface. Then, I held the metal as straight as possible while I cut down the line with some tin snips. Snapping the duct together can be frustrating, but if you keep working at it from both ends it will finally surrender to your will.
Both the straight sections and the elbows come with one side already crimped. This reduces the diameter so it can fit inside of the piece you are connecting it to. The most efficient way to install the duct is to face the crimped ends in the direction of the airflow, meaning crimped ends towards the ERV in exhaust lines and away from it in supply lines. This will eliminate the added turbulence that would be caused by airflow hitting the crimped edge as it leaves the fitting. For the exhaust terminations, I started with the uncrimped side, but for the supply terminations, I began with the crimped side. Sometimes you need to crimp the end yourself, and this can be accomplished quite easily with a pair of needle-nose pliers, as shown in the photo below.
After dry-fitting each section to make sure it was the right size or angle, I removed it and used tape or mastic along the seam of straight sections and along the seams between the adjustable rings of the elbows. For each section, I used three self-tapping sheet metal screws to secure it to the previous one, and then covered that seam with tape or mastic as well. In the photo below, you can see how the upper duct has been secured with the sheet metal screws, and the screws in the lower duct have been taped over to air seal the seam.