Blog

Posted from: MD, USA

After the building envelope, the key element of any Passive House is its ventilation system. Because the envelope is so tight, fresh air must be brought in mechanically during the heating and cooling months when the house is closed up. We do this with an energy recovery ventilator  (ERV)—a simple  fan system that continually brings in fresh air at very low velocity and exhausts stale air, exchanging energy and humidity between the two air streams in the process.  This is integral to every Passive House.  

The energy recovery ventilator is hooked up to a duct system that carries the fresh air to all the living spaces in the house and pulls stale air from all the bathrooms, the kitchen and the laundry. This creates basically a one-way trip through the house for the air, which is the reason why Passive Houses have proven to be so healthy—they don’t simply churn the air around and around as do standard forced-air heating and cooling systems.

The Zehnder Comfo-Air 350 ventilator we are using is 86% efficient in exchanging heat from outgoing and incoming air, and around 50% efficient in exchanging humidity. Its core is basically a matrix of straws that run two air streams past each other, as shown in the picture below of a small section of a core:

Its efficiency is further improved by the pretempering ground loop, described in an earlier post. To make up for the small amount of heat that is lost through the envelope and the exhaust air, we are running a water line from a heat exchange coil in the domestic hot water tank to another heat exchange coil in the fresh air supply line downstream of the ERV. This will transfer the small heat we need from the heated water into the air supply.

duct heat exchanger, above

water tank heat exchanger, below

In summertime, we must cool and dehumidify the air coming in through the ERV.  Most projects in the past have used a very small hi-efficiency Mitsubishi  heat pump (sometimes called a mini-split) to do this. They generally hang on a wall as shown in the picture below.

The difficulty with such a system in a house with five bedrooms is getting the cooled air to all of the spaces—after all, air doesn’t move through closed doors very well.  What makes our project unique among Passive House projects we know is that we are linking the heat pump to the ERV duct system so that we can distribute that cooled air to every living space. This took a lot of careful planning on the part of Dan Foley, our mechanical contractor, and Michael Lebeau, who led the mechanical component when I took the Passive House training.  

Because this is a large house on four levels, we have broken up the mechanical system into parallel systems. One handles the basement and first floor spaces; the other handles the attic and second floor spaces. This allows us much shorter duct runs and provides the ability to zone the house into two parts.

The final piece of the system is the hot water heater. Remember, it is giving us not just our hot water, but our winter heat as well. We are using a Viessmmann Vito-Cell hot water storage tank linked to two heating sources: a 96% efficiency Lochinvar gas fired burner and a solar hot water heat exchange coil.  

The ERV:

The water tank, with water boiler on the wall to the left. Lines not yet run:

And finally a couple of progress pictures.

Keith installing the stairway:

The siding going up:

Passive House Featured in USA Today

Wendy Koch wrote a nice piece on Passive Houses as the lead article in the Money section of Friday’s USA Today. That’s our project on the cover. Here’s the link: http://content.usatoday.com/communities/greenhouse/post/2011/02/passive-houses-aggressively-reduce-energy/1?csp=usat.me

Energy rater Chris Conway of Conway Green Construction visited the site last week with his blower door fan and infrared imaging camera to perform the first blower door tests of our house. It was a critical moment, determining whether all our efforts in taping and sealing had proven effective. It was important to do this before finishes were applied so that we could correct any problems without having to tear out anything.

The blower door testing protocol for a Passive House is quite rigorous. First of all, the number you must reach — .6 air changes per hour (ACH) at a pressure of 50 pascale — means that your building is virtually airtight. .6 ACH at 50 pascale translates to .05 ACH at normal air pressure. In other words, it takes about 20 hours for a single air change in a Passive House, whereas the rate is about 1 ACH for a typical 1950’s brick ranch house.

Second, the way the Passive House Institute requires that ACH number be calculated is more stringent. Whereas, most blower tests divide the total volume of air that escapes per minute by the total gross volume of the house. The PHI requires you divide that number by the net volume of the house, excluding all interior and exterior walls, and all floor framing areas from that gross area, so that you get a much smaller denominator in your equation, giving you a higher number for air changes per hour.

Our results
When we began, we were at about .67 air changes per hour at 50 pascale (As Chris explained to us, 10 pascale is the equivalent pressure to that required to raise the liquid in a soda straw about 1”). We then went around with Chris’ infrared cameral and quickly found a 2” hole that had inadvertently not been sealed and a few cracks under doors. In addition, Panelwrights had deliberately not completed all of the SIP taping so that we could learn what difference the tape actually made. It was clear from the first test and from the infrared imaging that the taping is essential.

After all taping was completed, we went around with the infrared camera looking at every exterior wall and roof surface in the building, finding every tiny bit of air infiltration. After this experience it is clear to me that thermal imaging is absolutely essential in getting a building tight enough to reach the Passive House standard.

What we found
The basement slab and walls were incredibly tight. We found virtually no leaks there, nor were there leaks at top of the basement walls, where the SIP panels bear upon the concrete.

We found several leaks in some of the windows and door. It is likely that these are places where the integral weatherstripping had pulled away from corners.Here is an example:

Differential temperatures between windows and their adjacent walls was between 4-5 degrees, just as expected. This assures us that there will be no drafts at our exterior walls waused by temperature differentials.

We found virtually no thermal bridging in the roof SIP in which we had inset a structural beam. We had set all beams carrying the roof below the SIP panels rather than in the panels in order to avoid  thermal bridging through the panel. In this one case, because of a headroom issue, we had kept the beam in the panel. The infrared image at that beam showed about a 1 degree difference in interior surface temperatures at either side of that beam.

One general weak point we noticed was at the taping of window frames. In every case, there were tiny leaks at the interior corners in the frames where the tape had not been pushed tight into the those corners. We went round the entire house and sealed those little gaps.

In the end we achieved a rating of .59 air changes per hour, just under the requirement. According to Chris, drywalling and trim will continue to increase our airtightness. Keith Kauffman, the construction superintendant, takes this personally and you can be sure that all finish carpenters will have caulking guns in their pockets as they install the trim at each exterior opening. The official blower door test will be performed when the building is completed.

Here are some more of the thermal images: See the scale at right to find temperature/color calibration. Here you see an un-taped SIP panel joint. The wispy lines of color in the corner indicate air movement there.

Above you can see a a wall penetration that didn’t get fully caulked.

And here is Chris, explaining how it all works in three short segments.

Posted from: MD, USA

The front porch framing just going up in this photo taken yesterday by Terry Hill. Work on the side porch and roof framing, (not shown) is now complete. Work on the west side deck framing is underway.

Thanks to Al Cobb for his clear explanation of the details of air sealing the SIP panels in the last two posts.

 This Friday, January 14, the last of our windows will arrive at the site! Construction has been held up for weeks while the manufacturer dealt with problems with the glass suppliers, but that will now be behind us and work will proceed apace.  Once windows are installed and taped next week, we will perform the first blower door test. Then, as Al noted, Panelwrights will tape all interior SIP joints and the test will be performed again. This double testing is not a part of the Passive House protocol for certification, but for our own verification of SIP performance.

 Once we are satisfied with the blower door testing results, we will schedule our close-in inspection with the County, and proceed with interior finishes. Suddenly big changes will become apparent, and as importantly, the workmen doing interior work will no longer have to work in sub-freezing weather!

Here is a series of photos of the out-sulation work which is now just about complete:

The first picture shows the nailer around a basement window before insulation is applied. You can see that this is basically a sandwich of two pieces of solid wood with high density rigid foam between them providing a thermal break. The outside face of the wood is in the same plane as the outside face of the EPS insulation that will later be installed.

In the picture above, that insulation has been installed above the window, not yet along the sides.

The window above has insulation installed, Tyvek housewrap installed over it, and taping all done. You can see two tapes here. The light colored tape is the flexible Tyvek tape that is adheres the Tyvek to the wood window framing. This is done before the window is set in place. The dark tape then seals the window’s metal mounting flange to the adjacent Tyvek. Bot these tapes are addressing moisture issues, not air infiltration issues. As Al pointed out earlier, we deal with the air sealing from the inside of the wall. Our primary goal on the outside of the wall is to keep the water out. The vertical wood strips to the right of the window are the furring for the HardiePlank siding, which will be the last step in closing in the wall. We fur the HardiePlank out from the face of the Tyvek to create an air space that allows for drainage of any water that gets through the HardiePlank.

Finally, below is a picture of the retaining wall at the rear (north east corner). In a typical home, the masonry wall would be tied into the concrete basement wall to make one integral structure. Here, we have kept it away from the basement wall so that the exterior foam insulation can run through uninterrupted. This required that we brace the retaining wall independently of the house. (The plumbing penetration has been foamed where it pierces the concrete and the gap in the visible insulation will be foamed as well before the exterior finish is applied.

The Outsulation is now being installed on the envelope of the entire project. The Panelwrights crew is battling windy conditions that make installing exterior sheet insulation (Outsulation) and house-wrap a difficult exercise. This stage of the process is critical to addressing all the potential areas where a thermal bridge could allow the loss of energy through the wall assembly.

All windows and doors get a perimeter of dimensional lumber and high density EPS. The foam has a compressive strength of 25psi and the dimensional lumber is screwed tight to the perimeter with Trufast screws. These heavy duty fasteners allow the dimensional lumber to act as a solid attachment for the window flange. In addition, the foam disconnects the thermal bridge of the imbedded lumber in the SIPs which surrounds the window and door openings.

This north wall is being covered with the standard two layers of EPS foam. The sheets are staggered and screwed in place awaiting a covering of house-wrap and vertical batons.

Notice the two layers of foam and how they cover the area where solid lumber would effectively bleed energy from the structure. The lower horizontal band is both the blocking for the transition trim between siding and foundation as well as the solid support for the vertical batons. This band is also held off the structure by high density EPS.

In addition to our work on the exterior, the other trades have begun their own installation. Due to a pre-construction meeting, all the trades are well versed in the correct way to approach this project. For a successful build, every member of the building force has to understand the importance of preserving the integrity of air-tightness in the envelope. We have identified every penetration in the entire envelope and then detailed the specification for both making the penetration and subsequent sealing.

This clearly shows how the plumber cut an oversized hole in the envelope before inserting the PVC pipe. This oversized hole allowed the Panelwrights to inject a single component foam in the space around the pipe and get a perfect seal.

The window package has been partially delivered and the installation is underway. If all stays on schedule, we should have a blower door test that will let us celebrate a very Happy New Year.