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A New Multifamily Will Put Passive House Performance to the Test

Half the Minneapolis building will be built to code, the other half to the Passive House standard

Posted on Dec 1 2017 by Scott Gibson

A planned multifamily project in Minneapolis will give designers a chance to see just how much of a difference Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. construction makes when it comes to energy consumption, if not tenant comfort.

Hook and Ladder Apartments, named after a nearby firefighting museum, is being planned as a pair of nearly identical looking buildings with a total of 118 apartments. Half the project will be built to a slightly enhanced state energy code, the other half to the PHIUS+2015 standard of the Passive House Institute U.S. According to a post at the Minneapolis Star Tribune, Hook and Ladder will be the first Passive House multifamily project in the state.

The $20 million project will provide housing for people earning less than 60% of the area's median income. The developers hope to show lenders and property owners that the added costs of Passive House construction can be offset by lower operating costs, the Star Tribune said.

Newport Partners, a California-based real estate firm, will own Hook and Ladder, but Kim Bretheim of LHB Architects in Minneapolis, which is designing it, said that the impetus came from a neighborhood association and Becky Landon, an affordable housing consultant in the area.

Construction of the 117,000-square-foot project is expected to start in 2018, Bretheim said by telephone, and take about a year.

The price of parking

Both sides of the apartment complex might have been built to the Passive House standard had it not been for the need for off-street parking, Bretheim said. The conventionally built side will have an underground parking garage, while the Passive House side will be built partially on a slab with no below-grade parking. Meeting Passive House specs will add roughly 10% to the cost of that building — about what the underground parking cost on the conventionally built side.

"Pricing we have now shows the cost of the buildings is essentially the same," Bretheim said. "The difference is that one is Passive House, and one has underground parking."

Sharing the same site, the two buildings will have the same mix of apartment types, and draw on the same pool of tenants. Because so many building and site characteristics will be the same, Bretheim said, the project will provide an unusual opportunity compare the performance difference between the two types of construction.

Costs for heating and cooling on the Passive House side should be considerably different than those on the conventionally built side, he said. In order to keep out-of-pocket expenses for tenants roughly the same, Newport Partners will pay all monthly utility costs.

Keeping construction simple

Designers looked for ways to keep construction relatively simple so subs won't be spending a lot of time and money figuring out unfamiliar building techniques.

"We wanted to make sure from a cost containment perspective that we were doing it in a simple way that does not leave the subcontractors who have never done this before filling uncertainty with cost margins," Bretheim said. "We're trying to build innovative walls consisting of components that people individually are used to building but add up to a cost-effective thermal and airtight barrier."

Differences between the two buildings were detailed by Jeff Hemer, the project manager for Hook and Ladder.

The exterior wall assembly for the conventional side of the project is a 2x8 stud wall with 8-inch fiberglass batts (compressed to 7 1/4 inches), commercial-grade Tyvek, and double-glazed fiberglass windows. Hemer said the cavity insulation would have an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of 24, with a wall assembly R-value of just under 22. On the Passive House side, walls will be framed with 2x6s and insulated with R-21 fiberglass batts. On the exterior is a 2-inch-thick insulated sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. combining plywood and polyisocyanurate foam. A layer of OSB on the inside of the stud wall will be an air and vapor barrier. In addition, the wall gets a service cavity on the inside for wiring. Hemer says that the total R-value for the Passive House wall is about 30.

Windows on the Passive House building will be triple-glazed vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate).. Windows for both sides of the project are coming from Pella. CladdingMaterials used on the roof and walls to enclose a house, providing protection against weather. on both buildings is a mix of masonry, fiber-cement, and metal paneling.

On both buildings, there's no cavity insulation in the I-joist roof framing but polyiso on top of the framing: R-36 on the conventional side, R-45 on the Passive House side.

Hemer said that modeling showed that R-30 exterior walls were enough to meet Passive House requirements. "One of the things we studied was the envelope systems of the buildings," he said. "The Weidt Group studied a lot of variations of R-values for wall assemblies, roof assemblies, and windows in order to find the sweet spot for those R-values and find that place where it made the most sense to invest in enhancing certain parts. You can build an R-50 wall, but the return on that is so minuscule that it's not worth the money to do that."

Heating and cooling

Hemer called the plans for heating and cooling one of the most unusual parts of the project. On the conventional side, through-the-wall Magic-pak units provide both heating and cooling as well as a fresh air intake.

For the Passive House building, designers chose a variable-refrigerant flow (VRF) heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump. system with a half-dozen rooftop compressors and ducted fan-coil units in each apartment. Fresh air will be provided by an energy-recovery ventilator.

As for renewables, Hemer said that's still being studied. The roof of the conventional building may get photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. panels while the Passive House side gets some kind of solar thermal system for hot water.

"Right now we're in the scoping out stage," Hemer said. "We want to see if we have enough money in our renewable energy budget to cover that system." With limited sunlight in winter, gas-fired hot water will have to be part of the mix even if the project finds money for the solar thermal system.

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  1. LHB Architects

Dec 4, 2017 11:13 AM ET

interesting points
by Erich Riesenberg

This sounds like such an interesting experiment.

The relatively modest increased amount of insulation to reach Passive standards, and the relatively modest extra cost of 10%.

Also, the side by side comparison. My house is two stories with a basement. Winter heat is supplied with a single heat pump on the ground floor, which keeps both the second floor and basement tolerable. A second heat pump in the basement for the coldest periods.

In this model, one building appears 3 story, the other 5 story. My guess is a vertical building would inherently require less heat than a shorter but wider building, because heat naturally rises.

Dec 6, 2017 1:55 PM ET

R-Value of the Walls
by Bill Burke

The article says: "The exterior wall assembly for the conventional side of the project is a 2x8 stud wall with fiberglass batt insulation, a commercial-grade Tyvek, and double-glazed fiberglass windows. Walls will have an R-value of about 25."

If you put high-density fiberglass batts in a 2 x 8 wood stud wall you get a u-factor of 0.054, or a little less than R-20. So I think the claim of an R-25 wall is mistaken and exaggerated.

Dec 6, 2017 1:59 PM ET

The White Group
by Bill Burke

As this project is in Minneapolis, the speaker probably meant The Weidt Group when speaking about the consultant that did analyses of various wall assemblies.

Dec 6, 2017 3:12 PM ET

Thanks, Bill
by Scott Gibson

Jeff Hemer has updated his estimates on the R-value for the 2x8 exterior walls, and I've corrected the spelling for the Weidt Group.

Dec 15, 2017 8:56 AM ET

Questionable Insulation
by Charles Leahy

This seems like a whole lot of work/complexity for not so great returns on insulation. And it is interesting that in such a cold climate they have chosen polyiso insulation for an exterior wrap - an insulation type that even Building Science Corp has noted LOSES thermal resistance (R value) as the temperature falls. So the whole wall R value of their wall assembly will fall when they need it to work the most. Plus you have leading architectural firms like KieranTimberlake coming out and saying they will no longer use insulation like EPS, XPS and Polyiso foam due to their overall negative affects on the planet. Plus with the materials selected fire protection is going to be another concern you will have to work against. It can be done simpler, stronger and safer with significantly higher insulation values at about the same cost.

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