Question-and-Answer Resource for the Building Energy Modeling Community
Get started with the Help page
Ask Your Question

Downtown mid-rise multi-family Net Zero?

asked 2016-11-04 13:20:16 -0500

Jim Dirkes gravatar image

Dear forum, We have a client who wants to make their vacant-for-a-long-time, 7 story, 20K square feet per floor, multi-family building Net Zero. We've modeled an assortment of energy conserving options and think 50% savings compared to 90.1-2007 isn't bad, given the site and occupancy constraints.

Nonetheless, the client is asking whether we can figure out how to get to Net Zero. I don't think I'm smart enough, or have enough NZE experience, or both to do that. The roof is about 24,000 sq.ft. We (not measuring carefully) put 250kW of PV in the model, just to see how much that would move the energy usage. That resulted in about 12-14% additional savings - not too close to NZE.

Here's the question: If money was not a problem and return on investment didn't matter (it does, but pretend it doesn't), what would you do to achieve Net Zero?

Thanks for your brainstorming efforts!

p.s., we can add wall and roof insulation, improve the existing ~30% WWR glazing and are currently using VRF + DOAS HVAC

image description

edit retag flag offensive close merge delete


  • Location?
    • Square footage of apartments?
    • South facade shading?
    • Water heating fuel?
aparker gravatar imageaparker ( 2016-11-04 13:31:49 -0500 )edit

Sorry not to have included these... Midwest (Michigan) 6 of 7 floors are apartments (1st floor is commercial office space) No shading exists, but could be added (Note South is the small dimension and the other elevations have partial shading from adjacent buildings) Water heating can be any fuel we wish.

Jim Dirkes gravatar imageJim Dirkes ( 2016-11-04 14:40:36 -0500 )edit

Sorry for the poor formatting. I'll get better at it.

Jim Dirkes gravatar imageJim Dirkes ( 2016-11-04 14:41:22 -0500 )edit

4 Answers

Sort by ยป oldest newest most voted

answered 2016-11-07 11:24:39 -0500

At some point for ZNE it moves away from "% better than code and then buy solar" to "can we stay under our solar budget?"

Your solar budget is:

24,000 sf roof * 90% roof area utilization * ~19 W/sf (assuming flat, high-efficiency panel) * (1 kW/1000 W) * 1,100 kWh/kW-yr = ~450,000 kWh/yr, or a roof production of 62 kBtu/sf-yr.
If you have 7 floors, that means your overall EUI target for the building is about 9 kBtu/sf-yr.

This will be very difficult to hit. You need more energy production. Some ways to do that are:

  • Elevate solar above roof, to get full area (increases target to ~10 kBtu/sf-yr)
  • Add solar to facades /shading / canopies
  • Figure out another energy source

I think you may be able to get down to ~20-25 kBtu/sf-yr through energy efficiency measures.

"Mandatory" measures:

  • All LED lighting
  • Energy Star Tier 3 appliances (refrigerator, dishwashers, etc.) + induction cooktops
  • Lots of insulation, good windows

Also consider:

  • Solar hot water heating
  • Heat pump water heater
  • Drain Water Heat Recovery
  • Sewer line-integrated heat pump/heat recovery
  • Compact Hot Water Distribution
  • Low-flow showers, common laundry
  • Each unit gets a Heat Recovery Ventilators instead of bathrooms exhaust fans for ventilation, with spot boosting for kitchen range hoods.
  • VRF heating/cooling or central heat pump serving baseboard hot water loop in each unit (+mass and radiant if you have the $)
  • Ceiling fans provided for each unit
  • Hit a target infiltration rate with airborne sealant deposition. (Aeroseal, the duct sealing company, now provides a service to seal multifamily units and hit a guaranteed infiltration rate. <1 ACH50 infiltration).

California is requiring all new homes to be ZNE in 2019, including multifamily (I'm not sure how they handle multi-story buildings):

You can see a list of the measures they are considering for this in presentations here:

edit flag offensive delete link more


My head is spinning! I think I'm way smarter than I was and thank everyone for the excellent input. I'm going into a quiet corner now to sort this out...... Thank you!

Jim Dirkes gravatar imageJim Dirkes ( 2016-11-07 11:58:40 -0500 )edit

answered 2016-11-05 12:18:52 -0500

updated 2016-11-05 21:51:58 -0500

You mentioned that you have achieved 50% savings from 90.1 2007, so I trust you have already looked at the ASHRAE Advanced Energy Design Guides (AEDG)? If you haven't, you may want to look at them. They don't have any large residential guides, but you may still get some helpful ideas from those guides specific to your climate zone:

Have you checked out Tom Hootman's Book 'Net Zero Energy Design: A Guide for Commercial Architecture'? Again, not specific to residential buildings, but gives a good overview of NZE strategies:

I'm pretty new to net zero design as well, but I'll give a shot at some other strategies that might want to researched:

  • Plug load/phantom load management (smart outlets/smart strips). If the homeowners have a green thumb, they might put their devices on schedules.
  • Smart thermostat. Again, if the homeowners are engaged (which they will have to be if this is going to work), they might put their home on a smart schedule for when they are on vacation, not home, etc.
  • Overall, you might want to insist that the occupants are trained on how to use their home upon moving in? I would especially do this if you have a lot controls.
  • Engage all homeowners with a dashboard to meter their performance. May want to try to encourage a competition among each unit on who can be the 'greenest' occupant by having a monthly prize for who has the lowest EUI.
  • If possible, maybe install a clothesline for drying clothes? It might save energy from the dryers during the summer months, but being in Michigan those months might be pretty short. This would especially work well if you have engaged homeowners. I have no clue how you would determine the projected energy reduction.
  • Low flow fixtures = energy savings from hot water system. Less hot water to be heated if less is used.
  • Simple operable windows to reduce ventilation load. If you want to get fancy maybe also use CO2 monitors.
  • Use light colored finishes to increase the brightness of the space.
  • Light shelves to redirect light deeper into space:

image description

  • Electrochromic Glass (warning: some designers don't like the change in light color; not sure how that flower is going to like it)

image description

edit flag offensive delete link more

answered 2016-11-06 16:13:22 -0500

Have you tried going down the passive route...

Lots of exterior insulation around the building, triple glazing (this should reduce heating to near zero), natural ventilation during swing seasons and rather than VRF a radiant system. I understand that humidity might be an issue, less so for residential so perhaps a mechanical ventilation system in the summer with heat exchanger (more important really for the winter) and some dehumidification. This is what I see for extremely well performing buildings in Germany (building built today climate zone 5a). They tend to hook up to the district heating system so have the advantage of thermal hot water delivered to the building but maybe a really good heat pump will do the job instead if you have to all electric. Also have you tried solar thermal?

Those would be my first thoughts, something to ponder if you haven't already.


edit flag offensive delete link more

answered 2016-11-04 14:08:30 -0500

shanti gravatar image

cool project! A few additional questions in addition to Andrew's:

  • Other locations for PV/Solar hot water? Parking?
  • An upgrade up to passive house standards possible? Hot water, cooking, and dryers would then be the biggest load. See the new large passive house apartment building as part of Cornell Tech new campus:

Also be creative about integration of large PV into the roof, overhangs, and into the facade. Most of the urban net zero projects i've been following all do this:

Finally, the architecture at zero design competition from a few years ago had a lot of good multifamily net zero designs:

edit flag offensive delete link more

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer


Question Tools



Asked: 2016-11-04 13:20:16 -0500

Seen: 452 times

Last updated: Nov 07 '16