Balancing embodied & operating carbon – wall assembly
Context
I’m building an apartment building in Minneapolis (climate zone 6) trying to balance three goals: 1) minimize the embodied carbon of the building 2) minimize operating carbon emissions and 3) make the pro-forma work.
The building will be all electric (mini-splits for heating/cooling) with a solar array sufficient to meet the needs of all six units.
The originally specified wall assembly was 2×6 studs @ 24 OC, dense packed cellullulos in wall cavity 1)and 3″ of continuous GPS exterior insulation.
Question: Which assembly would you choose given my goals?
Situation
In the midst of value engineering with the builder, it looks like I need to drop the GPS all together. The proposed wall assemblies (cost equivalent) are:
1) SIPS using FSC certified plywood and GPS insulation achieving R29
2) Stick framing with T-Studs and dense packed cellulose (R20/21)
Option 2 has lower embodied carbon by quite a bit, but has 30% lower R-value than the SIP assembly. That said, the building will be heated/cooled with solar power…so the lower operating efficiency doesn’t result in more carbon emissions.
I don’t have the knowledge, team or extra budget to pay someone to actually calculate the expected trade-off in carbon….so would love the community’s thoughts on this dilemma!
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Replies
You can also look at 2x8 24OC walls dense packed. This would not be much more than 2x6 and would bump up your assembly R value by ~25%.
Next set up would be 9.5" or 11 7/8" TJI for the wall studs dense packed. This would get you above your original assembly R value without any exterior insulation and mostly standard construction techniques. It would would require more engineering to spec the design details.
Looks like there is recycled 2.5" polyiso for $8/sheet available:
https://minneapolis.craigslist.org/wsh/mad/d/jackson-roofers-contractors-25x4x8/7162659667.html
There is a recycled insulation depot in Chicago with other thicknesses available; shipping is kind of steep unless you are getting a building's worth:
https://www.repurposedmaterialsinc.com
Consider 2×6 studs @ 24 OC, dense packed cellulose and then 1 to 2" of somewhat vapor permeable exterior foam. Unfaced EPS is a good choice, <= 1.5" might be easier to construct (no furring/screws according to this). With good air sealing and a Class II vapor retarder it's a good design.
I had the builder option 1.5 vs. 3" of GPS (cost comparable to EPS) and the difference was negligible. The trips around the building and details associated with the continuous exterior insulation were the cost drivers. The Building Science article "Incorporating Thick Layers
of Exterior Rigid Insulation on Walls" indicates that the method for attaching cladding through continuous exterior insulation is the same for the relatively shallow depths we are discussing. (just a long fastener that hits the stud).
Perhaps a better price with foam on the interior side?
If you are sizing the solar to meet the heating need, you'll need a bigger solar array to meet the heating needs with lower R-value. Then we get into asking about the embodied carbon in the solar array vs. the insulation. That's a good PhD project for someone, but if you go with some of the suggestions of reclaimed foam or thicker dense pack, you are likely to be able to have great insulation at low cost and low embodied carbon.
"That said, the building will be heated/cooled with solar power…so the lower operating efficiency doesn’t result in more carbon emissions."
It results in fewer expensive solar panels. Have you traded off costs here against those costs?
"1) SIPS using FSC certified plywood and GPS insulation achieving R29"
SIPS can be finicky. There have been some significant cold-weather failures (Lstiburek has talks about a whole series of failures in Alaska requiring thousands of new roofs). If the contractor is having issues with trips around the building for continuous insulation, will they have issues with the delicate task of properly air-sealing SIPS properly?
"2) Stick framing with T-Studs and dense packed cellulose (R20/21)"
Have you examined framing with 2x8 or 2x10 studs, or the TJI ("wood I-joist") equivalents, and cellulose? Or doing a double-stud wall with cellulose? MN gets some of the roughest weather in the country. This isn't a carbon issue necessarily (you're already way ahead of a single family home just by virtue of it being an apartment building), but it's a significant comfort issue for the rooms with an exterior wall. (Likewise triple-glazed windows).
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Most twin cities area apartments are now framed with 2x8 to meet current energy code. I would honestly steer you more toward that than your 2 options simply because it's familiar to all trades, commercial subs will get it more airtight than SIPs, it's probably similar or better in overall U factor to the Tstud assembly, and you'll get better pricing that can be invested elsewhere on the project.
Of course, I'm shooting from the hip and haven't done the exact math. Take with a grain of salt. 2x8 @ 24" O.C. dense packed is not too shabby if continuous exterior insulation isn't in the cards.
Best,
j
Edit: definitely use plywood and/or paperless gypsum sheathing on the exterior if you go this route.
Cmfisher,
I thinks it's great that developers like you are even thinking about this!
I'm going to go against the grain here a little and say that the r value of your walls may not actually be that important.
Especially if this building is to have several units that are smaller than 1,000 square feet and particularly if multiple people are living in each unit. Because multi-family units share walls,floors, and ceilings, there is less surface area to the exterior and therefore less heat loss. You just don't need massive walls to hit really low heat loss numbers.
You mention that you will be using ductless mini splits to heat/cool the units. There are few units available that can modulate low enough during 3/4 of the year to meet really low loads. For instance , your manual J calc may show heat loss per unit of 700 BTUs per hour at 25 degrees. Few (if any) units ductless mini splits will modulate that low so they will be short cycling and possibly using more energy.
It sounds crazy, but until there is a better solution (ultra-low modulating heat pumps) for multi-family units, you may actually use MORE energy with thicker walls than you would with your R-22 walls due to over-sized heat pump units. Then again, even with r-22 walls, your loads might still be too low to prevent short cycling.
Personally, I feel that when it comes to most multi-family projects, I think embodied energy is way more important than reducing heat loads. As a green builder, I would focus on solar (as you are) and including efficient appliances such as heat pump hot water heaters/dryers, induction cook tops, etc. Try to create floor plans that eliminate the use of steel beams. Use advanced framing.
I might still run some quick heat loss figures on excel to find a sweet spot for insulation. It's easier and cheaper to increase insulation at the ceiling and floor plane than the walls. Raised heel trusses and Super-Insulated slab will help.
One more quick note:
Thick walls are great if you ditch the mini splits and go with baseboard heat. This may actually save you tens of thousands of dollars as baseboard is inexpensive (and you'll likely still need some anyway). Yes-generally speaking- baseboard heat is far less efficient than heat pumps, but- your loads could be as low as 2000-4000 BTUs/hr per unit in the dead of winter. Mini split units will have lower COP then anyway and short cycling the rest of the heating year. Baseboard is a real option.
The only issue is air conditioning. You'll have to figure this out separately and this may negate any cost savings from opting to baseboard heat. Window units are an option but they're ugly. Also, it's hard to be super insulated/air tight using windows that accommodate window units. You can forget casement and tilt n turn style windows.
Rick,
For a project with more density than six units, I would agree with you to increase focus on systems and operational energy reductions versus more exterior envelope expenses. But for this climate and project size, walls are still a weak link.
Code minimum for wood framed walls here is R20+ 3.8ci or R13+7.5ci or U 0.051. Tstud gets there but not by much. They list U 0.047 at 16.3% framing factor with cellulose. 2x8 dense packed will get you more like U 0.041 at the same framing factor and should be a better bargain all things considered.
Jason,
How big is this project? Sorry if I missed it but I didn't see it specified. Does the code you you referenced apply to apartment buildings (3 units+)? I think you can get away with lower r values in the international building code?
I've run manual j calcs on a multi family building here in zone 6 (NH). I stand by my comments that low loads 2-6,000 BTUs/hr at 99% dt) are achievable for a multi family unit even with skimpy-ish walls and even in this climate.
Rick,
OP stated six units, but not overall project SF. There's definitely ways to "get away with lower r values". For example MN accepts Comcheck as an alternative to IECC prescriptive requirements, which allows some pretty egregious 'tradeoffs'. The numbers I quoted were direct from IECC and are a better baseline minimum.
I for one don't accept that the envelope thermal resistance should be sized to suit a standard heat pump peak capacity. Cart before horse. New Hampshire may also be in climate zone 6, but January 99% design temp out there is unfortunately not equivalent to a January here. Look up Minneapolis sustained low temps for 2019...
I will say I follow your logic though, and the cost-effectiveness battle has all the chips stacked against better envelope efficiency when there are truly no heating/cooling units small enough to match small, well-built multifamily units, apart from baseboard heat. Ain't it a shame?
Just seeing this now.... Jason, I think we are actually mostly in agreement here :-)
"New Hampshire may also be in climate zone 6, but January 99% design temp out there is unfortunately not equivalent to a January here"
According to the Energy star guide, Lebanon NH (where the building was located) has a 99% design temp of -18F while Hennepin County, MN is -11F. I used -7F for my Manual J which probably is a reasonable number for both climates. Minneapolis, to your point, may be slightly lower at between -8.6F to -11F depending upon the neighborhood.
https://www.energystar.gov/sites/default/files/asset/document/Design%20Temperature%20Limit%20Reference%20Guide%20%282019%20Ed%29%20-%20ENERGY%20STAR%20Certified%20Homes_Rev10.pdf
"I for one don't accept that the envelope thermal resistance should be sized to suit a standard heat pump peak capacity. Cart before horse."
I agree- especially for single family homes where even the most super insulated home in a cold climate will still likely have loads above 6,000 btus/hr in the dead of winter. Add as much insulation as you can afford and then add the smallest heat pump you can find (there are at least a couple to choose from).
But, if you add a lot of insulation to a multi-family unit (which is the subject of this question), then your loads will be so small that you have to question whether or not currently available mini-split units can modulate down to these micro-loads especially in the shoulder seasons. There may also be comfort issues as the equipment won't be able to run long enough to reduce the latent load.
If you go the mini-split route, what kind of COPs will be achieved? Is there much- if any- energy savings achieved if the COP is measurably lower for thick walls compared to skinnier walls (especially when we are dealing with such low loads anyway)? If you've already got your peak heat loads down to 3-4,000 btus/hr with R-15 walls then do we really need more insulation or should you focus on other areas? The energy saved (if there is any savings at all due to lower COPS) by getting your peak loads down from say 3,000 btu's/hr to 2,500 btus per hour is minimal. This is why I say that up-front embodied energy/carbon is more important than reducing HVAC energy load for many multi family units. As I said before, more attention should be given to hot water use, solar, induction cooktops, heat pump dryer, etc.
"There's definitely ways to "get away with lower r values". For example MN accepts Comcheck as an alternative to IECC prescriptive requirements, which allows some pretty egregious 'tradeoffs'. The numbers I quoted were direct from IECC and are a better baseline minimum."
Makes sense to me. If code requires thicker walls the the debate about thicker walls is a moot point. Ya gotta do it! :-)
Jason & Rick,
Your back and fourth here provided some incredibly rich material for me to dig into with my build team. Thank you!
Unfortunately cost is continuing to be an issue (and this is for an infill project that has an effective land cost of zero).
I may now have to go the PTAC route for heating and cooling in the units. 6 gaping holes in the exterior envelope, and all the other well known problems... I looked hard for a cold climate PTHP solution but there are none in my market. The Innova product (https://innova-usa.com/hpac-2-0/) seems perfect, but would be $45K delivered for the 12 units I'd need. Ice Air has a PTHP that works down to 17F before kicking on electric heat, but they don't seem interested in supplying me here.
Any thoughts on PTACs that are slightly less egregious from an air leaking and thermal bridging perspective?
I am going to do a cost exercise on the 2x8 framing option, but am guessing it will also be a bridge to far.
Here's a high performance ptac/pthp to consider.
https://www.hotspotenergy.com/hotel-air-conditioner/
Most PTACs are non modulating. Even the smallest one would probably be at least 4x over sized for such small places, never mind putting two into each unit as you propose.
The units are small enough that you don't need a hyper heat mini split even in your climate. You can go with one of the budget mini split that has a base pan heater (I've gotten these for about $800). Installed, this plus a length of electric baseboard in the bedrooms/bath would be comparable to the cost of 2x PTAC+sleeves and much better efficiency/comfort/noise.
It would be interesting to know if paying twice that for something like a Gree Sapphire would pay for itself in operation down to lower ambient temperatures.
Note that if the occupant sets the bedrooms/bath temperature to the same as the main room, then 100% of the bedrooms/bath loads will be supplied by resistance heat. All season long.
Note that mini-split over-sizing/cycling should be a < 15% efficiency issue. Switching to resistance heat is a not comparable 200-300% efficiency issue. But best to convert these to dollars.
Can you link to an example of a "budget mini split with base pan heater"?
>"I may now have to go the PTAC route for heating and cooling in the units. 6 gaping holes in the exterior envelope, and all the other well known problems... I looked hard for a cold climate PTHP solution but there are none in my market. The Innova product (https://innova-usa.com/hpac-2-0/) seems perfect, but would be $45K delivered for the 12 units I'd need. Ice Air has a PTHP that works down to 17F before kicking on electric heat, but they don't seem interested in supplying me here."
It doesn't have to work in heat pump only at your January mean temp to make a HUGE difference in seasonal energy use. Add up the hourly bins at temps below the unit' changeover temps, and compare that to the bins where it's operating in heat pump mode. MOST of the heating season hours and heating energy use (even in Minneapolis) is going to be above the typical ~ 25F crossover temp of a random low-cost PTHP:
https://www.climatestations.com/images/stories/minneapolis/mspcumt.gif
Even in January a large fraction of the hours are above 25F, and over the 10 coldest weeks of the heating season about half the total hours are above 25F:
https://weatherspark.com/m/10405/1/Average-Weather-in-January-in-Minneapolis-Minnesota-United-States#Sections-Temperature
For the long shoulder seasons MOST of the hours are above 25F.
In low load buildings in relatively low-cost and greener grid locations it's not necessarily going to be "worth it" to pay a premium for a low-temp PTHP or mini-split that fully covers the 99% or even 95% heat load in heat pump-only mode. This is even more true for non-modulating systems, where oversizing to cover an only slightly lower temperature bin results in significantly more on/off cycling during the much longer shoulder seasons.
@Akos - it sounds like this is your recommended solution "You can go with one of the budget mini split that has a base pan heater (I've gotten these for about $800). Installed, this plus a length of electric baseboard in the bedrooms/bath would be comparable to the cost of 2x PTAC+sleeves and much better efficiency/comfort/noise."
@Danna Dorsett - it reads like you are suggesting any low cost PTHP as a reasonable choice.
BTW - it is three stories, 1399 SF footprint. The largest unit size is 624SF. Attached is a snapshot of the floor plan.
I'm honestly swimming in this discussion a bit at this point, and am thinking I should just hire an engineer to design the system with cost in mind. Feels like the HVAC contractor is pushing an oversized system and isn't considering all the variables in this thread.
Any HVAC engineer suggestions?
Without somebody doing a load calc on the actual units, it is hard to say which way is the best and what is over/under sized. My recommendation was from my experience with less well insulated multi family units, the loads tend to be very low.
As for budget mini split, I've used the 12000btu version of this and works reasonably well. The capacity does drop off significantly near 0F, but it should still be able to heat the small units you are proposing especially if there is supplemental heat in the bedrooms and bathroom.
https://ashp.neep.org/#!/product/25289
@Akos - it sounds like this is your recommended solution "You can go with one of the budget mini split that has a base pan heater (I've gotten these for about $800). Installed, this plus a length of electric baseboard in the bedrooms/bath would be comparable to the cost of 2x PTAC+sleeves and much better efficiency/comfort/noise."
@Danna Dorsett - it reads like you are suggesting any low cost PTHP as a reasonable choice.
BTW - it is three stories, 1399 SF footprint. The largest unit size is 624SF. Attached is a snapshot of the floor plan.
I'm honestly swimming in this discussion a bit at this point, and am thinking I should just hire an engineer to design the system with cost in mind. Feels like the HVAC contractor is pushing an oversized system and isn't considering all the variables in this thread.
Any suggestions
cmfisher,
Looks like a great building! I like the design. You're on to something great here- don't give up! :-)
What windows were you planning on using? Are these high-end units? I am just wondering if you can get away with baseboard heat and window units in bedrooms (especially if the bedrooms face the rear for the building)?
Thanks for the encouragement and all the engagement on this thread Rick!
All windows are either fixed or casements. I spent a massive amount of time trying to find fiberglass frames with a sub 0.19 u-value that I could afford. I basically couldn't find anything available in my area that met those criteria. Original spec ended up being Kolbe fiberglass windows with a U-value ~0.21 https://www.kolbewindows.com/. That window package was ~$35K delivered....which went out the door in value engineering. Now I have a vinyl package from Pella with a slightly better U-value of 0.19 coming in at $17K.
I chose casements for air sealing benefits...that makes window units not an option.
The units will be what passes for high end grade in this market. Granite counters, LVP, limited tile. The bedrooms are in the rear, so switching to single hung with window units is possible.
If you happen to have ideas on windows, I'm still open. I've spec-ed and priced: Anderson, Marvin, Kolbe, Pella, Jeld Wen, Optiwin, Tanner, and ThermoTech.
A common strategy is fixed-over-awning. Awning windows are much like casements, the advantage being they're usually smaller so you can substitute more fixed frame glazing area into the same opening size to save some cost. Also the locking hardware on an awning doesn't have to work as hard to compress against fewer linear feet of gasket compared to a casement, meaning even better airtightness. This is assuming the windows are factory-mulled airtight and installed well.
Use fixed frames wherever feasible, as operable windows add $$ and are nearly always less airtight.