Performance / value: Building envelope question
I’m building a new home in Parker, CO and am researching building envelope options. I love all the great information I’ve read here, but I’ve found in my research that often the questions I’ve seen asked and the responses given involve products well beyond my price point. Our house is a large 2 story walk out – so building envelope costs really add up quick. Additionally, I’m looking for methods that are going to be able to be successfully implemented using average contractors (even if that means more oversight…that’s fine). House will have Marvin Ultimate Alu clad wood windows and a pretty high end hvac systems designed by a ME using JDST that has a few large zones and uses all modulating furnace and AC equipment.
The goal is the stick to a price point around 10%-15% or less higher than OSB + Tyvek + Fiberglass bat in 2×6 exterior walls.
So I’m learning toward one of three options:
1) Zip + Ownes Energycomplete/Knauf Ecoseal Plus + Fiberglass BIBS
Pros: Cost is reasonable, common skillsets for local contractors, great air sealing potential, descent insulation (r23 on 2×6 walls)
Cons: Low envelope permeability, questionable long term performance of zip system
2) Soletex Mento + OSB + Ownes Energycomplete/Knauf Ecoseal Plus + Fiberglass BIBS
Pros: Better permeability, not too complex of an installation, but slightly more so than #1. More predictable long term performance of the WRB
Cons: Envelope air infiltration will likely be higher than #1, but still should be very good with the internal air sealing
3) Barricade Thermo-Brace Red, Barricade Seam Tape + Ownes Energycomplete/Knauf Ecoseal Plus + Fiberglass BIBS
Pros: Non OSB sheathing offers better performance in the event of WRB failure. Don’t really know much about this type of sheathing in terms of other benefits. Should offer similar air sealing with taped seams similar to that of zip.
I’m in no way an expert in this area as probably evident from my post. Would love to hear what others would recommend in terms of the best value option for our build. Thank you all in advance for your insight, wisdom, and criticism!
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Replies
First of all, how did the ME design your HVAC system if your envelope hasn't been designed yet?
You mention that your house is a "large 2 story walk out." One way to hit your price point is to reduce the overall size of your house.
You really can put extra $ into insulation and air sealing and then reduce your mechanical size/complexity.
After doing my own (extensive) cost research, I settled on 2x6 framing (dense pack cellulose and Intello air/vapor barrier) with plywood sheathing and exterior rockwool as a full thermal break insulation layer. WRB is on plywood and rockwool is part of a rainscreen drainage plane.
Recommending a smaller build is controversial here at GBA, but it really is a valid method to get a high performance home down to an affordable price point.
Cyclist,
First of all, can you tell us your name? (I'm Martin.)
I was confused by the abbreviation "JDST," but I eventually figured out that you meant that your heating and cooling system design would be performed according to Manual J, Manual D, Manual S, and Manual T.
Here at GBA, we emphasize building envelope details -- paying attention to airtightness and installing insulation that exceeds minimum requirements for R-value. If you follow that approach, you can often install a small, inexpensive heating and cooling system.
So it strikes me as odd that you are committed to installing a "pretty high-end HVAC system" before your envelope details are finalized.
Cyclist,
I think your analysis suffers from a few misconceptions.
1. You mention "low envelope permeability" as a "con" (as opposed to a "pro"). But there is no particular advantage to high permeance in walls. In fact, high permeance is one of the factors in the disastrous failures associated with inward solar vapor drive. (For more information on this issue, see "When Sunshine Drives Moisture Into Walls.")
2. You mention the "questionable long-term performance of Zip system sheathing" as a con. Where did you get that idea? Do you have any evidence to support your theory?
3. You mention that when using conventional OSB sheathing, "envelope air infiltration will likely be higher" than with Zip sheathing. But the main variable that explains differences in air leakage rate is workmanship, not the brand of the OSB.
As a side note, we recently used that fiberglass BIBS system on a house. Worked like a charm until we went to put drywall on. Turns out the contractor way overpacked most of the cavities (they were just eyeballing it) and the drywall screws all popped shortly after installation. Caused a lot of extra headaches. So make sure you get a good installer who has some way of measuring how densely to pack it.
Thanks for the responses everyone.
First off, to answer the questions about HVAC design w/o having full details on the thermal envelope. We designed using some assumptions using the manual default infiltration tables -- we aren't able to apply for permit here without a manual jd&s and my budget isn't unlimited so I need to have an idea of what we're in for well before I'm able to do actual measurements. The house has lots of windows, corners, etc. This house isn't designed as primary goal of energy efficiency, but it's still a high objective. When modeled as semi-tight vs tight, there is only 0.15% cooling load difference and a 2.3% heating load difference so there's some room for modification to the thermal envelope without causing material difference in equipment schedule and duct sizing. True measurements trump all assumptions and we'll blower door test and adjust if needed. I'm not sure I can do much better than this, but I'm open to suggestions. The variable in my decisions is infiltration, as you'll notice that across options insulation is more of a constant (R-23 in walls) and we defined our wall and insulation structure in the non walk out sides of the basement.
1) Thank you for clarifying. I've seen 'inward solar vapor drive' mentioned in some sales literature for delta-dry and barricade plus, but didn't really understand what it was. Reading the article you linked makes sense so low perm systems like zip (1 per (3rd party) or 13 perm (huber) depending on what test data you rely on) aren't a concern?
2) None, and honestly I trust Huber has done their homework. That said, it's still relying on adhesive vs mechanical fasteners and I can't help but stand behind others that have some concern over how zip tape holds up in 30 years. Speculation...not fact, but relating to point #3 adhesive durability in a lab vs real job site conditions is more likely the cause for failure in the near and intermediate term. If preped and applied correctly maybe it'll be all good in 40 years.
3) I'm open to suggestions. Back to my original point...installation makes all the difference, but sadly I can't count on all our subs having the attention to detail that you or I may have. This, to me, is where the balance comes in between construction techniques that require specialized tradesmen vs things an ordinary crew can handle that have a reasonable margin of error.
Hope that helps clarify a few of my comments! So...back to my original three questions. Any feedback on those or additional alternatives for me to research? Would appreciate the ideas!
Yupster -
Great point. I liked Owens BIBS system for their proprietary external density measurement device. Thought this was a nice QA measure to help avoid over packing the walls. Even so, I'll jot that note down to make sure I address with our insulation contractor.
CoCyclist,
I'm curious, about how much will it cost for the "pretty high end hvac systems"? How did you come up with desiring better performing walls that are "10%-15% higher price point" above code-minimum or standard construction materials?
I usually think of these two as trade-offs. Reduce heat loss in various building components to get to an inexpensive heating (and/or cooling) solution. You seem to be doing the reverse, but really I don't know your thought process or design process, so am curious, and would like to hear more about how people arrive at these decisions.
All half-inch OSB is under 1 perm when dry, and still not more than 5 perms even at a fairly substantial moisture content in a high humidity environment. The 13 perms quoted by Huber is only for the factory applied WRB, not the whole product (which is still about 1 perm, or a bit less.) At 13 perms the WRB is tighter than most grades of Tyvek, not quite as vapor tight as some versions of Typar, but still not so tight to be of concern.
You don't have to count solely on the tape to be air-tight for then next 50 years- it's fine to use caulk on the interior side with an appropriate sealant for good measure.
Dense packed fiberglass in mesh is a significant up charge. R23 rock wool batts are cheaper than 1.8lbs fiberglass BIBs. A competent installer followed up by a truly obsessive inspector (that could be YOU), can make it perform. Rock wool has considerably better fire resistance than fiberglass too.
With back-vented or rainscreen siding you don't need an interior side vapor retarder (other than standard latex interior paint on wallboard) in your zone 5B climate. What does the intended material stackup from the WRB to the "great outdoors" look like?
Gas is cheap right now, but a properly sized modulating cold climate air source heat pumps might be a better high-end solution than your modulating gas burners, especially if you can avoid the additional cost of hooking up to the gas grid. The Rocky Mountain Institute has been all over the economics of the "electrification of everything" in the past year or so and it's generally favorable for new construction- heat pumps can be a net-win for both the project budget and the planet. They published their latest opus magus on the topic a few weeks back:
https://www.rmi.org/insights/reports/economics-electrifying-buildings/
Hi Robert -
Fair question. In summary, my goals for HVAC design are #1 comfort, #2 quiet, #3 efficiency. Because of that, we chose to go with modulating ac/furnaces using fully communicating zoning (large zones) with no bypass. The house is complex -- it's multi generational so there's an 1800 square foot apartment attached to around 6500 square feet of conditioned space on the main and lower levels. Additionally there's about 700 square feet of conditioned workshop in the garage.
So back to your question about defining the building envelope first, followed by Manual JSD. I did model a few configurations of wall assembly, window glazing, infiltration to explore diminishing returns of different options against our heating and cooling loads. What I found the data to support was very diminishing returns past R-23 in the exterior walls and above average air sealing (.3-.6ach). Windows are another story, but the next level in performance is nearly triple the cost (Alpen as one example of 'next level').
So as it stands, we have a structure designed around exceeding code insulation and air sealing, and as important to me, using methods I know are in budget and easily implementable. Should I discover on this journey a better way to skin the cat, systems and ductwork can always be downsized without impacting architecture or structure. Upsizing is another story.
Would it have helped to fully define the building envelope first? Yes...and I did (had to in order to arrive at an accurate jsd). I'd don't think any of the options I listed will yield a highly material difference -- I consider this optimization -- making the most of the dollars spent within the design and budget constraints present. Amoung options, I'm not looking at things like availability, labor skills, longevity, durability, and of course marginal gains in performance. I'm just trying to get some opinions on some not so sexy options compared to what's normally discussed and learn a bit along the way! In regards to how I chose 10-15%...well that's about what I could afford in the budget -- it's that simple.
Oh, and regarding the cost of the HVAC. It's about 36k (3 furnace/3 ac/4 zone mitsu hyper heat mini split).
Feel free to bash away...you learn so much building your first house, only to never build another one :(
Hi Cocyclist,
We might describe your home as a duplex with one apartment (maybe with its own separate HVAC) and possibly a 26x28 garage workshop with its own mini split? That would leave two HVAC systems for the very large home (for redundant backup, or separate suite HVAC?). There must be multiple justifications for 4 separate heating/cooling units, but it seems you could get by with less than four, and cut costs significantly.
Relatively uninformed opinions:
• We all have a construction budget limitation, but it’s for the whole project, not one specific to insulation only. I know little about HVAC, but I’m guessing that even without considering fuel, maintenance and replacement costs, there seems a large $36,000 budget allocated to HVAC compared to the 10-15% limit on insulation upgrades (which have zero fuel, maintenance and replacement costs). More allocated to insulation would reduce the cost of HVAC, fuel and maintenance, reducing the total cost of ownership, as well as providing greater resilience during outages or fuel disruptions.
• Many here start designing assuming the 60-40-20-10 “standard” for insulation in cold winter climates. That would suggest starting with an R-40 wall rather than your nominal R-23 BIBS (maybe R-20 with thermal bridging through studs, plates and headers, near code minimum R-19). In my limited experience, “pretty good house” examples usually had R-30 to R-35 walls and R-60 attics/roofs.
• A 2x8 wall would increase the BIBS nominal R-value from R-23 to R-30, a significant increase. That might cut your total heating load by about 6%. Most here would probably suggest at least a minimal layer of exterior insulation instead, to reduce thermal bridging (and provide a quieter wall, especially with mineral wool board).
• Cellulose may provide a heavier, quieter wall (your #2 priority) vs. fiberglass BIBS, but I believe a somewhat lower R-value. Can others here (who often suggest cellulose) provide more info?
• I’m not familiar with the Ownes Energycomplete/Knauf Ecoseal Plus, in all three of your alternative walls. It seems to be redundant air sealing but might provide more than this. I assume this product and installation is not cheap. What’s the advantages or your reason for choosing?
• Your area of CO is an ideal climate for PV, reliable wintertime solar heat gain, and reliable summertime overnight cooling. Would be interested in your thoughts or impact on your design, if you don’t mind writing about it. Not a very relevant comparison, but I almost eliminated HVAC costs for a small passive solar house I built 50 miles northeast of you ( https://www.greenbuildingadvisor.com/homes/passive-solar-home-1980s )