More Best Practices
In my last post on Pretty Good House (PGH) building details, I shared a concept house illustrating how to incorporate low-carbon building strategies. Here, I’ll highlight another approach to a PGH—one that is performance-based, with a focus on operational energy—as well as additional low-carbon building practices and a few software tools I find valuable.
Start with energy and carbon modeling
While you can build a decent house, or even a PGH, without energy modeling, using it has several advantages, and it’s not that hard to do once you learn how. What I find most useful is being able to optimize construction details by comparing the effect different assemblies will have on a building’s energy usage. Energy modeling also helps fine-tune window specifications for the lowest energy use, and determines heating and cooling loads for the house, among other options.
For anyone trying energy modeling software for the first time, BEopt, a free software from the National Renewable Energy Laboratory (NREL) using the U.S. Department of Energy’s Energy Plus modeling program, is good and relatively simple. It has an optional feature to automatically determine the most cost-effective path to reaching net-zero operation. You can also use it to balance expenditures and gains and to help calculate return on investment. In many jurisdictions you will need Manual J calculations for heating and cooling, and they are important for right-sizing HVAC equipment, but BEopt does not provide Manual J outputs.
Ekotrope, Wrightsoft, and REM/Design are a few professional-grade programs. Some BIM (building information modeling) programs also have energy plug-ins. Passive House practitioners use the PHPP or WUFI Passiv energy-modeling programs, which are labor-intensive to use but preferred for extremely low energy–use homes.
It’s also a good idea to see how your embodied carbon emissions stack up with different assemblies. My favorite tool for this is the BEAM Estimator, which Chris Magwood and Builders for Climate Action designed. It doesn’t provide a full accounting of all embodied carbon emissions because comprehensive, accurate data simply isn’t available for some things like electrical and plumbing systems. But you can compare the relative impact of anything building-enclosure related, from foundation systems to roofing types and interior finishes. (BEAM is available on a sliding scale to make it accessible to all; donations are well worth the value the tool provides.)
Determine targets
One way to design for PGH energy efficiency benchmarks is to divide the location’s heating degree days (HDDs) by 180 to get the R-value for a wall, and by 120 for a roof. For example, in a cold climate, such as climate zones 5 and 6, use:
R-10 SLAB (EPS or GPS expanded polystyrene, mineral wool, or recycled XPS or NGX sub-slab insulation)
R-20 FOUNDATION WALL (crawlspace wall and slab-perimeter too)
R-60 ROOF/CEILING
R-40 WALLS
R-5 TO R-8 WINDOWS (U-0.20 to U-0.13). Even the best windows make lousy walls, so don’t over-glaze.
1.0 ACH50 AIRTIGHTNESS (This is the maximum air leakage target many of us are using. Others say 1.5 or 2.0 ACH50 is tight enough. Tighter than 1.0 ACH50 may not add significantly to the home’s performance but high-performance builders are routinely getting as low as 0.1 ACH50.)
Meet PGH benchmarks with proven assemblies
R-40 pier foundation
A pier-and-beam system using helical metal piers or concrete piers have much lower upfront carbon emissions than a full concrete foundation. Piers are located to carry wood beams to support the floor framing. When the top of helical piers are more than a few inches above grade they need bracing, so I’m showing a bolster system to create the height needed to allow code-minimum 18-in. clearance from grade to the bottom of the floor system. (Pressure treated beams don’t need to meet that clearance requirement.) The beams are inset enough to allow for ventilated skirting—the more airflow, the better.
The floor system can be dimensional or engineered lumber; I prefer I-joists because the narrow webs nearly eliminate thermal bridging, allowing for about R-40 for the whole floor system when insulated with dense-packed cellulose, wood fiber, or fiberglass.
To keep out air and critters, 1⁄2-in. sheathing is installed below the joists and sealed to the beams. (If you’re in an earthquake-prone zone, this system may not work for you, but it pairs well with a partial basement that can provide additional lateral support and a place for utilities.)
R-40 double-stud walls
Builders have strong feelings for or against double-stud walls, but they have a long track record in New England, and the builders I work with tend to like them. In a cold climate, they need to have ventilated rainscreens so they can dry easily to the exterior. Dense-packed cellulose or wood fiber are ideal insulation materials for this type of wall.
In cold climates, I am most comfortable when a variable-permeance membrane is used at the interior, though many builders just use painted drywall. (Often called “smart” membranes, these products are made from plastics whose vapor permeability changes with humidity levels.) With a 12-in. insulation cavity, the wall will perform at about R-40. The taped sheathing doubles as the air control layer.
R-60 roofs with thick cellulose
The least expensive, lowest-carbon way to build and insulate a roof or ceiling is usually a raised-heel truss with loose-blown, bio-based insulation. I typically spec R-60 cellulose at 16 in. deep. Raised heels can make walls look tall, so I sometimes reduce the insulation depth at the eaves to as little as 10 in. The air control layer is at the ceiling, in the form of a variable-permeance membrane, but taped 1⁄2-in. sheathing works too, as long as any penetrations are carefully air-sealed. Inside of that, horizontal 2x furring creates spaces that can be used to install lights and run wires without puncturing a membrane air control layer.
Optimize the window package
European-style triple-glazed tilt-turn windows are significantly different from standard North American-style windows, but once you’ve experienced their quality and comfort, it’s hard to go back to comparatively flimsy American windows. They perform best installed at the center of the wall, but with a minor performance penalty they can be flush with the sheathing. They are usually installed with clips instead of flanges; specialty tapes are available to keep water out and provide airtightness.
Why shoot for PGH benchmarks?
Building energy-efficient homes makes for more comfortable, less-expensive living and better prepares the occupants for climate change. Doing so in a reasonably cost-effective manner, perhaps trading fancy finishes and complicated rooflines to better afford the upcharge, makes it accessible to more homeowners and homebuyers. Reducing your carbon footprint at the same time is better for the planet, and often better for occupant health and building durability, than building a house out of plastic or concrete. As Bruce King, author of The New Carbon Architecture and co-author of Build Beyond Zero once wrote (borrowing from Michael Pollan): “Build, but not too big, and mostly with plants. Nuff said.”
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Michael Maines is a Maine-based residential designer, co-author of The Pretty Good House, and longtime contributor to Green Building Advisor and Fine Homebuilding magazine.
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15 Comments
This is an excellent article. There is no doubt that the solutions for energy efficient and durable houses exist. The questions have been answered...
However - how do we get to the next stage, where this material is widely disseminated, especially to general contractors and their subs? My experience is that if you ask a GC to bid on anything other than what they are used to seeing daily they will inflate their prices to cover what they perceive as additional risk.
Thank you! I wish I had an answer for how to get people interested but that is something I have not found a solution to. My PGH co-authors and I hoped that our book would help with this, with Taunton Press' focus on making even technical books attractive inside and out, and I believe it has made a tiny dent, but nowhere near what is needed.
Great article, Michael, I really enjoyed it and really wish I had seen it and the Pretty Good House book before I built my house (about 18 years ago)! But I did get the windows right at least.
Edit for question
When I built this house in 2007, I added 18" of blown cellulose in the attic. No raised trusses because at the time I had never heard of them, but I do remember being puzzled about how to get better performance at the edges.
Fast forward to today and I needed to get into the attic to replace a dead bathroom ceiling fan. When I got up there I was surprised that my 18" of cellulose is now closer to 8". Is that pretty normal settling? I am thinking I need to contact a couple of the local insulation contractors to give me a bid on adding another foot or so on top of what's there now. Wish I had raised trusses though.
Thank you, Greg! I'm sure you did get the windows right. In fact I know you did because we've talked about them ;-)
Much of what I know about cellulose insulation is directly from Bill Hulstrunk, who probably knows more about the product and how to use it than anyone. In this article by Martin Holladay, he quotes Bill as saying that 13% settling is normal: https://www.greenbuildingadvisor.com/article/how-to-install-cellulose-insulation.
Older types of cellulose were only hammer-milled, but modern versions are also fiberized, which fluffs the cellulose bits up and makes them insulate and retain their loft better. Loose-blowing cellulose results in extra air space, not unlike whipping eggs to incorporate air to make them fluffier. But as with eggs, over time the extra air gets squeezed out, leaving a bit less air space. Heavy moisture levels or leaks can further compress the insulation but as long as it remains reasonably dry it shouldn't compress further.
Your settling of 56% is very unusual and I'm not sure what would cause that if you haven't had a flood in the attic. Perhaps it was long enough ago that the cellulose wasn't fiberized; I'm not sure when that change happened, but flat flakes will settle more than fluffy flakes.
Are you experiencing ice dams? That would be my main concern with not enough insulation at the eaves. If not, as long as everything is reasonably airtight, heat loss is mostly about the overall average insulation level, so a few extra inches in the middle of the space will cancel out not having enough insulation at the eaves.
Thanks Mike, that was really good info.
Near as I can tell, there are no leaks and no water of any kind ever happened up there. The space looks pretty tight and clean, although it's not somewhere I would want to hang-out for any length of time.
Never had an ice dam, in fact our roof is generally still snow covered when other homes around us are half cleared with icicles hanging off the eaves. We are in a rural area, so only three other houses are even visible from our location.
I plan on going back up there in a couple months when the temperatures are in the 50s and not the 80s and see if the settling is localized or the whole thing is like that.
Sometimes wind-washing will erode cellulose depths at the eaves, or electricians and HVAC contractors will move it away to access things. Otherwise it's a mystery, but adding more cellulose (or wood fiber) on top should be adequate.
I hope methods like these soon become the new 'status quo' for home construction. These assemblies create excellent homes without making things worse for the rest of us. And each of these preferred methods can be easily adjusted to reach Passive House performance levels. Thanks for this resource and for encouraging architects, builders, and homeowners to make better choices.
You're welcome! It's been a group effort from the beginning, with most of the ideas crowd-sourced. (I do add my own spin on things; I'm not a journalist!)
We (the book authors, contributors on GBA and elsewhere, and other PGH fans) like to think of the PGH approach as "Passive House Light." That ruffles a lot of PH feathers but we are also fans of PH when clients are interested and we can do it in a reasonably low-carbon manner. There's nothing like PH software to really dial in the performance details, and PH builders are ideal for PGH methods.
Michael,
One problem with the book I'm having trouble fixing is that I bought an extra copy so I could lend one out, and somehow now never have either at my place.
Haha, email me your address and I'll send you one of my copies ;-)
Micheal,
I know exactly where one is. I have keys to the house and he is going fishing tomorrow, so I should be good.
First, thank you for making simple statements about target R-values and clear photos showing reality. I am an ill-equipped advocate for the record.
I believe the PGH could be more transformative in architecture than PH or LEED, but we must overcome a few difficult hurdles to realize its full potential. I'm going to say out loud what too many are merely thinking.
Online discussions often delve deep into and get lost in the debate about the finer points of detailing, which is valuable, but we risk losing sight of the bigger picture: inspiring widespread adoption. We must intentionally shift the conversation from doubt to advocacy to achieve this. When builders (and architects) become enthusiastic champions of sustainable design, we’ll see Pittsburgh (my region) and beyond flourish. We must tap into others' values and make it work for them. Not everyone cares if we save the planet, as crazy as that may seem.
Another challenge is the steep learning curve associated with new software tools. Let's be honest: as a solo practitioner architect (like me) juggling multiple projects, it’s daunting to dedicate the necessary time to master complex programs like BEopt and Energy Plus. While free software like Beam V1.1 is a step in the right direction with a graduated fee (kudos to them), more practical support is needed. I don't have the time to master the software quickly. After six or ten projects, I may forget how hard it was. Saying "just do it" didn't work for Nike and doesn't work for us. Energy was hard enough; now carbon, too?
I envision a future where consultants are readily available to guide architects through the learning process or do the modeling for us. A hands-on approach would be invaluable in overcoming the initial obstacles and ensuring successful implementation. The online tutorials I scanned were overwhelming and painfully boring. Come on, make it interesting and easier. Get an architect to design the UI/UX to make data entry easier.
Ultimately, we need to bridge the gap between design and construction. It is crucial to make these methods accessible and economically viable for builders (and homeowners). By addressing these challenges, we can create a thriving community of architects and contractors working together to build a more sustainable future. Until then, the clash and rhetoric won't end.
Many architects share these concerns, and by openly discussing them, we can find solutions that benefit the entire industry. I'm willing to step forward and say what too many complain about under their breath. Some of the finest material is found on this site, so I believe the authors here could be a big part of the solution.
Forgive my soapbox. But if I move the needle forward, I'll take the hit.
I'm sorry I missed this. I think all are necessary. High-performance details do no good if they are not relatively easy to build, resilient and durable over time. As a design/engineering-minded person, that's where my interest lies.
Widespread adoption is a tough one. I believe that over the 12+ years that PGH has been a thing, we have helped inspire thousands of people to build better homes. That's far better than none, but barely a drop in the bucket nationally. When you have the NAHB pushing against anything that slows the building of more homes, that's a powerful adversary. And many of the existing above-code programs are great, perhaps better than PGH, with big budgets behind them (vs. zero budget for PGH advocacy--a few of our public appearances to promote the book were paid by the host, but otherwise all in-person or virtual promotions have been out-of-pocket) but they also face continual resistance.
I'm now over 50 and don't find it easy to learn new programs or other systems anymore. Just ask my wife about the words I use after a new computer update, new phone, etc.. But I found learning BEopt on my own relatively easy. If you don't have the time or interest to learn new programs yourself, you can hire it out. When PGH was first brought to life, we thought that energy modeling should not be necessary. But the reality is that houses are inherently so complex, that if you don't model it, you won't get optimum results.
I personally have spent thousands of hours and thousands of dollars trying to address these and the other issues you write about, including spending a few hours every week answering questions here on GBA. Others have done the same. Others get grants or are supported by other means. We all need to carry some load if we want to see a better future. Articulating the problems clearly, and advocating for discussion as you have done, is a good start. Now, what actions can you take to contribute to the future you want?
Did you have any issues getting 2x4 at 24" o.c. spacing approved?
IRC table R602.3(5) shows where 24" o.c. is allowed. The drawings and specs for the project in this article are all hypothetical, but I have designed similar assemblies with no issues. I don't recall every having pushback from building departments on 24" o.c. spacing when it meets IRC requirements. That said, 24" o.c. studs don't save much lumber and can cause problems if not addressed in other ways, so I usually discuss it with the builder. One that I work with only does 24" o.c., many others prefer 16" o.c..
Roof framing is always allowed at 24" o.c. as long as it meets span ratings. I almost always design for 24" o.c. there because it saves time and materials.
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