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How to Design a Passive Solar Hempcrete Home?

Convergence | Posted in General Questions on

Hello!

My wife and I recently purchased a plot of land in Bailey, CO (9,000 ft elevation). We are looking to build a passive solar home that utilizes hempcrete (12in walls).Our lot faces the street to the south and the mountains to the north. Ideally, this would be reversed, so we could have the solar gain come from the same direction as the mountain view, but what can you do. 

My question about design is primarily this: how do we maximize our solar gain while still being able to enjoy the view? I’m wondering if having the hempcrete walls (with R30 value) which have terrific thermal mass properties will allow us flexibility in our design? Instead of relying on a concrete slab with south-facing windows, will the hempcrete walls be able to absorb the sun on their own? Can we have less windows facing the street as a result? Also, with this type of design, how much will we be sacrificing when it comes to having north-facing windows? 

I’ve attached a picture of the type of home we’re thinking. My friend, who builds homes, stresses the ease of building a single-pitch home for cost efficiency. I’m still open to other ideas, but we’d like to finish the house as quickly as we can and as affordably as possible (outside of using hempcrete, obviously). 

Your input is greatly appreciated, thanks!

Colin

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Replies

  1. DC_Contrarian | | #1

    Serious answer: learn how to use an energy modeling program like BeOpt. What you are asking are not simple questions, and can only be answered through modeling.

  2. personusa1 | | #2

    What if you used heliostats aimed at your north windows

  3. Expert Member
    Michael Maines | | #3

    I would prioritize insulation and air-sealing over solar gain. Solar gain can be good in cold weather, but it's bad in hot weather and hard to predict in the shoulder seasons. As DCContrarian said, energy modeling is the best way to predict how your home will perform.

  4. brendanalbano | | #4

    You can almost certainly achieve a variety of high performance standards (net zero, passive house, whatever) while orienting your house and windows to maximize the views. It's really unlikely that you need to make view-related sacrifices (except that perhaps the windows might not be able to be quite as large as you want if you're shooting for a really stringent target).

    Either pick an efficiency standard, and use an energy modeling program like DCContrarian suggested to see if you are achieving it, or use the rule-of-thumb guidelines in the Pretty Good House Book https://www.tauntonstore.com/pretty-good-house

    Design the house to be a great house, maximize your views, create good connections to the landscape around you, etc. The one thing I would consider while designing is to see if you can keep the footprint and the roof relatively simple (not too many corners, not too many hips and valleys, etc.).

    Then adjust the r-values, window u-values, window sizes, overhangs, etc. until you hit your energy targets! Again, either use a program like BeOpt to do this, or just use the Pretty Good House rules of thumb.

    1. DC_Contrarian | | #5

      The thing about the PGH guidelines is you have to follow all of them. For climate zone 5 and above PGH recommends wall insulation of R40. So at R30 the hempcrete isn't going to comply.

      Energy modeling allows you to pick and choose, you can compensate for not meeting the guideline in one area by beating it in another. BeOpt in particular allows you to model solar gain and even put in your "thermal mass" so you can see how little it does for you.

      1. Expert Member
        Michael Maines | | #6

        DC, you don't have to follow any of the PGH recommend insulation levels, they are just what Joe Lstiburek and BSC determined was a baseline "high performance house" in 2011 or so, and varies by climate. Using energy modeling, here in climate zone 6, depending on the wall assembly, I often find that going above R-30 walls is not worth the added cost. In other cases--typically double-stud walls with cellulose--R-40 does make sense, because it's just a little extra labor and material.

        The Pretty Good House approach is a comprehensive way of looking at how we build, not just a list of R-values.

    2. Convergence | | #8

      Awesome, thank you for this advice! And everyone else, of course.

      Is this program fairly easy to use?

      1. Expert Member
        Michael Maines | | #9

        It's pretty easy as far as energy modeling programs go, but there is a learning curve: https://www.greenbuildingadvisor.com/question/what-is-the-learning-curve-like-to-use-beopt-for-a-new-user. One of its best features is that it's free, and another is that it has a built-in ability to show you the least expensive path to net zero energy (or another energy target) using national cost averages.

  5. plumb_bob | | #7

    In terms of single pitch roofs being cheaper to construct, it depends on the details. If the ceiling is vaulted you will end up with tons of angles and triangle to finish at height, high windows to install and finish, etc.
    Having done it both ways multiple times, I am a vented attic guy. It is an easier system to construct well for air tightness, more insulation, better ventilation, space for ducting, more efficient to heat, flat ceilings.

  6. onslow | | #10

    Colin,

    Not out to be the skunk at the party, but it might do to review these links and pdf at the end before committing to one method of building. Being green can sometimes require excessive amounts of green.

    Aside from the merits of easing our damage to the planet you will need to face the realities of finding builders willing to get to your site and work with the material you desire. You will also need to deal with local inspectors that accept the idea and find an engineer willing to work with you and design to snow and wind loads of your area. Being close to the front range you might be able to use one of the companies advertising the material. Their addresses still put them pretty far for building purposes. Travel time is money as well.

    Your efforts to model the energy requirements will be complicated by the rather variable R factors provided for hemp. Choosing your heat source will be complicated by the altitude and available contractors if ASHP is the intended method. Partitioning your budget may get complicated by surprises in the soil types and terrain of the lot you purchased. The building window typically available at 9,000 ft elevation can be tricky as well. And, though you may think this will be your forever home, life happens. Will it be a saleable asset in others eyes.

    I speak from having done my own build at 8,000' on the other side of the range. Wind and fire constraints will suggest some design choices. If you are in an area with visual impact rules, that could also dictate choices you might not otherwise make. You seem to have an actual road for access.

    I am not a fan of BEOpt, but I am easily put off by software. I did slog through calculations for wall and roof cross sections manually and made sure to get the highest performing windows I could afford. I have managed to hit a heating load of 1.4 btu/sf/degree day so I am basically happy with that. Siting of the house was indeed driven by views and spouse choices. I forwent any delusions of passive solar heating. That is a very long winded discussion that boils down to the influx available is trivial when needed most. My view windows are east and north and the east windows do penalize me for some excess gain during the summer. I chose the lowest SHGC glass for that reason.

    Short version of advice is not get married to a material before all the rest of earth, air, water and fire are at least reviewed.

    Here are some links:

    https://www.hpdconsult.com/hempcrete-advantages-and-disadvantages/

    https://www.westword.com/marijuana/why-hasnt-hempcrete-taken-over-construction-already-12748168

    https://www.hempbuildmag.com/home/hempcrete-demonstration-wall-built-in-boulder

    1. Convergence | | #11

      Just seeing this reply now. Thanks for all of that information. What building material did you decide to go with in terms of your walls? Anything unconventional?

      I'm attracted to Hempcrete for many reasons, though I am not fully married to it if it's not practical. We did a course with a local hemp building company, so sourcing contractors to work with would not be difficult in that regard. Ideally we could save some time and money by helping out that part of the build and having less contractors needed (insulation, drywall, painting all done by same contractor). Obviously the cost of the hempcrete will be the big factor in that regard. I'm drawn to Hempcrete for the fireproofing factors as well. We have very easy access from the road to our building site, thankfully, and we also do not have any visual constraints in the neighborhood.

      If we were to simply opt for Hempcrete walls and not rely on the sun for heat, would you go for radiant floors or a mini-split heat system? I like the idea of radiant floors, especially with the thermal mass of hempcrete, but understand the cost of installation could be more (although we would try to do this part ourselves).

      Any other thoughts on building advice is greatly appreciated, thanks!

      1. andyfrog | | #12

        This is good reading: https://www.buildingscience.com/documents/insights/bsi-081-zeroing-in

        As far as hempcrete goes, it doesn't seem to be load bearing. So maybe you can use it for partition walls, but for your building envelope, it seems like it would make things more complicated, i.e. first building a structure, then infilling with hempcrete, then figuring out a way to make that all airtight.

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