Image Credit: Thomas Bateman Hood Architecture
Image Credit: Thomas Bateman Hood Architecture The extensive lot coverage will require most of the paving to be pervious. At the rear of the lot, we have also included a rain garden where most water drains to. Note the sheathing on top of the second-floor ceiling joists, providing a solid air barrier with no penetrations. I will be able to test the house for air leakage before any drywall is installed. OK, critics: let's hear your complaints about the design.
More Green Building Curmudgeon
Carl Seville and his wife are building themselves a new home in Decatur, Georgia. This is the first blog in a series describing the construction project. Links to all of the blogs in this series can be found in the “Related Articles” sidebar below.
After two failed attempts at building a new house, I’m finally ready to get started. My first plan, started in 2009, was shot down by the historic commission.
The second plan was approved, but due to the recession, I found myself unable to finance the construction. At that point, fed up with my ratty cottage and having some spare cash, I decided to renovate.
Although I have been thoroughly enjoying my cottage, having recently gotten married, and with both of us working at home, 800 square feet just isn’t working. After reconsidering the last, approved design, we decided to, once again, start over with a new design.
We started design work in October, working with my friend, the architect Tom Hood. We received approval from the historic commission in March, and finally wrapped up all the details and obtained the building permit in July.
Ch-Ch-Ch-Changes
The previous plan was very typical for the neighborhood: a two-story Craftsman style house with a full width front porch, with a one-car garage between the new and existing houses.
Since the cottage had been expanded, and there is a limitation on the total area of accessory buildings, I was no longer able to build a garage. Instead we needed to build a carport. Wanting space for two cars, we decided to reduce the size of the front porch and move the house closer to the street, providing enough space for a larger carport.
We worked very closely with the historic commission staff during the design process, ultimately coming up with a plan that was approved over numerous (and I feel unreasonable) objections of several neighbors, many of whom could be considered either NIMBYs (Not In My Back Yarders) or possibly even BANANAs (Build Absolutely Nothing Anywhere Near Anything).
The most complicated issues in the project were working around existing trees to comply with a very restrictive tree canopy preservation ordinance, and permeable area requirements. The lot coverage requirements necessitated using pervious paving, currently planned to be combination of pavers and concrete. I look forward to working with the materials, if not the extra cost of installation.
So many green certifications, so little time
My city, Decatur, has a green building certification requirement. I would certainly certify the house whether or not it was required, and I am currently working on certification for several local builders.
The big question for me is which program or programs to use for my house. Most of my single-family certification work is with EarthCraft House. Multifamily projects tend to be either NGBS or EarthCraft. We do the occasional LEED project as well.
Ultimately, I have decided to certify the house as LEED Version 4 – the most recent version that will become the only one available for all projects registered after October of this year. I decided it would be useful to have experience in Version 4 before I have to work on someone else’s project, and will hopefully be able to work out any kinks in the process. I will probably also certify under EarthCraft and NGBS. Lots of paperwork in my future.
Decisions, decisions
I have decided to do some different things in this house, somewhat out of the mainstream, but not unusual for readers of GBA.
First, I am going to avoid spray foam insulation, and limit foam board as much as practical. I will be using Owens Corning spray-applied fiberglass in the exterior walls and on the attic ceiling. Exterior sheathing will be Zip-R with an R-3 polyiso layer. I considered using mineral wool for the exterior thermal break, but the added thickness and complexity of construction led me to the combined sheathing and insulation product.
I am going to use a passive-house type of detail on the top floor ceiling. The Zip sheathing will be installed on top of the ceiling joists, taped to the side wall sheathing, provide a complete air seal on top of the joists, instead of relying on the ceiling drywall. HVAC will be ductless mini splits from Mitsubishi, eliminating any ducts in the attic area.
The foundation will be stem walls, filled with a slab floor. Because of the low floor height required and the pervious paving, any crawl space would likely end up being a swimming pool, and I’m not a big fan of crawl spaces anyway.
My first draft with REMRate came up with a HERS rating of 43. Hopefully it will end up lower at completion. Since I have a full stand of mature trees surrounding the house, there are no opportunities for any PV, so I won’t be able to get anywhere near net zero.
More posts to come as construction progresses, including some time-lapse movies of the construction. Feel free to comment, complain, and argue with my decisions.
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16 Comments
unvented attic
The choice to vent your attic maybe because of Dekalb's building code. I would like to know what are your thoughts on venting or not venting your attic. Mr Joseph Lstiburek has given his view point. If you were to go with an unvented attic might give you a hedge against all those pesky critters that like to call your attic home. You made be far enough south to put a hot water tank heater in an unvented attic. Being a raider from Decatur, I salute you for your good articles.
Attic
I'm doing a vented attic because I will have no ducts and will not be using spray foam. It will be similar to a traditional attic. Nothing to do with the new Decatur ordinance.
Certification
Carl,
I understand you have no choice in Decatur, but could you explain why you would have sought some certification for the house even if that were not the case?
As you are in the field, and know how you want to build your house, what do you see the advantages are of picking one of the programmes?
Certification
Malcolm - If I were to choose a program for ease and simplicity, I would either go with EarthCraft or NGBS. In my case, I decided to start with LEED V4 primarily to to gain experience with the program for future clients. I will likely also certify under the other programs for marketing purposes and may use of of the other certifications to obtain my CO, since LEED typically takes longer to complete the certification process. In my opinion, all the certification programs are overly complex and administratively oppressive. I'm still waiting for a rigorous, streamlined certification. I think it is possible, but it won't be easy getting there.
Carl
I hope you are going to blog the build here on GBA. The design looks like it will be a very nice house.
Air tightness target?
With your 'service space' separated from the vented attic, you should be able to hit an excellent air tightness number.
Do you have a target in mind?
Excited for you! Quick question.
With regards to the slab. I didn't see a layer or poly between the foam and concrete. Will there be one?
I'm excited to hear about ZIP-R. I've seen ZIP going up around Atlanta on apartments and SFD.
Air leakage and poly
I'm not a fanatic about air leakage. I expect that I will be in the range of .15 envelope leakage ratio, or around 1.5 ACH50. May end up tighter. Yes there will be a layer of poly under the slab, on top of the foam board. Will definitely be posting about it. Regarding the Zip R, I'm excited to see how the continuous insulation performs as well.
Nice!
Very nice layout. It will be a great and well-loved home with a long life.
Zip-R perm value
Hey Carl.
Congratulations on your new build. Do you have any concerns about the low perm value (<1) of the R panel? It won't dry to the inside, correct. So it will have to dry to the exterior if any moisture penetrates into osb.
With ZIP-R the OSB always dries toward the exterior.
With the 1/4" rainscreen spelled out in the wall detail, the OSB should have no problems drying toward the exterior:
https://www.greenbuildingadvisor.com/sites/default/files/Sections.jpg
The proprietary weather resistant barrier coating on all ZIP products means that bulk water wicking into the OSB would always be minimal, and with any rainscreen at all the amount of exterior moisture blowing by the clapboards reaching the ZIP would also be minimal, and offers an inimpeded drainage & good drying path for both the siding and the ZIP.
Thanks Dana
I have been traveling all day and wasn't able to respond to Steve's question. As Dana, said, the combination of the weather barrier on the Zip and the vented rainscreen will provide any necessary drying of the sheathing, as well as help limit any vapor drive from the exterior.
Thanks, Dana and Carl. I
Thanks, Dana and Carl. I would assume both of you would still want to foam any screw tips that ended up exposed to air on the interior side.
Response to Steve Knapp
Steve,
Are you talking about screws used to attach the furring strips that happen to miss the studs -- so that the screw tips penetrate the sheathing and are visible in the stud bays? (These screw tips don't really have access to "air on the interior side," a confusing phrase.)
Or are you talking about the heads of trim screws that are used to secure baseboard or window trim?
Martin. Since Carl is using
Martin. Since Carl is using blown-in fiberglass, I was wondering about fasteners that failed to catch a stud. I remember reading something about foaming exposed metal so moisture would not condense on them. Maybe this is a bigger issue in colder climates. Or maybe I am mis-remembering that particular best practice. Perhaps this is one of those "never-mind" posts.
Response to Steve Knapp
Steve,
Your suggestion might make sense in Fairbanks, Alaska -- but not in Atlanta.
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