PERSIST or modified PERSIST in Zone 4A?
I am building a small (800 sq ft) efficient cottage for my wife and I in SE Tennessee (4A zone). I own a tiny house building company and have a background in residential construction. The design is centered around simplicity, energy efficiency, and cost, but I am willing to spend a bit more money on things that will pay me back in the long run like a really well insulated envelope. It will be a simple A frame with metal roof and metal siding I am considering building a full on PERSIST method wall or at the very least adding 1″ of polyiso on the exterior. I have a few questions.
If I go the PERSIST route:
1. What foam type in walls/ceiling (I have a lead on some cheap polyiso)?
2. What thickness (R value) for walls/ ceiling in zone 4A?
3. Is going full monty PERSIST in 4A going to be cost effective compared to the option below when you factor energy cost savings (we are planning on owning this home for a long time).
4. Is there any specific implementation details I should follow with metal siding?
5. Is there any need for wall coverings on the interior with PERSIST or could I use the space for built-in shelves? As I understand it in the PERSIST method, more ventilation to the interior is a good thing.
6. Good cost effective vapor barrier to go on the outside of the bottom layer of sheathing.
If I go with 1″ foam exterior and fiberglass/mineral wool batt interior:
1. What thickness would I use for this? Seems like 1″ is adequate for my climate to ensure the dew point is not in the wall?
2. What are some good (cost effective) non-persist options for insulating a vaulted ceiling?
3. If I use foil-faced polyiso, do I still need a vapor barrier or housewrap?
In general I would also be interested in your insights about the pros and cons of how the two options compare in terms of cost and energy efficiency in zone 4A.
I will be chronicling the build on my personal website tallmantinyhouse.com
Thanks!
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Replies
The "Full monty PERSIST" approach was developed for super cold climates. Is there a reason you were considering it? The "standard" way to do it these days seems to be to use both fluffy cavity insulation and exterior rigid insulation, as long as you use the appropriate ratio of exterior insulation to total insulation. This article has the numbers: https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
For a given budget, using cavity + rigid seems like its always going to be more cost effective than PERSIST (which only uses the exterior rigid). The point of PERSIST was for durability in super cold climates.
For your climate, the above article recommends a minimum of 16% of the total insulation to be in the rigid layer. If your walls are either 2x4 or 2x6, 1" of most any type of rigid insulation works. If you have more money, you just add more rigid.
Big caveat: all this is talking about walls, while your A-frame is sort of half-wall, half roof. If you want to be conservative, you could use the recommendations for roofs, which recommends R-15 of rigid insulation for your climate. Hopefully someone with some A-frame experience can chime in here!
Jeremy,
You have a long list of questions. Many of these questions can be answered by reading articles on GBA (or using the GBA search box).
Q. "If I go the PERSIST route, what foam type in walls/ceiling (I have a lead on some cheap polyiso)?"
A. Cheap polyiso is fine. For more information, see Choosing Rigid Foam.
Q. " What thickness (R value) for walls/ ceiling in zone 4A?"
A. Minimum code requirements are probably R-20 for walls and R-49 for roofs, but to know the requirements in your area, contact your local building department.
Q. "Is going full monty PERSIST in 4A going to be cost effective compared to the option below when you factor energy cost savings?"
A. Only you can perform that calculation. Read this article to figure out how to make the calculation: Payback Calculations for Energy-Efficiency Improvements.
Q. "Are there any specific implementation details I should follow with metal siding?"
A. You'll need to install furring strips on the exterior side of the rigid foam, and you'll need to follow the installation instructions provided by the siding manufacturer.
Q. "Is there any need for wall coverings on the interior with PERSIST or could I use the space for built-in shelves? As I understand it in the PERSIST method, more ventilation to the interior is a good thing."
A. Talk to your local building department to see if they will accept the use of your OSB or plywood sheathing as a thermal barrier to protect your rigid foam. It's possible that your local building department will require the installation of 1/2-inch drywall.
Q. "Good cost effective vapor barrier to go on the outside of the bottom layer of sheathing."
A. This question is unclear. The traditional PERSIST method uses a peel-and-stick membrane on the exterior side of the sheathing. Is that what you're talking about? For more information, see Getting Insulation Out of Your Walls and Ceilings.
Q. "If I go with 1 inch foam exterior and fiberglass/mineral wool batt interior, what thickness would I use for this? Seems like 1 inch is adequate for my climate to ensure the dew point is not in the wall."
A. One inch of foam will work for a 2x6 wall, but you will need at least R-15 of rigid foam for your roof assembly. For more information, see Combining Exterior Rigid Foam With Fluffy Insulation.
Q. "What are some good (cost effective) non-persist options for insulating a vaulted ceiling?"
A. Read this article: How to Build an Insulated Cathedral Ceiling
Q. "If I use foil-faced polyiso, do I still need a vapor barrier or housewrap?"
A. No and probably. Read these two articles:
Do I Need a Vapor Retarder?
All About Water-Resistive Barriers.
Wow. Thanks for the detailed responses Martin and Brenden. I will do some reading per Martin's recommendations and if I have more questions I'll check back in here.
Okay I think I've settled on the following wall system for the above build. I've read the articles about foam as a WRB and where the WRB should go and still have a question or two. LP Smartside lap siding > ring shank nailed to 1x3 strapping > screwed through seam taped Tyvek OVER 1" rigid foam > through 7/16" OSB into studs.
My questions are these:
1. Am i correct in assuming the strapping should outside the Tyvek? (screws travel through it, the foam and into osb/stud).
2. Can I forgo the strapping and air barrier and Nail the siding directly through the tyvek and 1" foam to the OSB? Is this advisable?
3. I know "only I can make this decision", but what would YOU change about the above layering? (looking at you Martin) :)
Jeremy,
Q. "Am I correct in assuming the strapping should outside the Tyvek?"
A. Yes. It's also possible to install the Tyvek between the OSB and the rigid foam. For more information, see "Where Does the Housewrap Go?"
Q. "Can I forgo the strapping and air barrier and nail the siding directly through the Tyvek and 1 inch foam to the OSB?"
A. To answer a question like this, you have to look up the installation instructions published by the siding manufacturer. According to LP:
"Insulated Sheathings. LP SmartSide Sidings may be installed over low-compression rigid foam or exterior gypsum. The following precautions must be followed:
• Adequate bracing of the wall in accordance with the International Codes or other ruling building code is required.
• For rigid foam sheathing up to 1 in. (25.4 mm) thick, siding may be nailed directly to the foam sheathing unless a drainage plane is required by the local building code. Nail length must be increased to ensure a minimum 1-1/2 in. (38.1 mm) fastener penetration into the structural framing."
Q. "Is this advisable?"
A. I wouldn't do it. I would include a rainscreen gap.
Q. "I know 'only I can make this decision,' but what would YOU change about the above layering?"
A. Two-inch thick rigid foam is better than 1-inch-thick foam -- but in your climate, 1 inch foam will work.
Jeremy, your second question mentions forgoing an air barrier. Did you mean to say strapping and rain screen? You want to treat some layer in your build-up as the air barrier. In your case, it would probably be the OSB sheathing. It definitely would not be the 1x3 strapping.