Amish shed-to-cottage project: Air/vapor barrier necessary for simple construction?
alanferg
| Posted in Energy Efficiency and Durability on
Converting a 12×20 Amish-built shed on skids into a small 240 sq ft cottage for full-time living. It is 2×4 framing with dutchlap exterior cladding (no sheathing and WRB). The floor joists, stud bays, and ceiling are insulated with mineral wool batts. I’m ready to finish interior walls with pine boards or plywood sheets (no drywall and paint).
Do I need an interior air or vapor barrier? I’m told to install 4 mil plastic and tape seams and edges–but I fear this advice is based on a typical wall assembly. My construction details are simpler and I’ve spent hours researching this topic. Will an interior air or vapor barrier do more damage than good?
Details:
– Northern Maine (climate zone 7): cold dry winters, warm humid summers
– 2×4 framing with dutchlap exterior cladding (no sheathing and WRB)
– Mineral wool batt insulation (floor R-23, walls R-15, ceiling R-45)
– Interior wall cladding will be pine boards or plywood (no drywall and paint)
– Vented attic space with gable vents (no ridge vent). Soffit vents to be installed.
– Electric heat
– No idea if an air conditioner will be used, or if it will be necessary
– Propane range and oven (frequent cooking)
– Bathroom exhaust fan installed and range exhaust hood to be installed
– Composting toilet
– Presently no plumbing but planning to install potable water tank, water heater, and RV pump for sink and shower.
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Replies
HI Alan -
25 - 40% of total heat loss in a building is the result of air leakage. And when air leaks, it carries moisture with it--a double penalty.
So, yes, air control layer is essential throughout your enclosure.
Peter
Peter, thanks for the advice.
"So, yes, air control layer is essential throughout your enclosure."
-- even though my enclosure is very basic? (exterior wood cladding attached to studs with batt insulation.)
I guess what I'm trying to ask, should my enclosure be allowed to leak moisture-laden air due to it's unusually basic wall assembly? Am creating moisture problems by the very act of stopping moisture problems?
I've been confused about the use of air and vapor barriers. Some authors say a vapor barrier isn't necessary while others say it is. Joe Lstiburek seems to contradict himself about using interior poly sheeting for air barriers in cold climate structures (hopefully I simply misunderstood his article, https://buildingscience.com/documents/digests/bsd-104-understanding-air-barriers).
Thanks,
Alan
Alan,
There are situations where vapour barriers can cause more problems that they solve, but that isn't the case with air-barriers. They are always a go0d idea.
My understanding is polyethylene sheeting functions as both air and vapor barrier. But I'm now realizing an air barrier isn't necessarily always sheeting. Would taping seams, joints, and penetrations of the interior plywood make an effective air barrier while avoiding possible problems of a vapor barrier? It would look terrible, but this project is on a very tight budget and aesthetics aren't a factor.
Thank you for your input,
Alan
Alan,
Q. "Would taping seams, joints, and penetrations of the interior plywood make an effective air barrier while avoiding possible problems of a vapor barrier?"
A. Yes, interior plywood with taped seams is a good idea. You don't need a vapor barrier, but in northern Maine (Zone 7), there aren't any disadvantages to interior polyethylene.
Your house cries out for an air barrier. Without any exterior sheathing, your exterior will leak air like a sieve. Without any interior drywall, the interior side of your walls will also leak air like a sieve. Your energy bills will be very high unless you create an air barrier. Either interior plywood or interior drywall would work. Once you've installed your interior plywood or drywall, you can cover the plywood or drywall with tongue-and-groove boards if you want.
For more information, see this article: "Questions and Answers About Air Barriers."
Why are you using electric heat? Are you on grid or will you have solar PV? If you are using a propane stove top and oven get a brand that doesn't need an electric "glow bar" in the oven. Premier makes a propane stove that doesn't have the glow bar.
https://www.premierrange.com/gasranges20.php
Also, will gable vents be sufficient for your attic? I've read that the entire length of both the eaves and ridge should be vented on a residential building.
"I've read that the entire length of both the eaves and ridge should be vented on a residential building."
This is true of cathedral ceilings, but the overwhelming majority of the housing stock in North America have attics with roof or gable vents and experience no problems.
Thank you all for the advice! I'm finishing the plywood interior and will be working next on an air barrier.
Update:
Last winter (February/March 2019), I finished the interior walls and ceiling with plywood and taped all seams with 3M All Weather Flashing Tape 8067 to create an air barrier--preventing moisture-laden air to reach cold surfaces. No plastic vapor barrier was installed, as I understand from my research the plastic sheeting is likely to cause more harm than good.
A single $40 1200 Watt electric oil-filled radiator is able to keep the interior comfortably warm around 65 to 70 °F even on freezing -20 to -30 °F nights.