Protecting framing and sheathing from humidify in PERSIST-wall greenhouse
Vebjorn
| Posted in General Questions on
I’d like to build an all-year greenhouse in Vermont, detached from the dwelling. (Well-insulated walls, triple-pane windows, mechanical ventilation, and a rocket mass heater.) The PERSIST wall has been recommended here before for greenhouses, and I’m trying to understand that recommendation.
The greenhouse will be warmer and more humid than the outside almost all the time, so it makes sense that the partial pressure of the vapor will drive humidity past an imperfectly sealed vapor retarder. And the PERSIST approach, with the vapor retarder external to the framing and sheathing, makes it easier to seal well. So far so good.
But what about the framing and sheathing? In a dwelling, excessive moisture (e.g., from a shower) will soon get a chance to dry back into the interior of the house. But won’t the humidity of the greenhouse cause mold and rot on pine framing and plywood sheathing?
I suppose I could frame and sheathe with cedar, but it’s weaker, more expensive, and not as easily available. We’d like to avoid pressure-treated wood where we’re going to grow food.
Am I right that covering the inside (e.g., with cedar T&G) will keep bulk water from watering away but otherwise do nothing to protect the framing and sheathing from humidity? Am I better off leaving the framing exposed to it can at least get some airflow?
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Replies
Hi Vebjorn -
I think it would help to see a plan for your greenhouse, in that most greenhouses:
1. are not sheathed but braced for shear, since wood sheathing blocks all light
2. are steel-framed.
Having said that, if I were to build a wood-framed and sheathed structure with greenhouse conditions, I would definitely do it open-cavity, and I would use Huber Advantech for sheathing, since in my experience it really resists mold growth under high humidity conditions. Not sure what Huber would say about the warranty though!
Vebjorn,
i share your reservations about growing food in beds made of most pressure-treated lumber, but don't see much risk when the wood will not be in contact with the soil or plants. You might want to look at the newer formulations of Pt being used, Like MPS that do not leach their preservatives (in the way CCA and ACQ did) before deciding to frame with pine.