Hemp panels between vapor retarder & sheet rock?
I was given some hemp panels which are 3/4″ thick with a claimed R value of 2.4. I’d like to use it on the inside of my exterior walls to boost their efficiency. However, I have already installed the vapor retarder…and I’m not sure it would be wise to sandwich this hemp between that and the sheet rock. My fear is that moisture could cause problems in the hemp and or wall assembly before it was able to dry.
The attached sketch is my wall as it currently is. I plan to use a clay plaster finish over the sheet rock…1/8″-1/4″ thick. No paint of any kind. So, it will be quite permeable.
I could use the hemp panels on the exterior side and put the rain shield over that, then the wood clap board siding. However, I’m not sure if that’s any better or worse than putting it on the inside. I’m guessing it would be more prone to getting wet on the outside like this but…would probably dry out faster than sandwiched between the vapor retarder and sheet rock.
Thoughts?
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Replies
I don't see any issue with putting 100% hemp panels on the inside of your double-stud wall. You will still have most of your insulation R-value on the exterior side of the vapor barrier, so there's no risk of condensation.
Dean,
As Frank said, I don't think it matters much where you put them. They don't amount to a large enough proportion of the wall insulation to affect the dew point.
Thank you, Frank and Malcolm.
Perhaps I'm still not understanding this side of building science. My thought was that if there was enough vapor that the Membrain closed off to, moisture would build at the point of contact between hemp and Membrain. But, if I'm understanding correctly now, that vapor would have to hit a cold surface before it condensed into moisture, and it won't be cold at the location.
Is that roughly correct?
Dean,
Yes. Moisture moving through walls only becomes a problem when it encounters a surface which it can't easily move through, and is cold enough that it causes that moisture to condense, or is taken up by adsorption. In a typical double wall the point where it can get that cold is roughly 2/3rd of the way in from the exterior. So the hemp layer you are adding doesn't affect anything.
To add to what Malcolm said, the hemp is also very vapor open, so it's not going to trap moisture, so there is no risk of the "double vapor barrier" problem here.
I think you'll be fine. The only issue I'd be careful of is the tendance for the drywall to "dimple" in the immediate area of fasteners when you fasten the drywal through a squishy material. This is easy to deal with by being careful when driving the fasteners. Using 5/8" drywall will also help (and that has other benefits too).
Bill
To put some numbers to Malcolm's explanation, suppose you have 90% of your insulation R-value on the exterior side of the vapor barrier and keep the interior at 70 °F and 50% relative humidity.
Air at 70 °F and 50% RH has a dew point temperature of about 50 °F. If it is 0 °F outside, the vapor barrier will be 63 °F (0° + 90% of 70°-0°), so still well above the dew point temperature.
Thanks to each of you for your replies. Appreciated!