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dixie53
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# Posted: 1 Sep 2025 01:20pm
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Im building a small cabin, the floor joists are 2x6, i nailed 2x2s at each bottom and ill cut plywood to go between each joist and put insulation in each bay. do i need a vapor barrier and if so where? thanks...
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Brettny
Member
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# Posted: 1 Sep 2025 07:43pm
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What type of Insulation and how close to the ground is is?
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dixie53
Member
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# Posted: 1 Sep 2025 08:00pm
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It's about 3-6 inchs off the ground. Insulation is unfaced batting from menards, bottom plywood is 1/2 inch 3 ply, top is 3/4inch bc.
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Brettny
Member
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# Posted: 3 Sep 2025 05:52am
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To close to the ground. Your going to trap moisture and rodents. Either way use foam board as it's more moisture resistant.
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travellerw
Member
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# Posted: 3 Sep 2025 05:28pm
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Ohhh.... Gonna be a mouse house for sure. Especially with batt insulation.
I concur about the foam. It won't totally keep them out but will be much better.
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MtnDon
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# Posted: 3 Sep 2025 09:15pm
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I agree, it is too close to the ground.
If the bottom is tightly closed by 3/8” plywood that part can work. The bottom of our floor joists has been enclosed with 3/8" plywood for 16 years. No rodents have entered. But the joist bottom faces are a minimum of 20 inches above grade.
That allowed the plywood sheets to be air nailed in uncut sheets from underneath.
FYI, joist bottoms should be a minimum of 18” above grade and girder bottoms at least 12 " above grade, or they should be PT wood.
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spencerin
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# Posted: 3 Sep 2025 09:39pm
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Yes, build it much higher off the ground. Not just for ventilation, but also so that you can easily get underneath it should you need to in the future.
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DRP
Member
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# Posted: 3 Sep 2025 11:08pm
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"do i need a vapor barrier and if so where?"
On the ground, which should be a "pitchers mound" draining out. Clean sandy dirt on it is best for keeping it there and working later.
It brings a dew point condensation story to mind. The 35-year-old house was on a crawlspace. When I was asked to look at it the plastic under there had pools of water in places on top of it and very damp soil under the plastic, which was bunched up in places, pulled back in others, years of not a care.
Below the insulation the joists were rotten, the insulation was glistening. The fully exposed centerline girder beneath the joists was completely rotten. pulling the insulation aside, the wood was like new where covered by the insulation. They had AC and is was cool down there. The drainage problem kept the humidity close to 100%, the AC had the exposed wood and surfaces below dew point. Moisture condensed on those surfaces, and the moisture level of the wood went into the rot zone.
That was an extreme case, but, the underside of the floor system is radiating into the cold dark reservoir of earth. If the wood lingers below dew point for the conditions under there, its a place where condensation is more likely and treated might be a good idea.
You do need to be able to exclude or remove leaves and debris (tinder).
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