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Community and Q&A

air/vapor barrier in first floor

[email protected] | Posted in Energy Efficiency and Durability on

should I put an air/vapor barrier  under the first floor ( between the crawl space  and the subfloor) such as Intello plus? This is a stem wall unvented crawl space foundation with 12mill  plastic over the rat slab crawl space floor.By code I have R23 Rockwool inbetween the joists and I will add  comfort board to the bottom of the joists. All the stem walls are insulated on the inside with either comfort board or polyisocyanourate foil faced foam board. I am at understory inspection stage now . This 25 miles north of San Francisco climate region 3a maritime. Subfloor will be T&G 5/8 inch thick. I want to avoid any more foam as there is a termite problem in the area.

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  1. Expert Member


    One of the main differences between vented and unvented crawlspaces is that unvented ones are included in the conditioned area of the house, and they are heated and supplied with ventilation like the rest of the house. That means the air and vapour-barrier is at the perimeter of the crawlspace, not at the floor above, as is all the insulation. Typically the poly is placed under the rat slab to protect it and keep the slab dry, and the required insulation is attached to the inside of the stem-walls and rim-joists.

    Are you sure you aren't confusing the code requirements for vented crawlspaces concerning insulating the framed floor above?

    1. [email protected] | | #2

      California code require 25 cfm fan exhaust and an inlet vent from the inside of the house with R-5 insulation on the walls . I have already poured the rat slab and placed the joists. I have the floor vapor barrier going up the inside of the stem wall 9 inches ( stem wall is 31 to 33 inches in total height from the rat slab). The poly insulation is foil faced 3 inch poly ( 115 linear ft) and a 30 linear ft comfort board. I can easily take the comfort board off and tape a piece of the vapor barrier up to 3 inches short of the top of the stem wall. The poly ( about 115 linear ft) is glued onto the concrete stem wall . Should I tape a piece of the vapor barrier (Perminator 10 mill) to the floor vapor barrier and then glue it to the inside foil face of the poly insulation to extend the vapor barrier up to 3 inches short of the stem wall? Then I should just leave the floor insulation alone ( Rockwool batts). I did not see anything in the code about heating, this is San Fransico area a very mild climate never gets below 34-36 in the winter and only for a few days at night.

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