Floor insulation in elevated buildings
Dear GBA Crew,
We’re building an elevated single family home in Brooklyn, NY, climate zone 4A, humid, subtropical, FEMA A Zone, about a quarter mile from NY Harbor. Per NYC code it is a non-combustible building with a steel chassis and cold formed steel studs above. The building is required to have a fire rated wall system and is immediately adjacent to other buildings so a 4″ mineral wool insulated metal panel provides fire separation and continuous insulation over 6″ CFS studs. The floors are made of perforated 12″ CFS joists with a 3/4″ USG Structocrete deck. The house is elevated on steel columns, with beam and joists 7’6″ clear above grade.
My concern is moisture accumulation in the floor construction during the summer. I’m planning to spray a fluid applied air vapor barrier to the underside of the structocrete deck. 12″ CFS joist cavities would be filled with two 6″ layers of mineral wool with a continuous 3″ semi rigid layer covering the joists. All would be held in place with hat channels screw attached though the continuous insulation with PVC spacers. Hardie soffit board with open 1/4″ joints would be screwed to the hat channels.
Do you think that this assembly needs a dedicated air water barrier below the insulation or between the insulation layers? Can you see a better way of doing this? I expect to have some air leakage through the insulation and a small amount of condensation under the deck during the AC months. Would an AWB reduce the air passage through the insulation? Would it be better to let it dry to the outside unimpeded?
Thanks so much for your help!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
You shouldn't have to worry much about a condensation issue in that assembly. Per your assembly description, you will have a vapor retarder at the interior surface of the floor (Structo-Crete and the fluid applied air and vapor barrier) plus plenty of insulation R-value (around R-60 by my calculation) to keep the first condensing surface above the dew point temp. Plus none of the assembly is moisture sensitive.
All that said, you will want to be sure the floor is airtight. If air can move through the floor system because of holes that are needed for mechanical systems, the air will carry moisture which then can condense on cold surfaces as it moves into the home. If you are diligent with sealing structural or any mechanical/electrical/plumbing penetrations that enter through the floor system, you shouldn't have any issues.