HOW TO INSULATE FLOOR JOISTS FOR ATTACHED ADDITION
BEAM AND PIER WITHOUT A PERIMETER WALL
QUESTION FROM : Tom Hagood, Birmingham, AL
Question concerns floor joist insulation for attached addition that is beam & pier and does not have a perimeter wall; it is only 125 sf and I feel it necessary to insulate the joist even though we live in a cooling environment because there is no weather barrier (no perimeter wall). It is infrequent but winter weather here can typically get to 10 degrees or less
My thoughts are to cover the bottom of the joist with chicken-wire and to stuff in unfaced fiberglass from the top and to place Tyveck on top of the joist before the plywood sub-floor is nailed in.
So in summary, what is your opinion of fiberglass bats (unfaced) separated from the conditioned space with Tyveck; I know that is exactly backwards, but I do not see why it would not function well. Am I wrong; is there anything I'm not seeing there?
Thank You and Regards
ANSWER FROM THE INSULATION DOCTOR:
Thank you for your question. What I would recommend is that you install a kraft-backed insulation in the joists with the kraft paper facing the conditioned room first. You may not be able to staple it in which case using chicken wire to hold it would be an excellent idea.
Next install the Tyvek on the bottom of the joists thus allowing the kraft vapor barrier that you first installed to act as a vapor barrier and the Tyvek to perform as a wind barrier (as it is designed).
Then install a layer of 1/4" Thermax 4'x8' sheets on the exterior of the joists. This will create a seal from critters, bugs, etc.
In reference to your original plan, you would trap moisture between the Tyvek and the unfaced insulation causing the insulation to become a sponge. With no ventilation, mold and mildew could have occurred.
I sincerely hope this recommendation works for you.
The Insulation Doctor