Retrofitting exterior sheathing or vapor
Clyde
| Posted in Energy, Heating & Insulation on
I am in the process of renovating an older home in the southeast that never had insulation in the exterior wall. We have taken everything down to the studs, and now have exterior lapboard directly on the 2 x 4 studs with no exterior sheathing or vapor barrier between the studs and exterior cladding. Short of removing and replacing all of the lapboards (which is not economically feasible), how can I best provide an after-the-fact vapor barrier? Building codes normally require that wall insulation be installed with the paper side to the interior of the house. I’d thought of trying to get the inspector to let me do the opposite and then attach an extra vapor barrier to the inside walls prior to hanging sheetrock. Any other ideas?
Replies
In the southeast, you really don't need a vapour barrier but an air barrier that is permeable to vapour flow. Since you may have air conditioning that makes the inside wall board the cool surface in the summer, it may be the surface that condensation occurs on. A vapour barrier just behind the interior wall board would not allow the condensed moisture to dry to the inside by diffusion.
The best place to have the air barrier is on the exterior under the siding. The reason fdor this is to stop moisture laden summer from entering the wall cavities where it may condense on the back of interior wall board. You don't need a vapour barrier on the exterior. There are 2 ways (other than rain/water leakage) that moisture gets into walls: (1) water vapour in air leakage and (2) vapour diffusion through permeable materials. In your area, the air leakage will most likely constitute 95-98%+ of the moisture movement into the wall cavity so if you stop the air leakage , you stop the most important part of the potential problem.
Also by stopping uncontrolled air leakage, you will reduce your cooling bills and increase comfort levels. It make no sense to pay to cool/dehumidify the air and then lose it through a loose building exterior shell. The air leakage may be responsible for from 20-40% of your cooling bill!!
Okay, so how would you go about this? What air barrier would you install? Would fiberglass insullation be sufficient? Should it have a paper coating on one side or not?
The best way would be the economically "not feasible" way- remove the clapboards and install an airsealed permeable building sheathing membrane/housewrap such as Tyvek/Typar or similar. This would give you the best chance to get an airtight exterior wall that wlll not allow leakage of air with high relative humidity into the cavity.
Barring total removal of the exterior boards, the next best approach would be to cut, fit, attach and seal the membrane in between the open studs and wall plates at their exterior edges where the boards are attached. All wiring and other service penetrations through the membrane should be sealed by using red sheathing tape, foam or non-hardening caulking. Where the main house walls rest on the foundation, it may be possible to remove the bottom 2 or 3 exterior boards and seal here with membrane, tape, foam or caulking.