I am using 2x6 studs with R-19 batt insulation accompanied with Kraft-faced vapor retarder, a Visquine vapor barrier, and drywall. Is this the best option? Or should I instead use an R-21 and no vapor retarder? Will I need the R-21 for southern WI? Or will it not add much in terms of insulation over and above the R-19? And seeing how I will have a Tyvek housewrap and a plastic barrier already, how important is that Kraft-faced vapor retarder? I would like your recommendation on the best wall insulation option for this case.
According to the International Building Code 2000, Wisconsin is mostly within the 6,000 Heating Degree Day (HDD) belt, except a small corner in the Northwest portion. Six thousand HDD is considered cold-climate condition. The recommended vapor barrier for the interior wall is either to use a 3-mil. polyethylene (Visquine) or a vapor-retarder paint on the interior. As such, you do not need both the Kraft-faced paper and Visquine for the R-19 design you mentioned. Housewrap is not a vapor barrier, it is primarily an air infiltration barrier. If you use housewrap, follow the manufacturer's installation instructions. As far as using R-21 vs. R-19, it depends. You should look at the payback for this upgrade and ask a heating contractor to evaluate it using Manual J software or perhaps your local electric or gas utility. Also consider if you use the extra insulation, it may allow you to install a smaller furnace thereby off-setting the expense. The links below will give more in-depth information on the topics discussed here: