Drainable house wrap under exterior foam insulation
Salesi
| Posted in General Questions on
We are building a new house and my plan was to use a fully adhered WRB like Henry Blueskin VP100 on the 1/2 plywood sheathing. Then use 2″ of polyiso foam insulation with 30 lb roofing felt (belt and suspenders) followed by a 3/4″ rainscreen and siding. However if water would ever get between the Henry Blueskin and the 2″ of polyiso, how would it drain? Wouldn’t it be better to use a drainable house wrap, Hydrogap, and then the polyiso. The small gap would allow water to drain away verses being trapped between two flat surfaces.
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Skip the Blueskin and felt. Air seal the sheathing, install draining housewrap, then rigid insulation, followed by the rainscreen.
How would you air seal the 1/2 plywood? Tape the seams? To me the felt is cheap insurance. I could skip it, but since we are doing a lot of the labor ourselves, it would be an extra layer and low cost to keep water out.
Zip tape seems to be the most cost effective acrylic adhesive based tape. Sega Fentrim tape is recommended for sealing sheathing to the foundation. STPE (silyl-terminated-polyether) fluid applied flashings like Zip Liquid flash, Henry Air-Bloc LF or Prosoco R Guard Fast flash are good for penetrations and problem areas. You can even dispense with tape and use fluid applied exclusively, but at a higher cost.
Seal the sheathing but not the windows. Install the housewrap. Cut the house wrap flush with the top of the window. Make diagonal cuts at the top corners of the window and temporally fold up the flap and secure with a small piece of tape. Cut the housewrap 1 1/2 inches form the window opening sides and bottom. Tape or fluid flash around and fully into the window opening including the top. overlapping the housewrap at least 1 1/2 inches. If you use tape for the sill pan use stretch tape. (Don't forget to angle the sill with a length of clapboard or frame it that way.) A good hybrid approach is to use fluid applied flashing in the window corners extending it 3 inches in all directions, then finish with tape. The tape should overlap the fluid applied at least 1 1/2 inches. This avoids the need for expensive stretch tape. Install the windows per manufacturer's recommendations Tape or fluid apply the window flanges at the top and sides, but not the bottom. Install a metal drip edge to the top of the window and tape it to the sheathing. Fold down the housewrap flap over the drip edge. Tape the diagonal cuts. The final air sealing is done from the inside. Typically this is done with low expansion foam. Do not fill the cavity, just the inside inch. However, fluid applied flashing makes a superior air seal. Shove foam backer rod into the gap about 3/8 inch deep. Cover the backer rod with fluid applied and tool with a finger or radius tool. Prosoco makes a dedicated STPE product called Air Dam for this purpose but I found that Zip Liquid Flash works fine. You didn't say whether the windows are innie or outie. If outie everything is the same except the window buck must be fully flashed. The fluid applied or hybrid approach is recommended because of the complexity of the corners.
Between the houswrap and the rainscreen there is nothing to deteriorate with water, but some people tape the seams of the rigid foam as extra water infiltration protection.