Is it wise to put a product like Drainwrap behind a layer of rigid insulation?
talusscree
| Posted in GBA Pro Help on
We are getting ready to remove the existing layers of siding from an old (100yrs. +) home that appears to have diagonal board sheathing attached to the studs.
Our intention is to blow cellulose into the stud bays, then cover the board sheathing with Drainwrap integrating the window flashing into this layer, then adding 1″ rigid foam with seams taped, and finally adding 1x furring strips and either Hardi or Lp lap siding.
The project is waterfront on a coastal bay in Maryland. The windows in the home are vinyl replacements in original wood frames.
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Replies
Mike,
There is no reason I can think of to use Drainwrap (wrinkled housewrap) rather than ordinary flat housewrap. In theory, wrinkled housewrap may slightly reduce the thermal performance of the wall.
The type of wall you are describing should be able to dry inward, as long as there is no interior polyethylene.
Your biggest challenge concerns window flashing. Vinyl replacement windows are often installed without providing good flashing on the rough sill. You'll need to assess the quality of the water management details (and each window's exposure to wind-driven rain) to determine whether the window sills are detailed correctly.
The windows are definitely the challenge. We often wrap the sill with aluminum after we cut a saw kerf at the bottom of the vertical trim and the stop so we can tuck it under everything. The reason that I was thinking of the wrinkled housewrap was to manage the water around the windows if we flashed them to this layer. Would we be better served to create the secondary drainage plane for the windows at the face of the foam? What would be your recommendation on foam? Foil faced polyiso or xps?
With innie-window flashing you DEFINITELY want to use DrainWrap (tm) rather than flat stuff, otherwise there is no capillary break and the bulk water dumped onto the housewrap just spreads via capillary action across the foam/housewrap plane and most of it ends up in the wood. With the crinkly stuff gravity does it's work (however slowly), and most of it ends up leaving at the bottom.
The thermal loss to convection with those extremely narrow channels is miniscule- it's like trying to blow through nearly micro-scopic straws. Over 8-12' it's pretty air-retardent.
If you mount & flash the windows outie (which you might do, if it's only 1" foam) the housewrap goes on the exterior facing the rainscreen gap, and flat stuff is fine.