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Community and Q&A

I-cut vs. modified I-cut in housewrap for window installation

DarkNova | Posted in General Questions on

Preparing for window installation where the housewrap has already been installed and we will use Dupont FlexWrap for the sills.

Dupont’s instructions specify a standard I-Cut in the housewrap where the bottom is cut flush with where the sill will be, but it says that a Modified I-Cut is also acceptable.

Joseph Lstiburek shows in his instructions the Modified I-Cut which involves folding the housewrap inward at the sill so the FlexWrap would go on top of the housewrap.

I’m thinking that the pro of the Modified I-Cut is that the window itself is holding down the housewrap so water can’t likely get behind the housewrap, whereas the I-Cut relies more on the adhesiveness of the FlexWrap to stay attached on top of the housewrap to prevent water from getting behind it (although the nailing flange will also hold it)

But for the Modified I-Cut, if you are not attaching a sloped sill piece on top of the housewrap, isn’t the FlexWrap basically just attached to the housewrap, not to the wood buck itself? It seems like you would want the FlexWrap actually stuck mostly to the wood buck.

Have people found one way works better than the other? Thanks.


  1. dankolbert | | #1

    I'm not sure who is responsible for this obsession with cutting house wrap. I remember reading instructions back in the 80's so presumably it's as old as Tyvek at least. But of all the waterproofing details you need to decide on, it is by far the least significant. Make sure everything laps in a way that makes sense to you and don't worry about how you cut out the house wrap.

  2. GBA Editor
    Martin Holladay | | #2

    I couldn't agree more. Remember the "upside-down martini glass" discussions? Lots of variations have been offered over the years.

    If anyone wants to read more on this topic, here are some links:

  3. dankolbert | | #3

    I'd forgotten that one. A future social anthropologist can do a cultural analysis of deeper meaning in house wrap cutting designs and names.

    I guess it's good if it encouraged people to think about how their details went together, but bad if people took it as gospel and began and ended with cutting the house wrap.

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