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Back caulking window nailing flanges

Is it common practice to use a specific window flange caulking for back caulking the flange before installation? Does flashing tape and house wrap suffice (assuming that it installed properly) or is caulk important in the case that the tape fails. Or, are you just adding a chance for water that would happen to enter this area to be trapped and rot the framing without knowing? I would like to spare the expense and labor of more caulking if I could but don't want to sacrifice air and water sealing at this crucial junction. Thanks in advance.

Asked by Matthew Michaud
Posted Jun 1, 2014 8:52 AM ET
Edited Jun 1, 2014 8:53 AM ET


5 Answers

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Matthew, Not everyone uses caulking at their flanges. Some, as you say, rely on tape. But in both cases there should be no fear of moisture being trapped as the bottom of the flange is left open. Here in BC our code goes further mandating clear drainage path for any moisture that gets under the sill by either drilling holes in the bottom flange or mounting the window outboard of the rain screen strapping. The sill must also be lined and sloped at 6 degrees. The air sealing around the window occurs not at the WRB layer but more usually between the window frame and the framing.

Answered by Malcolm Taylor
Posted Jun 1, 2014 2:31 PM ET


Malcolm's answer is spot on.

Answered by aj builder, Upstate NY Zone 6a
Posted Jun 1, 2014 3:08 PM ET


So if I use caulk, none is placed on the bottom window flange? This is to let water drain downward?

Answered by Matthew Michaud
Posted Jun 1, 2014 7:56 PM ET


A similar situation occurs at the head flashing which is installed after the window is in. While the WRB is taped at the jambs and sill, no tape, or intermittent strips are used at the head to allow moisture which has got behind the WRB to escape.

Answered by Malcolm Taylor
Posted Jun 1, 2014 8:34 PM ET


Most window installation experts agree that you should never install caulk behind the bottom flange. Unfortunately, a few window manufacturers still provide bad advice in their installation instructions on this point.

As long as the window manufacturer doesn't advise you to install caulk behind the bottom flange, you should strive to adhere to the manufacturer's installation instructions.

GBA has a series of videos that show Bill Robinson's method of installing flanged windows: Flanged Window Replacement.

Answered by Martin Holladay
Posted Jun 2, 2014 5:29 AM ET

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