I am preparing to install a new pre-hung double hung. Stopped in Home Depot for adjunct supplies, and advice on what materials to layer. I understood to start with a full-width overlapping material to cover the studs. Home Depot Guy directed me only to the peel-and-stick membrane I see mentioned in various threads here (reflective silver backing, very sticky). I was unfamiliar, and I know now that I did not understand it's usage completely by his description, and he never spoke to me about the underlayment. Result is that I covered the exterior edges and folded it to cover the window-contacting surfaced of the studs/sill. It is tucked under the clapboard siding, and over the exposed edges of the crumbly sheathing. It is, I believe, only 5" wide, so it only covers about 2.5" of the stud depth. Did it very neatly, overlapped junctions, etc. But, as I really knew going in, I should have covered the rough-in with building paper; the peel-and-stick is meant just to cover the nailing flange.

Should I clear it out and start over, or can I attach the window now over it? Since the nailing flange will provide the force to keep it uniformly adhered, and I can tape over the flange after, won't I achieve the same watertight seal as with the builders paper? Why does the portion of the builders paper that overlaps the interior edge of the studs/sill matter, when this part is clearly behind the water seal created by the flange, sealant and tape? Or should I just slap the builders paper over the peel-and-stick? No big deal for me to tear it off (about half of an $ 8 roll wasted), but I wondered if my time and meticulous effort could be salvaged. I am almost sure I should start over with builders paper, but I mainly want to understand why this is important.

Thanks for any input. I do realize it is just about Christmas Day - the holiday provided a convenient time to tear the wall open, since I'm stuck at home for work and the house is otherwise empty! Your time is appreciated...

Two other potentially relevant facts:
1. it is a bathroom remodel
2. I have an extra 1/2" of rough-in width (only) to play with (1.5" total, which I think won't be a problem - will fill gap with foam).

Happy Holidays to all!

Mike
Ithaca, NY

 

Wrapping the rough opening with your peel and stick membrane certainly won't hurt anything. But you are right... the window should actually be installed on top of the building paper. The way I do it is:

Install the building paper right across the rough opening of the window. This ensures the sheathing will be protected. Make a horizontal cut across the top of the rough opening. Make a vertical cut down the middle of the rough opening that extends from the top of the opening down to within 12" of the bottom of the opening. Then make a diagonal cut from each of the bottom corners up to that last spot. Fold and staple the bottom and side flaps into the rough opening and tape the edges with contractors' tape. (not the peel and stick). At the top, make 2 diagonal incisions up and away from the corners of the building paper so as to create a 6" flap that is temporarily folded up and out of the way.

I might mention that if you are using tar paper (felt) it is not compatable with most peel and stick membranes. The solvents in the tar paper will quickly repel butyl rubber membranes rendering them useless.

At any rate, make a pan flashing out of your membranes along the bottom, then apply sealant to the sides and top of the opening (and the bottom if you prefer- but some companies such as Pella demand the bottom be left uncaulked), then install the window. Apply peel and stick over the side nailing fins... then apply peel and stick onto the sheathing on top so that it extends over the top of the top nailing fin and side pieces. then fold the flap of building paper back down, and tape it (or skip tape it, as you prefer).

 

I had a lengthy answer for you, but sleep man beat me to the submit key. Good work!! Depending on your siding, you need to also take care of the top of the window. Generally vinyl siding has its channels, but wood, Masonite, etc, you will need to fabricate a drip on top. It can be done with sheet metal, extending well up under your siding and about 1/2" past the outer molding of the window. This keeps infiltration of water from the top.