Ice & water barrier on vertical wall
patrickcmuldoon
| Posted in General Questions on
My project calls to install an ice and water barrier at one side of my roof to prevent ice dam. Plans call for metal roof decking running to the edge of my mono-pitched roof on the lower side, then then down a vertical wall. The drip edge is at the bottom of the vertical wall where the metal decking ends. All the details I’ve found for an ice and water barrier show it terminating at the drip edge. In my case, running this underlayment to the drip edge adds a lot more material. Is it necessary to run this to the drip edge or could I run it just past the pitch and into the vertical maybe 6″ to tie into some zip-board panels.
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Replies
Hi Patrick.
If I'm understanding this correctly, you must have no roof overhangs and metal roofing transitioning into siding where the roof and wall meet. If that is correct, I think it is fine to turn the ice-and-water barrier down the wall a few inches to protect the transition and stop it there. In fact, you probably don't want an impermeable membrane over the ZIP sheathing anyway.
One thing to note, ice-and-water barrier don't prevent ice dams. They may mitigate some damage caused by ice dams, but it is proper insulation and air sealing that prevents them in the first place.
Yes you are thinking of my roof correctly. I'm planning on no ZIP sheathing under the section that will have ice and water barrier. There will be just regular plywood with the barrier adhering to it.
As for the proper insulation and air sealing... how does BATT, rigid foam, and spray insulation work with ice and barrier products? I read on a previous forum post BATT doesn't perform well. When you speak on air sealing, do you mean ensuring your attic space is properly sealed from interior?
Thank you for the support!
Ice and water is a vapor barrier. You should only apply it over walls that have sufficient exterior rigid insulation for condensation control. I would keep the section on the wall short and rely on the wall WRB after that.
Generally, you can have a cold side vapor barrier if:
-above vented roof
-installed over sufficient amount of rigid insulation
-closed cell spray foam bellow the deck
You should never install it directly over an assembly with just batts.
Specific to this situation, air sealing would done on the ceiling, whether it is a flat ceiling with an unconditioned attic above or a cathedral ceiling that follows the roof line. If you are going this route and insulating the roof line, have a look at the details presented in this article: How to Build an Insulated Cathedral Ceiling.
And I agree with Akos. Keep the ice-and-water membrane only where you need it. Lap the roof to wall joint a bit if you'd like to protect that vulnerable area, but don't carry it down on the wall very far.