preventing moisture issues with new metal roof

Ryan Meyer
6/15/2014
I'm looking at doing a new standing seam steel roof on my 4/12 ranch with 5hips and one valley. My house was built in the 1950's in Wisconsin. Its has insulated 2x4 walls with brown fiberboard sheeting outside. Ceiling has two layers of fiberglass batt insulation and 11" of blow in cellulose free on top of that. We have boiler heat with no ductwork in the house and have moisture issues in winter. I run two whole house dehumidifiers in winter to keep humidity at 30-40% I installed new siding, windows and vented soffet last summer. I put a rain screen behind my siding to allow any moisture escaping to get out through soffet and ridge vent. With my high ceilings and low pitched roof there is very little insulation at the outside corners due to the lack of room for it and still allow air flow under roof deck from soffet to the ridge vent. This last winter I had ice forming on my drywall ceilings in the corner with the new airflow past them. Soffet was previously unvented. Before next winter I plan on tearing off the roof, roof deck area above outside walls, cutting out the little fiberglass batt insulation that is there and replacing it with 3" of closed cell spray foam from outside corner to about 3ft in and placing rest of insulation over it while maintaining the air space under roof deck for venting with baffles. Now to my question- What is the best possible way to install standing seam steel roof and be able to avoid any issues of it rusting from the bottom side? I have tongue and groove 1x8 board sheeting and replacing bottom 4ft I'm removing to spray foam with 3/4" plywood (not osb) Idea 1 Deck armor breathable underlayment to allow any moisture to get out from under sheets through the decking to attic? I'm afraid this will also allow moisture through the decking and possibly condense on back of cold metal roof. Idea 2 non breathable underlayment to protect moisture going through decking from reaching underside steel roof in first place. Idea 3 non breathable roof underlayment and 1x4's installed verticly over sheeting where steel roof fasteners will be placed to allow air gap to be vented at top under ridge vent for any moisture to get out. Second question- What is the best most breathable ridge vent for standing seam roofs? I was set on shinglevent II before I decided against installing regular shingles again and finding out I can afford the steel and hopefully never have to do this roof again if I build in 15-20 years. Any insight or comments would be appreciated. The amish that will most likely be doing the job are difficult to get ahold of for questions and they will do the best job they can with whatever materials I choose. Thanks, Ryan Ps I'm also a little worried about falling snow and ice over doorways with my hip roof. Any recommendation on catches? I don't really care for the little leaf ones. I like the looks of straight angle iron looking ones, but unsure what's best or how many rows to put on my 16ft sides from ridge to gutter.
Eric Novotny
An informed customer is the Best Customer!
6/15/2014
If you have decking boards, air seal the decking with a peel and stick membrane (i.e. ice/water). Air seal the attic floor to be certain and when you remove the roof is the time to do it because of the limited access at the eaves. Closed cell or open cell foam across all the top plates is the way to go. Don't worry about the Open Cell vs. Closed Cell permeability ratings as the much bigger issue is to stop the air movement. I wish we had spoken about the siding program before because there are some things you could have done differently there but 20/20 hindsight is always perfect. I do like the rainscreen though. So...back the roof...seal up all the penetrations and top plates of the attic. This will keep a majority of the moisture out of the home. You have plenty of ridge length so that high point being opened, with continuous and unobstructed soffits will serve you just fine. You don't need to use a specific ridge vent and they are usually formed onsite out of coil stock. You, if you have critters you are worried about, can install some sort of closure across the ridge to keep them out. After the deck is air sealed (i.e. with ice/water), you can run your panels right over that (be sure to use non-granulated ice/water and high temp). As long as the attic is vented, there isn't much benefit (in your climate) to putting foam over the deck or doing any above deck venting. Putting some synthetic underlayment over the ice/water may not be a bad idea either as it will give you a bit more seamless look as well as a better slip layer for the metal to move and expand. Once that is all done, insulate the attic to R-60 and use a good, borate only cellulose. Snow retention options are numerous and depend largely on your aesthetic preference but there are all types.
Guest User
6/16/2014
I don't have the time, money, or patience to take out two layers of batt insulation after trying to somehow take out 11" of fluffy blow in insulation in order to air seal the ceilings. I do not have a floor in the attic, just drywall and ceiling studs. The first layer of tarpaper like faced batt insulation was stapled to stud face before drywall ceiling was applied when built so there is a moisture barrier already, just not working as well as it could. If I'm going to do more air sealing, my light fixtures would be much easier and more worth the time than removing all my attic insulation. Why would I need all the ice and water under the roof sheets? If installed correctly it shouldn't leak. Besides running it in the valley and from bathroom fan to gutter, I don't see any benefit from using any more elsewhere. Couldn't I just use normal non breathable underlayment and save the cost? What would you have done differently with siding? I'm trying to learn all I can so when I build someday everything I'd as good as it can be. I would like some type of screen and baffles in ridge vent to prevent wind driven rain, snow and insects. Is there a reason for wanting r60? I was under assumption r35-r40 was more than enough. My windows are weakest link. Wish I would have gone with triple pane. No snow catches are better than others? I looked at menards standing seam metal tonight and it looked like garbage. Monday ill call up a couple places and find out info on whatever metal was going to be on the quote and on my roof. I like the idea of mechanical seamed, but doubt amish do that and I bet its more expensive than whatever the guys were going to quote me. I looked at one standing seam that one of my quote places did last year and it looks very nice. Not sure on any details of materials used other than its hidden fastener roof and they normally do mostly exposed fastener roofs. I'm hoping to find out and keep doing research till I figure out what the best route to take is. Thanks for the reply, Ryan
Eric Novotny
An informed customer is the Best Customer!
6/16/2014
Ryan, The reason for the air sealing recommendation has nothing to do with the vapor barrier. It has everything to do with the air barrier as air will carry about 100X as much moisture with it than will diffusion by comparison. My recommendations were based on your concerns goals for keeping the moisture out of the attic. As is usually the case, the most comprehensive approach is the most cumbersome. The reason that you would potentially put Ice/Water over the entire roof deck would be to air seal the roof deck and prevent any air loss to the underside of the metal roofing. By sealing the roof deck up, you will be controlling the ventilation and ensuring that it is controlled via the ridge vent and the soffits. This is more more of a potential issue when it comes to Tongue and Groove ceilings that serve as the insulation layer by decking board can still vent, unintentionally, between them. R-60 is the recommendation because it is cheap as compared to R-49 (Energy Star recommendation). Screening on the ridge is fine and they make several products that will work. The wind driving rain should be handled by the metal application and have them fold up a dam leg on the back edge if you are concerned.

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