15# felt rather than newer WRB between plywood sheathing and Hardie clapboard in rainy western Oregon?
Our local contractor with 20 years experience has suggested using 15# felt on the outside of the plywood (not OSB) sheathing on our new vacation home. We will be using Hardie smooth lap siding for the exterior. I doubt that our guy keeps up with building science. Should we be considering one of the newer weather resistant barrier products and also have a rain screen? The house: ~2,000 sq ft two-story, standing seam metal roof, crawlspace, lots of windows on the view side (Milgard Tuscany vinyl). The house will be unoccupied but somewhat heated for days up to a couple weeks in the winter in an area of 70″ of rain/year. Also, he wants to do the layer soon even though our windows won’t be ready for two months (early August). Our summer weather is mostly dry. Do we need to be concerned about damage to the felt/WRB before the windows and siding are in place? So far the plywood sheathing is darkening from UV but I assume it will be OK during the summer. Thanks for any thoughts about this.
Replies
Hey there,
There's nothing wrong at all with what your builder is proposing, but I assume his approach is purely based on cost saving measures. I would use Zip sheathing in place of plywood and felt. Felt works fine, but won't last long before siding with wind and rain. It's also more difficult to integrate window and door flashing and drainage with felt than Zip or other more modern house wraps. I would advise against rainscreens with Hardie lap siding. There is plenty of room behind the siding since the siding is not beveled. It automatically bridges away from the sheathing behind it and really makes minimal contact with the sheathing at all. A rainscreen behind Hardie only makes for more hollow spots where impact damage becomes more likely, or in the case of Homeslicker, or similar drainage mats, Hardie will tend to draw in where it is nailed and look terribly wavy. If it was my house, I would offer to pay for the extra money up front for the Zip sheathing and save later on the felt and labor to install it. The added benefit of Zip is a much more airtight assembly as well.
Hey there again... just rereading your post and seeing that the plywood is already installed, so my advice about using Zip is not valid. I still wouldn't recommend installing felt months ahead of the window install. The plywood can be exposed to the weather, for a long time if the roof is sheathed and papered too. A good product to install now if you are concerned about how much weather the plywood is seeing is Henry Blueskin. Like Zip it adds some air-sealing benefits too, but is obviously a lot more money than felt. Again with Hardie lap siding, no need for rainscreen.
I would, and always do, use a rainscreen for fiber cement clapboards, and, although I use smart WRBs nowadays, I’m also a proponent of asphalted felt. It’s especially handy in smaller projects where you don’t need to buy a big roll of WRB, and it’s an easy one-person job. I resided most of my old house over the last few years, one wall at a time, and used felt. I had no problems at all with taping around windows. As for the rainscreen, although manufacturers don’t require it, it’s still a good idea. Clapboards that are nailed against sheathing/WRB don’t drain, water and/or moisture is blocked every few inches from draining. But a good rainscreen doesn’t just allow liquid water to drain away, it also creates a vertical draft to help moisture get away from the clapboard backs. Paint failure on fiber cement doesn’t happen on the paint surface, it happens from moisture penetrating the clapboards from the back. That will give you years more paint life on your siding.
I have a vacation house on the Oregon coast. I resided a few years ago - #30 felt, taped with butyl flashing at all openings, 1/4" rain screen, cedar shingles. It's performed great.
A number of building science experts I follow have admitted they have / would use felt on their own homes. It's a "smart" membrane with great drying properties. You have to be careful about the details, but it does make a good WRB.
Best of luck to you.