Exterior foam, WRB, & new siding in the PNW
I’m going to soon be doing a fairly extensive remodel to my home here in the PNW. It’s zone 4-marine. Right now I’m drawing the plans to submit for my permit. I’d just as soon get it right from the start. Of immediate concern (for this posting) is the stuff and method I use to increase my exterior insulation.
Currently the walls are 2×4, some fiberglass, no internal vb, no external wrb, 5/8″ plywood sheathing, with vertical cedar shiplap for cladding. Near as I can tell, all the walls are dry with no problems. No leaking windows. But the house is under insulated. Our furnace runs too often.
We want new siding… this is mostly an aesthetic decision but installing new means we will have an easy time of making things look nice where we remove ugly rock facing, move windows about, remove windows where they don’t belong… you get the idea. So it seems a no-brainer to add insulation by adding the exterior foam. The interior drywall is in very good condition. I don’t want to remove the existing cedar cladding. I think this would be a bear of a job. It appears to have been fastened with 10d galvanized nails.. no lie. It’s in good condition, so why not leave it? Over the cedar I’m going to add 1″ of foam. I’m uncertain which to use: XPS or polyiso? Then I want a good WRB. Can I simply tape the foam or should I use Jumbo-Tex? If tape… which tape? Over the WRB will be vertical battens of treated ply then horizontal cladding. Probably Hardi brand. The good looking lap siding so I can miter the corners.
Answers to these questions or other thoughts will be greatly appreciated. I’ve ideas on how to detail the windows but there may be a future posting regarding that.
Jeff
Replies
What's your minimum winter temp? Coldest temp you'll see five-ten nights running?
In the Spokane area I've seen them use polyiso for exterior. But they also have a vapor retarder interior. You might resolve that with vapor retarder paint ... a lot of the people liked that vs poly. Easy, cheap, etc. If you have concerns over interior moisture, maybe consider a reality check on your exhaust systems to make sure they are doing a good job.
What is 'zone 4 marine' mean?
Spokane has quite a bit different climate than here. They are interior: quite a bit colder in the winter and very hot and dry in the summer. I'm right on Puget Sound. We get quite a bit of rain... usually slowly not the thunder storm downpour kind of rain. Clouds and fog are frequent.
The US Department of Energy classes all area of the country into zones. Ours is Zone 4 which covers most of Washington and Oregon. Since I'm on the water, we get a sub-category: marine.
Interior vapor barriers are generally not used here. Our summer is actually quite dry and hot. Its more of a Mediterranean climate than one would think.
I'm generally quite familiar w/ the Seattle climate. Twice the rain as Spokane (34 vs 17") and yes, milder winters. Rainfall along the sound varies a lot, I think. I know that parts of the Olympic Pennisula get 500 inches of rain.
If VB aren't used much there, I'd be comfortable w/ the polyiso for the sheathing.
There are
several issues you need to consider if adding exterior foam. You need to determine what thickness/type to use, whether/how to tape it, where to install the windows and flashing, what/where/how to install your WRB, how to insulate and finish the rest of the wall to the inside, etc.
It would be a good idea to read this short article http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing and the other various exterior foam info there. You should be able to find several articles and question/answer threads.
There probably isn't any issue with leaving the existing cedar in place, from a building science perspective.
As with any project where you are tightening the house, a blower door test and a review of ventilation and combustion air is a very good idea.
Whereabouts are you?
David, I'm on Bainbridge Island. We are near neighbors.
Thanks for the link to the GBA article. I'd seen a variation of that article by M. Holladay. That's where I came up with the 1" of foam... to give me at least R-2.5. But your link led me to another that deals with using foam sheathing as the wrb. I've concluded from that reading ( http://www.greenbuildingadvisor.com/blogs/dept/musings/using-rigid-foam-water-resistive-barrier ) that it is not a good idea. This confirms my gut feeling. Relying on tape for joint sealing isn't such a good idea. So, I'll be using Jumbo-Tex 60, the product commonly used around here for wrb. Now the question becomes how to attach it to foam. Since a hammer stapler would be ineffective I guess using plastic washered nails is the way to go. A few of these to hold the Jumbo into place then slap on the battens. We'll simply not work if there is too much wind.
There is still the question of using XPS or Polyiso. Perhaps I'll first have to cost out the material then make my choice. My reading suggests that ultimate performance is about the same.
Detailing the windows and doors is an ongoing concern. I've seen it done a variety of ways here. Most guys seem to rely on the fact that the Vycor tape will stick forever. I'm not so sure about that. I'm open to any and all suggestions. My house has fairly good overhangs but still receives a good deal of wind driven rain on its south-west (Sound facing) side. The good news is that that rainy side also receives the most sun, so it dries equally well.
I'm wondering about the blower door test. Does it really make sense to get one before the remodel? I know there are many gaps in the air barrier... I can see them. My plan is to go through the entire house with foam and caulk. Nearly every window will be replaced, bathroom fans will be moved, the kitchen moves.... on and on. Its almost like I'll be building new. So wouldn't it make more sense to get a blower test done after... to confirm that I caught all that needed to be done?
Thanks again for the help.
Jeff
Which foam
If you are trying to maintain a somewhat vapor-open wall then you would rule out foil polyiso, maybe lean towards EPS. I agree with your conclusion to use a separate WRB layer, it makes a LOT more sense to me than simply hoping that moisture never migrates thru foam joints and tape never fails. As you no doubt found there are a lot of competing ideas and criteria for using and detailing foam.
Re the blower door test, I would do it after you have done all the exterior shell work and before you close up the walls (if you are opening them). My assumption would be that taped foam is going to be very effective at air-sealing, and that some canned foam will get you the rest of the way. I would also be paying close attention to air-sealing the attic/ceiling areas. Once you are tight, you run the BD to determine if there are still bypasses so you can fix them, and also to determine whether you have enough ventilation. You may find you need to add mechanical ventilation in order to maintain IAQ. Once you have that foam up you need to control interior humidity carefully.
I met a guy from Bainbridge in BPI training a few months ago. He has a blower door and the other tools. Rick Blumenthal. I have his email address somewhere around here if you want to ask him about the testing.
Since starting this discussion, I cut a hole into one of my walls to verify the existing insulation and look for any water infiltration. There is fiberglass with the reflective paper layer toward the drywall. It is in good condition. I saw no water damage of any kind. My test hole is right below windows on the south-west corner of the house... a place for wind blown rain.
Does the presence of the foil covered fiberglass rule out the use of polyiso or XPS in favor of the more permeable EPS?
I've seen the Blumenthal name on construction trucks here on the island. I believe that they are one of the premier builders. I'll certainly consider them for the blower test.
I'm going to try to not open too many walls. As I said earlier on, the drywall is in good condition. It's even smooth wall as I like it. This being an older (1978 vintage) home, perhaps there's enough layers of built up paint to act as an effective air barrier? Or not. I suspect that I'll empty more than a couple of cans of spray foam before I'm done with this.
Actually, 1978 isn't that old, and there may not be more than 2-3 coats of latex on the walls. Older homes (prior to around 1960) are more likely to have a layer or three of oil paint on the walls, and that makes a more effective vapor barrier.
But paint creates only a vapor barrier, not an air barrier (which the drywall creates with or without paint). The problem is that drywall is rarely fully effective because of air leaks at the bottom edges, around windows, etc.
Lots of controversy as to whether you should or shouldn't have the dreaded "multiple vapor barriers". This is where detailed specifics about the local climate can be critical.
I agree with Dan
the presence of the foil FG could be considered a problem in terms of drying potential to the inside, if you foam the outside. If it's face-stapled continuously over the entire wall then I would be concerned. I understand your desire not to remove all the drywall, but if you're really going for it you could get a lot of benefit out of opening the walls, removing the FG, performing air sealing, and then using dense-packed cellulose before replacing the drywall.
Everything is always more work than you want it to be. And more money.
Well, actually I don't agree... or disagree. I'm fairly agnostic with regard to the "double vapor barrier" issue. Depends on hairy details of construction and climate.
David & Dan
OK... I was
David & Dan
OK... I was really just kidding re the paint as air barrier. I do know better than that... really!
About the foil facing: I was actually disappointed to find it there tho not surprised. I worry that it may inhibit internal drying even though it is usually mocked as an actual vapor barrier. One of its original selling points. But when I realistically look at this project and all the places I'll be working on the outside walls anyway, perhaps removing just a few more square feet of drywall won't be so much extra work. And I like the idea of using dense pack cellulose.
Know of any good cellulose blower guys down my way?