Cover with house wrap or not?

[Savvy]

I am planning to reside and replace the windows my Tahoe cabin.




The plan is to remove all the battens, and resecure the existing paneling where it has become loose.
Bury the existing paneling under 1/2" OSB, taping all joints.
Bump out the window and door frames 2.5" using sistered 3" wide OSB furring strips.
Install 2" Rmax (foil faced) rigid foam board, taping all joints.

Here's the issue:
Rmax tech support states that no house wrap is needed if all joints are taped.
They say I should flash the windows directly to the foil backed board.
A contractor neighbor of the cabin, who has no horse in this race,  says to cover taped foam board with house wrap, tape joints and flash windows to the wrap.
An engineer friend says to wrap the house ontop of the OSB, flash the windows to the wrap, install the foam board, taping all joints.

Ultimately, the foam board will have 2" wide 1/2" thick vertical battens placed above the stud positions to create a drain scape, with pre-stained cedar shingle paneling as the final top siding.

The question:
Should I listen to the Rmax tech and leave out the wrap?
Listen to the contractor and wrap the foam board?
Listen to the engineer and wrap the OSB first?

Thanks in advance.
Cheers!!!

[MountainDon]

There probably is not just a single correct answer. 

But I have a question first.  What is the reason behind covering the existing siding with OSB before carrying on with foam and new siding? 


A few short weeks ago JLC posted an article on weather barriers.  That would be a good first read on the subject. It can be found at the end of this link.

Than a greenbuildingadvisor article from a few years ago also addresses the position of the housewrap   They take the type of windows into their advice. There is a link in that article to some window info.

Building Science is a gold mine of info on weather barriers and insulation. A couple of my bookmarks include this one and this one.   The climate zone where the project is located can also make a difference in the choices. Climate zone info



FWIW, we added 2.5 inches of foam to the exterior of our home a few years back. We are in a relatively dry climate, 8" annual precipitation.  We added the foam in 2 layers staggering the seams. The inner thickest layer is XPS foam with the outer layer being polyiso foam with silver foil facing outwards. We taped the seams in the outer layer. The foam and foil covered polyiso with taped seams is our weather barrier. If we were in a rainy climate I might have done that differently.  1x furring strips placed vertically over that, secured to the 16"OC studs with long structural grade screws. We used Fastenmasters HeadLOK screws. They have tables available that help determine how many are needed to carry the siding load.  Then the final horizontal lap siding was applied over the furring.


A wall like that will dry to the interior which is fine for our hot, dry and air conditioned location.

[Savvy]

Fair question and thanks for the reply, MountainDon.

The existing siding is only 3/8 inch. Though 95% sound, it is quite flimsy. It's the only thing on top of the studs. Placing new OSB gives, for my piece of mind anyway, a clean flat surface to begin with....not to mention something more secure to drive into if I miss a stud later down the process. Removing, has the probability of exposing more issues. Issues I'm more than happy to be unaware of .

Definitely installing "outie" windows.
Climate zone 5B (washoe cnty, nv).
I left out my details which include peel and stick graco membrane OSB to foundation and osb to deck ledger board, peel and stick around windows and doors.

My gut says to throw out my engineer friends recommendation.
It's between contractor and Rmax tech.
I'm leaning towards my neighbor contractor using the pretense, one can't be too protected, but I also don't want to just toss cash away.
I, too, at first was thinking 2 layers of 2" foam....but 5 inches of bump out seemed a bit extreme.

Cheers!!!

[NathanS]

Two layers of foam board, with joints offset and taped, would give you a better chance at a good air barrier if you are up for the extra labor.

I agree with Mountain Don to spend time going through buildingscience.com. They also have case studies on retrofitting houses with foam board.

You want to be careful to not trap moisture in your walls. Foil facing is a vapor barrier. Water vapor moves from warm to cold. If the condensing surface of the rigid foam board is not sufficiently warmed, the water vapor will condense here. This water in your walls can not dry to the outside because of the foil facing on the foam board. It can only dry to the inside (hopefully there is no poly in the way, you definitely would not want vapor barriers on the inside and outside walls).

In my opinion, your engineer friend gave you the best advice in putting the house wrap behind the foam board. The foam board will help to protect the house wrap from the elements, and if any rain ever gets passed the taped foam board, the wrap is there as a backup drainage plane.

As long as your current siding isn't rotten, I'd consider just putting the house wrap right on that, then installing the foam board in 2 layers with the joints offset and taped.

[Savvy]

I'm in Climate Zone 5.
The minimum R-value for exterior foam is R-5 for 2x4 walls.
I need 35 panels to cover 1 layer of foam around the house perimeter.
2" Rmax Rigid foam is R-13 and costs $32 each single sheet
1" Rmax Rigid foam is R-6 and costs $20 each single sheet
There is discount for purchasing more than 30 sheets.
One 2" layer has better R-value and 2/3rds the cost than two 1" layers.
So the question is....
What benefit is realized by doubling the layers?

Cheers!

[NathanS]

I want to add a disclaimer that all my knowledge about thermal/vapor/water/air barriers is from reading papers and books by Joe Lstiburek from building science. I'm not an engineer, and until I started researching the best practices on building houses 6 months ago, I didn't know much. I think I have a good understanding of the subject, but don't just take my word for it. For the price of one 2" rigid foam board you could buy his book on amazon and read it in a week.

Air leakage is a huge source of energy loss. I've read that 1/3 of your energy bill can be from leakage. By having multiple layers, you're more likely to actually create a good seal. That being said, if your roof is leaky, or sill plates, or windows, it doesn't really matter if the walls are air tight. You could try taping the building wrap and then doing 1 layer of rigid foam.

Air leakage also conveys the vast majority of moisture into walls. Kind of counter intuitive, but that is probably the greatest source of mold and rot in walls.

Do you know what is in your wall cavities right now? Also, if you have gypsum on the inside, what kind of paint? The reason that r-5 is the code minimum for exterior sheathing on 2x4 walls in zone 5, is because during the coldest month of the year, on average, the condensing surface of the foam board will be warm enough that water vapor will not condense on it.

Here are a couple cross sections of retrofits on houses... the third picture has a 'hot roof' and is a good way to end up with ice damming in zone 5 no matter how much insulation you pack in there... i'd ignore that part.


      

[Savvy]

Wow!
That's some detailed stuff.
Thanks!!
As for your questions.....
My roof is metal on top of 2x8 T/G pine that is exposed on the interior.
There is NO plan to insulate the roof anytime soon....probably ever.
There is fiberglass batten insulation in the wall cavity.
The interior walls are drywall of 2 layers.
First layer is unpainted and untaped 5/8 fire rated. (this layer WAS covered with 1/8" 70's walnut paneling).
In removing the paneling, some of the glue was left behind, as well as some of the paper tore.
This layer was covered with 1/4" sheetrock, that will be mudded and taped, and finished.
There is no paint....yet.
On the exterior of the studs is building paper with more than 1/4 but less than 3/8 cedar 4x8 vertical panel siding.
(birds have been able to peck holes in it)
Again.... this is a recreational second home that my multimillion dollar year round neighbors would like to see torn down.....but it suites my needs just fine.
I should also mention......
I have bear issues....
They like to live in my crawl space.
And if they can't get there......they try by peeling siding.

Cheers!!

[NathanS]

I went way overkill on you and didn't realize this is a vacation cabin. My head was thinking the only 'cost effective' time to add insulation is when you're re-siding.

Of your 3 choices, I think any of them would work. In general, house wrap is used in conjunction with foam board in areas that get lots of rainfall. Since the Sierra Nevadas are so dry, you'd probably be fine without it.

There must be existing bracing behind the current siding. I'm not sure what adding a layer of OSB would accomplish.

By the way, I camped on Blue Lakes Road just south of Lake Tahoe in Toiyabe Humboldt last fall. One of the most beautiful places I've ever been. Didn't get to see any bears, though.

[Savvy]

Curious question....
In this article, http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/window-installation-tips-deep-energy-retrofit
it states that they used 2 layers of 2"rigid foam.
It says to secure the 3/4" thick Dudley Boxes using long screws or nails.
There is a pic depicting this.

What Bostitch framing nailer shoots 5+" nails?

Cheers!!

[Savvy]

Nope, no bracing.
Going from most exterior.... 2"x 3/8 vertical battens on top of 1/4" 4x8 cedar panel, on top of builder paper, on top of 2x4 studs, filled with fiberglass battens, covered with 5/8" fire rated sheetrock, covered by 1/8" walnut paneling...which has been removed and replaced with 1/4 sheeetrock.
Aluminum framed single pane windows with some sort of thru n thru vent holes ( 2 near each corner.

The OSB will give me a rigid flat secure surface to nail to if I miss a stud along the way.
At least that's the story I'm sticking with.

Cheers!!!