Residing a house with black fiber board sheathing. Best practices?
Not sure what category this question should be under:
A friend of mine was about to replace all windows and deteriorating wood siding with Hardie or LP smart side in Colorado. Climate zone 5a. Obviously it’s a great time to add 2″ of XPS and a rain screen. But we discovered that the sheathing underneath is this black fiber board they used back in the day. Removing/replacing this fiber board is not really an option, as there’s loose cellulose in the studs (as a retrofit sometime ago) and the fiberboard would be beneficial to stay as a sound barrier (house next to busy road).
The simplest brute force solution would be to nail the thickest Zip R on top, through to studs, and then siding over the zip (with or without rain screen depending on budget and buildout thickness)
Or, can he just tyvek this fiberboard, and then foam, rain screen furring, then siding?
Or, foam directly on fiber board, tape the foam as the WRB, and then furring, siding?
Or do we accept the black tar layer (very compromised) as the WRB and just put the foam/rain screen/siding on top?
I guess the new windows will have to go on wood extension bucks anyway so the flange will just be taped to the new 2x lumber and not interfere with the fiberboard?
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I like this plan
“he just tyvek this fiberboard, and then foam, rain screen furring, then siding? “
Seems to me that CO is pretty much a dessert and wild fires could be a problem.
I see no benefit to a vented rain screen in the dessert it is more for people that live in a rain forest.
Vented rain screens and attics are very risky in wild fire country as they may allow embers inside the building.
Walta
blamus20,
I live in wildfire country Colorado and have to disagree with Walta100 about rain screen gaps being very risky. Just like the problem of low to no air flow when trying to vent roofs with less than 1 1/2" venting, rain screen gaps of 1/2 to 3/4" do not constitute a hazard from chimney effect behind the siding. I can't quickly find my references on this subject. A properly done rain screen should have insect screen top and bottom. Various methods exist, Cor-a-vent or BattenVent are two plastic ones that don't rot. Perforated metal J strips and other formats won't melt and might survive rodents better.
Embers tend to collect in corners of roof to roof or roof to shed dormer kinds of corners and of course in gutters. Gutters should be screened. Ember intrusion into attics is a problem that is greatest with end gable venting screen failure. Under eave venting is still a risk. Winds passing over the roof can create sufficient pressure drops that pull embers up. Depends a lot on the venting design. Just the same 1/8" screening is now called for for roof vents. ERV, dryer vent and combustion vents are now also to be covered against embers in my locality. I do have a problem with dry vent covers because of lint build up risk.
Back to your wall question. The tar coated fiber board makes me think of what we called beaver board back when I lived in that day. It has little to no structural value in my experience. Let in bracing was used to provide wracking resistance. How much resistance compared to modern code is a good question. When new, the tar coating at least kept the rain from soaking it long enough to get the siding on. Not sure if any claims were made about air or vapor resistance as no one thought like that then.
Zip R-12 would get you to a reasonably safe dry to interior wall if the stud cavity is 2x4. Structurally it is a question whether you are gaining with this choice, though taped off Zip would give you a better air sealing than you have now. Even in new construction this thickness is tricky to fasten to code and warranty requirements. Two inch Low GWP Foamular or foil-face polyiso insulation directly over the beaver board then taped off could be a possible choice. One by stock for rain gap or ripped 5/8" plywood strips (1/2" will be too wavy) then siding of choice.
Don't forget to cap the foam top and bottom against insect/rodent invasion. You might even find something that can provide the air vent with small enough holes. Plan carefully for all the new distance and how you plan to trim out the new windows. Plan on ripping 2x stock to add to the opening before the foam or if shrinking the openings you might be able to apply to the existing stud faces. Pitch your bottom pieces or add beveled siding if sizing allows.
For final fire protection. No vegetation of any kind with in 5 feet of house, restricted vegetation out to 30 ft (if you have that much room) and no combustible fencing attached to the house. Tempered glass for all windows and if you are doing the eaves over consider metal or other non combustible materials. No wood piles near the house and so on.
The road noise comment leads me to think you are in a more in-town context with fire hydrants near by, but as a Coloradan we are all going to find home insurance more costly and more difficult to acquire. Proactively working to reduce urban wildfire risk may well save your coverage future.
So you would prefer taped polyiso directly on top of this fiber board as opposed to tyvek (on fiberboard or on top of foam).
This is a house of my friends in the city of Boulder. No permits needed for residing. I do have a new construction project going on the the foothills (county) that I ended up doing 1/8" steel mesh screens for attic venting. Im worried that I now need a UL listed fire resistant vent. Fingers crossed the inspector finds regular mesh acceptable. On the plans they approved Hardie vented soffit, the ones with holes in the Hardie. But I switched to metal siding and changed the venting strategy. Everything on the exterior here has to be ignition resistant or class A.
onslow,
Joe Lstiburek discusses the risk (or lack of) of fire in rain-screens in this article.
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-098-great-fire-london
If you don't have a ventilated cavity (no opening a the top) and include a mesh barrier at the bottom, they don't appear to be much of a of a problem in wildfire areas.
Rain-screens have a number of functions. In dryer climates their primary one of providing a capillary break isn't as important, but the increased ability of the wall to dry the exterior helps everywhere.
blamus20,
The tyvek or similar does not provide any additional value in either location if you are using foil faced polyiso. If you are trying to use roofing polyiso with fiberglass faces, then maybe a bit of extra water resistance would be useful.
Taped foil faced polyiso will act as your water and air barrier for the wall field. Be sure to tape the window bucks carefully. Remember to plan your trim to siding and rain screen offsets now so you don't find a problem down the road.
You are actually in 5B according to the IECC chart for building. The wall you are creating is making it a dry to interior wall. If your friend is an aquarium type or raises lots of plants, he will do well to look at an ERV or HRV if not already in place.
Good luck with the other build. I would suggest erring on the side of over fire proofing though the insurance situation is very fluid now. Locally, we have a movement to set standards and certify wildfire resistance in an effort to keep insurers. The problem is a certain information aggregator that is clueless yet remains a major source of underwriting rating info.
I'm now wondering if taped EPS can also work. It's. Lot cheaper and sounds like over time, and at low temps, the R values become similar.
Home Depot has these R tech EPS sheets with a vapor barrier?
blamua20,
Why do you want the exterior foam to act as a vapour-barrier?
I guess not vapor barrier.
It was first tyvek under foam vs taping the foam as the WRB
Now I'm wondering if I can tape the foil faced EPS as the WRB.
The R tech EPS at home Depot with "metallic-reflective polymeric facer" And half the cost as polyiso for 2".
Your friend has shared a link to a Home Depot product they think you would be interested in seeing:
https://www.homedepot.com/p/R-Tech-2-in-x-48-in-x-8-ft-R-7-7-EPS-Rigid-Foam-Board-Insulation-310891/202532856
blamus20,
Ah - got it.
I've used that foam. It was pretty easy to tape. My one reservation is that the facing is pretty thin and insubstantial. I'd be comfortable taping the seams to stop incidental water from getting through, but for things like flashing over windows, which are typically lapped by WRB (and when tape get used instead it's to a solid surface like Zip sheathing), I don't know I'd trust just taping to the foam facing there.