Doing an OSB sips build, spoke to several manufacturers, metal roof, some say use 30lb paper, others say just the rain screen another said furring strips.

Personally I like what read about this mesh looking rain screen.

then of course what use under the siding, I'm going to use some corrugated and some hardie.

Any experience or suggestions?

Posted By gosolar on 05 May 2015 04:06 PM
Doing an OSB sips build, spoke to several manufacturers, metal roof, some say use 30lb paper, others say just the rain screen another said furring strips.

Personally I like what read about this mesh looking rain screen.

then of course what use under the siding, I'm going to use some corrugated and some hardie.

Any experience or suggestions?

BEST practice is to use a furred out roof and create an air vent channel. See pic:




If you place the metal roof directly onto the SIP roof you created a vapor impermeable layer and when the SIP skin gets wet, it cannot dry to the exterior since the metal roof prevents that. You will develop SIP rot and eventually roof leaks.

The 30# paper is a BAD idea because it will saturate and let water through. You are better off using an engineered product like SIGA Majcoat that will not absorb or let water through but it is still vapor open. Whatever $ you save by using paper will be lost tenfold when your roof fails.

Posted By gosolar on 05 May 2015 04:06 PM
Doing an OSB sips build, spoke to several manufacturers, metal roof, some say use 30lb paper, others say just the rain screen another said furring strips.

Personally I like what read about this mesh looking rain screen.

then of course what use under the siding, I'm going to use some corrugated and some hardie.

Any experience or suggestions?

As far as the walls go. Same approach. You MUST create a furring channel to allow for air movement. Create the furring channel and then place the cladding.

Use a vapor permeable but water resistant WRB. See pic:

Your question really can't be answered properly unless we know two things, and you've only told us one and a half.
Where are you building? without knowing the climate zone, it's unwise to specify a design that's appropriate for both performance and durability. Secondly, what type of cladding are you installing? You stated corrugated and Hardie. If the corrugated metal roofing is a full and continuous corrugation, you have a self ventilating roof cladding and you probably won't need to install a furred roof. A self ventilating roof can be installed directly over 30# felt and it will perform wonderfully! This material has a proven track record and it is vapor open and will further open when it gets wet. This is a fantastic property of 30# felt and it's priced right. Again, my final recommendation will not be offered until I know the answer to the two questions!
Hardie comes in two main styles. Plank and panel. I would never use panel as it can create problems unless it is vigorously back-vented (Also referred to as a rain-screen). The Hardie material is cement and that makes it a reservoir cladding. This kind of cladding must address vapor drive issues that result when physics and the sun's warmth conspire to push moisture into your wall assembly.
In closing, I need more information. For instance, if your building in Phoenix, back ventilation and the lot are a waste of time and money.
Al Cobb

I like 30# felt with the details that lbear shows (in the Pacific Northwest). I used wood furring strips under my Hardie siding but after doing so I found a metal furring strip on line for a reasonable price and I wish that I would have used that. I think I posted something in regards to that in 2010.

Nw Ga just north enough in to zone 4, rain is about 5" annual.

Metal standard or common roof. rib type not standing seam

Metal siding is corrugated the wavy type

• Plain galvanized
• Corrugated panels have a rib height of 1/2-in high

Posted By acobb on 05 May 2015 06:57 PM

If the corrugated metal roofing is a full and continuous corrugation, you have a self ventilating roof cladding and you probably won't need to install a furred roof. A self ventilating roof can be installed directly over 30# felt and it will perform wonderfully! This material has a proven track record and it is vapor open and will further open when it gets wet. This is a fantastic property of 30# felt and it's priced right.

In closing, I need more information. For instance, if your building in Phoenix, back ventilation and the lot are a waste of time and money.
Al Cobb

Metal roofs are non-permeable, they don't breathe. The OP never mentioned a corrugated ROOF.

The roof buildup you mentioned that has a "track record" does not have a track record when it comes to SIPs. Building paper is "old school" and sure it works on older buildings but we are talking 1950's technology here and building science has advanced since then. In older buildings when OSB sheathing was covered with building paper (actually asphalt back then), the assembly could dry both ways (to the interior and exterior). SIPs are one way drying and that is to the exterior. So when the exterior SIP skin gets wet, it cannot dry to the interior. Even more so on polyurethane SIPs or polystyrene SIP of greater than 6".

Building paper is inexpensive but it has it's drawbacks and there are way better WRB out there. 30# is really "30 minute" water resistant. It does and will let water past it, especially in wind drive rain. Building Paper rips easily. Ever see what happens to building paper in extreme heat? It falls apart and dry rots.

You mentioned Phoenix and back ventilation being a "waste of time and money" and I, along with the experts in the area would strongly disagree with your statement. Phoenix experiences a monsoon season in which there is about 3-4 months in the summer (July through October) where humidity levels are high and rain events are frequent. Rain rates of 2.00 inches per hour are common. An associate of mine works in the Phoenix area and his company basically goes around fixing wall and roof leaks and wall sheathing rot because they used building paper and failed to back vent. Many of many lawsuits out there against builders who failed to properly design a wall build-up.

I leave with these quotes from the scientists out there:

Building Science expert Joe Lstiburek states, “I’ve seen building paper rot, even if you have two layers,” building scientist Joe Lstiburek.

Wesley Page, a retired waterproofing consultant from Novato, Calif., Grade D paper cannot withstand repeated wetting. “Grade D building paper will fail completely if it gets wet,” said Page. “It just disintegrates and disappears.”

All experts agree that any paper or felt will be less likely to rot if it is installed behind an air space that permits drainage and speeds drying.

“Everyone seems to think of building paper as a moisture barrier,” said George Tsongas, a former professor of mechanical engineering at Portland State University. “In fact, they are not moisture barriers. If you get any significant amount of water behind the siding, the building paper will not hold back water — not even 15-pound felt. All the papers will allow liquid water to go through them in one day.”

note

Not a corrugated roof.

corrugated siding

Why would the furring need to be a 2 x 4? If the object is to create an air channel, couldn't a 1x accomplish that? Based on this discussion by a leading expert ( http://www.buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh ) a gap as small as 3/8" might be sufficient.

The two main issues I see at play here in regards to a furred out roof are: structural/holding power of the strapping, and then the air gap, which is the suspenders to the top layer of roofing belt.

A reduced air gap would be less costly in materials for both the roof build up and also fascia trim. Any real downside to not going with something smaller than a 2x strap?

I read something like that about the airspace, 1/4 - 3/8, but that's about what the rain screen material is.

Another idea is not to use any type of wood strap, it will absorb water perhaps rot, not treatable for termites.

A plastic strap if one could be found may work as well,

I'm trying to get info from the rain screen manufacturers

Posted By 3cityblue on 06 May 2015 12:11 PM
Why would the furring need to be a 2 x 4? If the object is to create an air channel, couldn't a 1x accomplish that?

The two main issues I see at play here in regards to a furred out roof are: structural/holding power of the strapping, and then the air gap, which is the suspenders to the top layer of roofing belt.

A reduced air gap would be less costly in materials for both the roof build up and also fascia trim. Any real downside to not going with something smaller than a 2x strap?

If you used a 1x there would be NO WAY to hold down the roof sheathing. Not enough hold down strength in a 1x.

just screw right thru it

Posted By gosolar on 06 May 2015 01:37 PM
I read something like that about the airspace, 1/4 - 3/8, but that's about what the rain screen material is.

Another idea is not to use any type of wood strap, it will absorb water perhaps rot, not treatable for termites.

A plastic strap if one could be found may work as well,

I'm trying to get info from the rain screen manufacturers

Plastic strap would not work for a roof because it wouldn't hold the roof down.

A wood furring strip would not rot because it is part of the venting channel so there is always airflow in that area. So when it got wet, it would dry.

As far as the gap of air space goes. A 1/4" is pretty tiny and it wouldn't take much to block the channel and the drying capability would be very, very slow. I wouldn't go below 1" for the air channel on a roof and below 1/2" for a wall.

The vent space for a SIP roof and SIP wall is a detail that needs to be done correctly or the results can result in a catastrophic failure of the SIP walls and SIP roof. I don't understand why one would want to skimp and cut corners on the wall and roof vent channels? If this isn't done right you will have rot and damage to your SIPs. One cannot fix a SIP in the field by just replacing the sheathing. It doesn't work that way.

It's like building a supercar that will go 200mph and then cutting corners on the brakes.

You are spending a lot of $$$ on a SIP home, why cut corners on something that can be the downfall of the entire structure? If you do the venting properly with the appropriate gaps, a proper WRB and detail, you will have a structure that will be standing 100 years from now. You cut corners, skimp on the WRB and details, you will have a structure that will be failing 10+ years from now.

Installing a 1/4" gap, cheap building paper and using 1x's or plastic is not a proper build-up.

By-the-way, a 1x will actually be .75 or .50 inches tall. Just like a 2x4 is only 1.5" x 3.5" in diameter.

"If you used a 1x there would be NO WAY to hold down the roof sheathing. Not enough hold down strength in a 1x. "

It seems to me that the weak link in the connections is the strapping to the SIP OSB. I know some of the strapping will hit on the SIP panel connection, but at least half or more will only be attached mid-panel, depending on strapping spacing and panel width.

If a 2x is deemed to be adequately attached at that interface, why wouldn't a 1x also be deemed adequate? The second layer of sheathing (I would use plywood, not OSB) attached to the strapping would only need a closer fastener spacing or longer fasteners to pass muster, IMHO.

I don't see it as a matter of cutting corners, I see it as a deterrent to folks using SIPS. Every additional step or detail needed to address every perceived shortcoming of a SIP panel is an added cost and detrimental to folks (or contractors) adopting the system. Hard enough to get folks to look past and pay for better than 2x4 construction with batt insulation.

If a SIP system has so many caveats and additional "but then you need this and then you need that", I don't see how it will ever reach critical mass.

I don't see why you think it's the strapping that holds the metal to roof, the strapping is merely a spacer in my mind.

For argument sake, the strapping can be lose just laying there for the air space, the metal gets screwed thru say felt paper or another breathable barrier into the SIPS osb skin.

Have to disagree to that gosolar.  The uplift in a wind storm would probably rip off the extra layer of plywood along with the metal.  You would still have the SIP panel in place but all the rest would be gone exposing the SIP OSB to the elements.  OSB is a notoriously poor fastener holder.

I never did or would consider a layer of plywood on this job.

OSB with the correct fasteners can hold a metal roof.

The whole house is put together with screws in OSB.

I found these three products.

http://www.vaproshield.com/products/wrapshield  for siding

for roof

http://www.vaproshield.com/products/slopeshield

Plastic battens for siding
http://www.vaproshield.com/products/rain-screen-components/vaprobatten


What about these products

Posted By 3cityblue on 06 May 2015 03:25 PM

If a 2x is deemed to be adequately attached at that interface, why wouldn't a 1x also be deemed adequate? The second layer of sheathing (I would use plywood, not OSB) attached to the strapping would only need a closer fastener spacing or longer fasteners to pass muster, IMHO.

I don't see it as a matter of cutting corners, I see it as a deterrent to folks using SIPS. Every additional step or detail needed to address every perceived shortcoming of a SIP panel is an added cost and detrimental to folks (or contractors) adopting the system. Hard enough to get folks to look past and pay for better than 2x4 construction with batt insulation.

If a SIP system has so many caveats and additional "but then you need this and then you need that", I don't see how it will ever reach critical mass.

A 2x4 is 1.5" x 3.5" dimensionally true. A 1x2 is .75" x 1.5"
A 1x will split quite easily if you run a nail gun through it or overdrive a screw. A 2x4 is just $3.00 and a 1x2 is $6.00 per board.

I go back to my original point which is people think they are saving $$ but in the end they end up paying more $$ and coming out with an inferior product and result. If venting the SIPs properly is seen as a "deterrent" then the person should either:

A - Don't build with SIPs
B - Don't build at all

Each wall and roof system has it's pros and cons. No matter which method you choose; SIPs, ICF, CMU, SCIPS, wood frame, etc., if you cut corners, your home will suffer the consequences.

Wood frame walls should also have vented channels to protect the sheathing from rot. If you clad the home with siding or stucco and fail to leave a gap between the exterior cladding and the sheathing, you will have problems later on. Not as bad as with SIPs since the wood frame sheathing can still somewhat dry to the interior (most of the time). The roof on a standard truss roof home doesn't need the roof furring because it can dry to the interior via the attic. Unless you spray foam the attic ceiling, then that is another topic altogether.

As far as plywood vs OSB goes. AdvanTech OSB outperforms plywood by leaps and bounds. Typical standard OSB, even Exposure 1, is not on the same page as AdvanTech OSB. You can leave AdvanTech OSB outside in the rain for a year without any edge swelling or delamination. Try that with plywood and it will peel back in layers and rot.

Posted By gosolar on 06 May 2015 04:29 PM

What about these products

The plastic battens for the siding seems doable although I have personally never seen it used.

The VaproShield for the roof is something I have not seen and I don't understand how this would work on a SIP roof that has a standing seam metal roof on top. The laws of physics do not change and how a vapor IMPERMEABLE roof (metal) when attached directly to a SIP with a membrane sandwiched in-between will somehow let the OSB exterior skin dry to the exterior is beyond me. It defies the laws of physics.

I know you are trying all ways to avoid doing a proper vent channel on the roof but there is no way around the reality that if you want to create a "cold roof" vent channel above the SIP, you have to install furring strips and use the detail that I uploaded before.




Even the Building Science corporation created a similar detail:


SIP ROOF DETAIL - BUILDING SCIENCE