Hey folks, My project is rolling along. I have one thing im struggling with. I have a siding crew showing up on monday. I have discussed with the salesman placing 1/4 inch foam ripped 2 inchs wide at each joint and everywhere it is nailed leaving a airgap. I fear this is going to become a cluster and possible compromise the quality of the install. I am balancing the risk of problems with the foam against problems with water behind the siding. We have been VERY careful with our tyvek and window flashing. The house has 22 inch roof overhangs and no matter what we are useing battens under the metal roof. What would you do if it were your house. Any help or opinions appreciated. thanks michael

Depends on how much rain you get, how big your roof overhangs are, etc. If you live in a windy high desert with few rainy days, and rapid drying there's no issue to speak of. But for the rest of us... well...

Fiber cement board can take on a lot of water in a driving rain, and when the sun hits it the vapor drives are huge- an air gap allows the vapor to end up in the gap air instead of the skin of the SIP. Also the odds of saturating moisture spots occuring between flush-mounted siding an the housewrap is significant, which will create wet-spots on the skin of the SIP. For the housewrap to function as a drain-plane, there needs to be a gap. 9-10mm or 3/8" is usually the minimum for rainscreen gaps, but 6mm or 1/4" is way better than flush.

Foam shouldn't be used as a rainscreen spacer in general, but especially with the heft of fiber-cement siding. Foam isn't mechanically strong enough from a compressive strength point of view. Use wooden furring strips or 3/8" OSB/plywood ripped to 2" width, screwed to the SIP at a reasonable spacing for fastening the siding to the furring. (Use Hardie-tolerant screw materials for fastening the furring to the SIPs.) With this method you end up with fewer holes in the housewrap too.

From a "...if it were your house" point of view, that's what I'd do.With 22"' roof overhangs on all sides, if you live in a low-wind area and can control splash-back of eave-drippings from wetting the siding you may do just fine without it, but the upfront cost of a 3/8" rain screen is cheap insurance against a much more expensive "what if" series of repairs. (It gives you another couple of R in average thermal performance too.)

I agree with Dana1.  The only thing that I would consider doing different is to use 3/4" thick x 1.5" wide furring strips.   This size of furring strip is more readily available and does not require ripping.  In the past I have seen furring strips like this for sale at either Lowes or Home Depot.  If you want to use something thinner you may have to pay more for plywood or osb and then rip it.

I like to use the furring strips to build out around the openings in the wall before the windows and doors are installed.  With this system one has to be careful not to trap water on the horizontal pieces above and below openings in the wall.

If your windows & doors are already installed then be aware of how much you cover up the window or door frame when you install the furring strips.

Michael,

Nice to hear you are coming along with your house! I would like to visit again in a couple months to see how it looks!

I agree with Dana and Alton. Use the 3/4 x 1.5 furring strips and make sure the sub contractor follows the installation guide by Hardie. Make absolutely certain they follow everything to perfection. If you ever have a problem the manufacturer will look to see if their installation guide was followed; if not, you will be on your own!

Make sure no butt ends of the siding are on top of a SIP joint; otherwise it could telegraph and cause a slight "peak" at the joint. Won't look bad until the job is done and then it will stick out boldly!

"If it was mine" I would watch every detail with a copy of the installation instructions in my hand! I read most of the installation instructions on line and one detail I did not come across yet is: all raw ends of cuts should be painted (or sealed) with a matching paint from Hardie. We have done this very successfully with a small container with a cotton rag in the bottom kept wet with the paint finish and dipped each cut end in the container and just blotted the excess so it would not run. In my experience it is extrodinarilly important to seal all the exposed cuts and MAKE SURE THE NAILING IS DONE EXACTLY AS SHOWN IN THE INSTALLATION INSTRUCTIONS!!!!

Deatails, details, details!!!

Best to you,
Rich Melius

Thank very much for the thoughtful replies. My biggest concern is a good install over battens. Thanks again. Michael

Do any opinions change on the need for an air gap or furring strips under the Hardie siding if the SIP skin is steel rather than OSB?

Jelly,

My understanding is that the furring strips help preserve the fibercement planks by not allowing water to trap in behind the fc planks.  In other words, I think it is best to use furring strips so the backside of the fibercement planks will have a chance to dry.

An air gap is very important on any type of cladding. There are many new types of rain screen products on the market today to address moisture management issues when cladding.
I just read a report about the moisture management industry growth through 2030. The codes hopefully will be changing to require an air gap behind all types of cladding as they have in Canada and Europe. The new 703.1 WRB code requires a "Means of Drainage" so you will start to see many more products come out on the market.

Below is the report.

http://www.nemionline.org/downloads/MoistureMgmtReport.pdf

Does this still apply?

R703.1.1 Exterior Wall Envelope. ...
Exceptions:
1. A space is not required where the exterior veneer is installed over a water-resistive
barrier

Posted By Jelly on 27 Mar 2010 09:28 AM
Do any opinions change on the need for an air gap or furring strips under the Hardie siding if the SIP skin is steel rather than OSB?

Not needed, I would be more concerned with the unsupported space behind the siding

Posted By Cladding Guy on 27 Mar 2010 11:10 AM
An air gap is very important on any type of cladding. There are many new types of rain screen products on the market today to address moisture management issues when cladding.
I just read a report about the moisture management industry growth through 2030. The codes hopefully will be changing to require an air gap behind all types of cladding as they have in Canada and Europe. The new 703.1 WRB code requires a "Means of Drainage" so you will start to see many more products come out on the market.

Below is the report.

http://www.nemionline.org/downloads/MoistureMgmtReport.pdf

R 703.2 ..............states that 15# felt meets that requirment

I again remind you to not use OSB, you can/will experience swelling in its unprotected state which will telegraph through siding. OSB has enormous swelling capacity, I have a chunk of 1/2" that is over 1" thick in my shop to demonstrate this fact. If it does swell it will not be uniform and as a result give your walls a wave when looking down the plane of a wall.

Tomorow morning we are going to start ripping down 1/4 inch plywood and installing it on the house. 2 inchs in the field and wider on the corner for corner boards and around the windows for the window trim. Any Advice on install would be great . Thanks for the help. Also if anyone could tell me how to add photos to posts i would be greatful. The new system has me baffled. Thanks michael

Posted By Brawler on 28 Mar 2010 03:07 PM
Tomorow morning we are going to start ripping down 1/4 inch plywood and installing it on the house. 2 inchs in the field and wider on the corner for corner boards and around the windows for the window trim. Any Advice on install would be great . Thanks for the help. Also if anyone could tell me how to add photos to posts i would be greatful. The new system has me baffled. Thanks michael

it might be a good idea to call the Hardie tech support line and get their "expert" advice , as they are warrantying your siding. Sometimes we try to reinvent the wheel and it comes back to bite us.

I still maintain it is not necessary as it is not shown on the hardie installation

Jelly,

My own house has Hardie Plank directly applied to steel SIPs. The install is over 10 years old and in Florida and there are no problems.

Posted By Cladding Guy on 27 Mar 2010 11:10 AM
An air gap is very important on any type of cladding. There are many new types of rain screen products on the market today to address moisture management issues when cladding.
I just read a report about the moisture management industry growth through 2030. The codes hopefully will be changing to require an air gap behind all types of cladding as they have in Canada and Europe. The new 703.1 WRB code requires a "Means of Drainage" so you will start to see many more products come out on the market.

Below is the report.

http://www.nemionline.org/downloads/MoistureMgmtReport.pdf

Cladding Guy;

I took a cursory look at the 217 page report that appears to be funded by govt. as many reports are generated by grants.

The residential codes have  required moisture management under claddings for a long time, an air gap is not required by code or the siding manufacturer.

The assumption that it will someday be required be code is speculation, if the air gap was important it would be noted as part of the installation instructions

I am not as concerned with protecting the siding as i am with protecting the structure itself. $100 in materials and hopefully it will take me and two carpenters less than a day to complete. I'll report back.

Posted By Brawler on 28 Mar 2010 10:09 PM
I am not as concerned with protecting the siding as i am with protecting the structure itself. $100 in materials and hopefully it will take me and two carpenters less than a day to complete. I'll report back.

I think we all understand your objective, but it is a little more prudent to take care in properly protecting your sheathing with a correctly installed moisture barrier.
What you are doing is protecting your sheathing, then applying a 1/4" bare wood furring that in unprotected from rot because it lies on the wrong side of the moisture barrier all the while adding more fastener penetrations in the process of installing an un- approved siding install. I believe you are shooting yourself in the foot with the future potential of the furring strips rotting away and then leaving an unsupported 1/4" gap behind siding.
The "report back" should be in 10 years to prove my theory.
It is great to think outside the box or we would not subscribe to this forum, sometimes we dwell too much on "non-issue" problems and create new ones.

Agree - Hardie installation instructions show no furring strips with OSB or foam and in the case of concrete block and furring strips, the wrap is outside the furring strips. Not sure I'd innovate on this.

Here is a great article by Joseph Lstiburek (Building Scientist) about gaps behind cladding:
http://www.buildingscience.com/docu...the-gap-eh