Best Type of Furring Strip for Vertical Metal Cladding
Hello, my wife and I are planning on building a custom steel home this year. Our wall assembly (From outside to inside) consists of; 24 guage metal cladding with hidden fasteners installed vertically, furring strips installed over 1″ XPS with taped and sealed joints (…to act as a rainscreen and air gap) both directly to the steel wail girts and/or steel framing members, (The XPS will be fastened with Trufast Thermal Prong washers), the wall cavity will be filled with 8″ mineral wool insulation without a vapor retarder (…due to the hot and humid climate in Houston TX), 1/2″ painted drywall.
We are trying to determine which type of furring strips would be the most economical and functional for this type of application. We see that most furring strips are installed horizontally when installing vertical siding; however, this seems to cause a water dam effect in a way. Wouldn’t it make more sense to use a furring strip that is vented? And then, if it is vented, which type of strip would be best so that you don’t have a compression issue when you install the metal siding?
Any advice on this subject would be greatly appreciated.
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Depending on your metal siding panels you may already have corrugations that provide a space for drainage. For furring you can rip 1/2" plywood or similar at an angle to drain any water out towards the siding corrugations, though that's possibly not even necessary as a flat surface will mostly drain. If you panels are totally flat between seams, you may want to install it with a spacer behind the center of the panel to bow them slightly to control any oil canning that might occur in heat. This could also create a drain/vent gap from the furring. Some installers use backer rod down the center of the panel for the oil can control.
Some will say if you are drain bulk water from behind your siding you have really screwed up on your flashing details elsewhere, so there's little reason for complicated solutions other than allowing for airflow to take care of condensation. I do think this problem should be though of more as trying to provide ventilation rather than drainage to the rainscreen.
Hi Tim, and thank you for your input. You bring up some good points and suggestions.
I have attached our chosen panel profile that shows the rib openings every 2 2/3". I agree with you regarding bulk water issue, and this approach would mostly be for airflow/ventilation. So, is there a reason you would suggest the wood furring strips over high density plastic ones? Also, I am interested in learning more about your recommendation of using a 'spacer behind the center of the panel'. While oil canning can occur with extreme weathering, it can also happen when there is other issues present when there is enough of a change that it puts tension or stress on the metal panels. While we are concerned about the parts of the flat panels sitting directly on top of the XPS and/or the furring strips, we are also concerned with oil canning.
I don't have any engineering reasons for wood over plastic, it is just what I am familar with. I used 1/2" PT plywood for my furring strips for vertical rough cut shiplap siding. I suppose I'd rather make PT sawdust than plastic chips in my yard.
I was imagining you were using a more traditional standing seam type panel, with a wide, flat section, when I suggested the backer rod. I do not think a corrugated panel would need any oil canning mitigation, since they can "accordion" as they grow and shrink, but that is my armchair opinion. The backer rod technique is recommended by some roofing manufacturers to control oil canning. The idea is it puts a slight arc through the whole length panel, directing any thermal growth into increasing the radius of arc rather than getting bound up from starting flat and expanding irregularly. FHB #272, "Fast Path to a Standing Seam Roof" mentions this technique.
That profile should provide all the ventilation/ drainage you need! I would just detail the bottom edge in a way to keep out critters but still allow ventilation/drainage. Perhaps Coravent in the the appropriate thickness across the entire bottom edge, with the manufacturer's closure strip to seal the corrugations. You could have your solid furring strip just above the coravent to use to attach your panel clips.
Some info FYI: https://rdh.com/wp-content/uploads/2017/07/TB-11-Cladding-Attachment-Solutions-Web-Reduced.pdf
Tim has given you good advice.
While plastic furring strips like Cor-A-Vent would provide a better drainage path, in your climate and with the profile of the panels you will be using, plywood or lumber will work fine. They also have the added advantage of providing a solid surface for cladding fasteners, precluding the need to screw back into the sheathing.
For the bottom I'd use a perforated J flashing that covered the whole depth of the panels. That will keep out pests but allow both ventilation and drainage. Something like this: https://www.menzies-metal.com/metal-flashings/perforated-j-channel-rain-screen-low-back/
Thank you gentlemen for the fantastic advice and information. In regards to installing the metal cladding panels directly onto the XPS, it is only rated for 165 degrees. We chose a matte black finish and we are not 100% sure that it's still a good idea to attach it directly to the XPS. Also, what about the flat parts of the back side of the panel that will be constantly compressed into the XPS...wouldn't that cause a degredation of the metal over time?
Lastly, Malcolm wouldn't the 24 gauge cladding panel 'have' to be attached to the actual wall girts and/or steel framing vs simply just wood furring strips for wind load reasons, etc?
Thank you all for your input!
In the article that Daniel presented, it seems as though they suggest to have minimal penetrations through the exterior insulation panels. I believe the point is that the penetrations will reduce the efficiency of the insulation over time and the R-value will decrease. We are using the XPS for our project as a inexpensive way to create rainscreen behind the cladding. We will be installing the XPS to the wall girts with the TruFast Thermal Grip Ci Prong Washer which will provide a good seal. However, any other penetrations made once the cladding goes up cannot be sealed. I would think if we get the flashing details right, then the little bit of water that makes it back there wont get through the through holes in the insulation panels. Thoughts?
There are a few siding products that require more that 3/4" fastner penetrations, but I've seen a metal panel that did - although it's worth checking the installation instructions of whatever you use. This provides a good overview of fastening rain-screen furring through exterior insulation starting on page nine: https://www.victoria.ca/assets/Departments/Planning~Development/Permits~Inspections/Example~Plans/Illustrated-Guide-R22-Effective-Walls-In-Wood-Frame-Construction.pdf
Thank you for your input Malcolm, and the very informative article. We found a product called Corrugated Lath Strip from MTI which looks promising in regards to function and cost. We plan to discuss this with our builder along with fastener lengths, detailing, etc.