More Energy-Smart Details
Building codes are moving towards the use of continuous exterior insulation and drainable rainscreen systems. Moving beyond drainable rainscreens, vented rainscreens can add even more durability to a wall system. These are all topics that have been discussed many times here on GBA; the links below provide more information. Wood shingles (and vertical cladding systems) add a wrinkle to these assemblies and will be the topic for today’s Energy Smart Details.
- Continuous Insulation, Part 1 – GreenBuildingAdvisor
- Continuous Insulation, Part 2 – GreenBuildingAdvisor
- All About Rainscreens – GreenBuildingAdvisor
When installing wood shingles or vertical cladding systems on a rainscreen over continuous insulation, the biggest challenge is having something for the cladding system to attach to. When shingles are installed directly over sheathing, the sheathing provides a nailer wherever the shingles land.
Unfortunately, once you add continuous insulation, attaching the shingles to the sheathing becomes ineffective. Nails cannot provide enough strength to secure the shingles through the insulation. You must provide another nail base, outward of the insulation. Unlike common siding materials, which have long lengths running horizontally, vertical furring strips don’t provide nailing where it is needed for shingles or vertical cladding systems.
To address this, you have two main options that could provide this secondary nail base. The first is either to install a second layer of sheathing over the exterior insulation or to use a nailbase panel, which combines the insulation and sheathing in one product. (Huber’s Zip R-Sheathing is a readily available nailbase panel.) This option provides a surface that offers nailing pretty much anywhere and allows the rest of the work to continue almost exactly as if there were no continuous insulation layer. After you install and detail the WRB, windows, and flashings as usual, a woven rainscreen product can be used to create the rainscreen gap behind the…
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10 Comments
This kind of wood strapping grid is probably what I'll create on my house after adding exterior insulation as part of my energy retrofit.
One thing that concerns me is the strength of the connection of the furring strips + cladding to the house. It seems like a system that would get ripped off of a house pretty quickly in high winds. I don't live in an area with lots of high winds, but in the Washington DC area we do occasionally get the tail end of hurricanes after they've made landfall a few hundred miles away and weakened, and we do occasionally/rarely get kind of "small" tornadoes that can damage properties. Basically, wind is usually not a problem, but the possibility of high winds does exist. It's a low risk, but not a non-zero risk. Any recommendations for wind-resistant rainscreens? Maybe the answer is that wood shingle siding is by definition not built for high winds, but the furring strips also make me wonder. The horizontal strips are connected only in a small number of places, and then there is airflow from top to bottom, which I imagine could generate a lot of uplift when subjected to high winds.
Maybe adding wood glue at all the connection points would be one way to go, though obviously that would increase the labor time and the cost.
And from what I understand, screws provide more tension strength, but are weaker than nails in shear strength. Maybe it would be a good idea to use some of each?
Maybe all this is overkill, but I'm just trying to think through the best way to create a robust structure that can last a good 100 years. The house has already lasted 100 years, and I want it to last another 100!
Different question: would you recommend dimensional lumber for the furring strips (maybe 1x4, for example), or ripped plywood? I've seen both methods, and maybe the answer is that either method is fine, but is one better than the other in terms of reduced splitting during nailing, or in drying potential or dimensional stability, ease of nailing the shingles, or some other factor? I'm guessing that each method has advantages and disadvantages, but I'm curious to hear if there are recommendations.
Paul,
I'd say the rule of thumb is that plywood strips work best when installed on a solid substrate (as the first layer against foam or sheathing), but you are better off going with the rigidity of solid lumber if they are spanning between attachment points.
Very nice article. There's one more option I think it would be appropriate to mention.
Shingles can be bought "panelized" in horizontal strips. These can then be attached over a single layer of vertical furring, just like lap siding.
Here are a few links. I have no commercial connection with any of these companies.
https://www.vintagewoodworks.com/cedpan.html
https://ecoshel.com/ecoshel-products.html#wrcshingle
http://shakertown.com/cedar-products/cedar-cove-shingle/
This one has the vertical furring incorporated into the panel.
https://premierforestproducts.com/pages/cedar-shingle-panels.html
JGSG,
That last one is really interesting!
I've never used penalized cedar shingles, but have installed a fair amount of Hardi-shingles, and they go on really fast.
“[Deleted]”
Great article, Grant! I would note 2 things. I realize you don't specify the type of exterior insulation, but (as happens with drawings) the pink exterior insulation makes one's brain think XPS. All fine and good if it is thick enough, but if one is using a rigid plastic (XPS/EPS/Polyiso) it is a really good idea to use a bumpy membrane behind it to allow any incidental water to drain away. This isn't an issue with products like rockwool that water can drain through. Water can get hung up in between these layers and the hydrostatic pressure can drive water into the wall system.
The other is to note that one issue with the cross strapped rainscreen systems is fire. Anything over 3/4" gap can lead to fire spreading, so those in fire prone areas or the WUI may want to consider the 3/8" mesh products.
There are always trade-offs. Energy performance versus fire safety and wind safety. It would be more convenient if all these needs were perfectly in alignment, but they’re not.
Exterior walls that face the sun in the hot seasons could benefit from essentially as large a gap as possible, to provide shade and maximum airflow. I mean, as a thought experiment, imagine a gap of a foot or two feet, or some other large number like that. The airflow would be great, and the heat from the sun wouldn’t affect those walls much.
But that same airflow could spread fires or cause damaging uplift in extremes winds.
If the insulation is mineral wool and the criss cross furring strips are painted with intumescent paint, that might be enough to mitigate the fire risk.
The wind risk seems a little harder to address.
Paul,
"There are always trade-offs. Energy performance versus fire safety and wind safety."
True, but the point Josh was making is that fire spread in a cavity becomes a problem at a specific width, which from Joe Lstiburek's advice is 3/4".
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