Fastening Furring Strips to a Foam-Sheathed Wall
A screw manufacturer provides advice on how many screws you need
UPDATED March 1, 2012
If you’re building a house with foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , and your siding is installed over vertical rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. strapping installed on top of the foam, how should you attach the strapping? Most builders screw the strapping through the foam into the studs; so far, so good. But what length screws should you use? And how closely should you space the screws?
Answers to these questions can be found in a new technical bulletin published by FastenMaster, the company that manufactures Headlok screws. The bulletin, “Attaching Exterior Wall Covering Assemblies with Foam Sheathing to Wood Wall Framing,” is available from FastenMaster.
If you follow FastenMaster’s prescriptive recommendations, your furring strips will adequately support the weight of the siding and will secure the furring strips to the wall in a way that resists wind and seismic forces.
First, the basics:
- You have to use 1x4s, since 1x3s could split. (If you want thicker lumber to allow for more secure siding attachment, you might consider using 2x4s.)
- Fasteners used to secure furring strips must penetrate studs by at least 1½ inch.
- FastenMaster provides prescriptive advice for walls with foam up to 4 inches thick; if your wall has thicker foam, you’ll probably have to consult an engineer.
- FastenMaster’s recommendations work for Type II ESP, Type X XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation., Type 1 polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. , or for any foam with a density greater than one of these listed foams.
I have prepared two tables that incorporate FastenMaster’s recommendations. (My tables are more logical and easier to read than the table in the FastenMaster document, shown as Illustration 2, below.)
The first table below is for houses with studs spaced 16 in. on center; the second is for houses with studs spaced 24 in. on center.
Table 1: Fastener spacing for attaching furring strips if studs are 16 in. o.c.
|Foam thickness||Fastener spacing (assuming vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). siding)||Fastener spacing (assuming fiber-cement siding)||Fastener spacing (assuming stucco claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. )||Fastener spacing (assuming adhered manufactured stone veneer)|
|4 in. or less||24 in.||24 in.||16 in.||8 in.|
|3 in. or less||24 in.||24 in.||24 in.||12 in.|
|2 in. or less||24 in.||24 in.||24 in.||16 in.|
|1 in. or less||24 in.||24 in.||24 in.||24 in.|
Table 1 notes:
Fasteners spaced 24 inches apart give you an allowable design wind pressure of 49 psf.
Fasteners spaced 16 inches apart give you an allowable design wind pressure of 73 psf.
Fasteners spaced 12 inches apart give you an allowable design wind pressure of 98 psf.
Fasteners spaced 8 inches apart give you an allowable design wind pressure of 147 psf.
Table 2: Fastener spacing for attaching furring strips if studs are 24 in. o.c.
|Foam thickness||Fastener spacing (assuming vinyl siding)||Fastener spacing (assuming fiber-cement siding)||Fastener spacing (assuming stucco cladding)||Fastener spacing (assuming adhered manufactured stone veneer)|
|4 in. or less||24 in.||24 in.||12 in.||Don’t do it|
|3 in. or less||24 in.||24 in.||16 in.||8 in.|
|2 in. or less||24 in.||24 in.||24 in.||12 in.|
|1 in. or less||24 in.||24 in.||24 in.||16 in.|
Table 2 notes:
Fasteners spaced 24 inches apart give you an allowable design wind pressure of 33 psf.
Fasteners spaced 16 inches apart give you an allowable design wind pressure of 49 psf.
Fasteners spaced 12 inches apart give you an allowable design wind pressure of 65 psf.
Fasteners spaced 8 inches apart give you an allowable design wind pressure of 98 psf.
More information on siding weight
FastenMaster assumes that the weight of different types of siding (including the weight of the furring strips) are as follows:
- Vinyl siding plus 1x4 furring: 2.3 pounds / square foot
- Fiber-cement siding plus 1x4 furring: 3.5 pounds / square foot
- Traditional stucco plus 1x4 furring: 12 pounds / square foot
- Adhered manufactured stone veneer plus 1x4 furring: 26 pounds / square foot
Note: if you are attaching plywood or OSB rather than 1x4 furring, add 0.5 pound to the above weights.
If you know the actual weight of the siding you intend to install, you can substitute that weight for FastenMaster’s default values. Remember to add 1 pound / square foot for the weight of furring strips, or 1.5 pound / square foot for plywood or OSB sheathing. If your siding weighs less than FastenMaster's default values, you may be able to space your furring fasteners farther apart. On the other hand, if your siding weighs more than FastenMaster's default values, you may have to space your furring fasteners closer together. In either case, refer to the FastenMaster tables.
Design wind pressures
In the notes below each of the above tables, I have listed the design wind pressures (in pounds per square foot) that these wall assemblies are designed to resist. More information on design wind pressures can be found in Table R302.2(2) of the International Residential Code.
Most homes in the U.S. are designed to resist wind speeds up to 90 mph. However, in coastal areas, the design wind speed is much higher; it is 130 mph in some coastal areas and 150 mph in south Florida.
If your design wind speed is 90 mph, and your home is located in a typical suburban or wooded location, your design wind load is 19.5 psf. If your design wind speed is 90 mph, and your home is located in open, flat terrain, your design wind load is 29.1 psf. For these locations, all of the FastenMaster recommendations are more than adequate. If you live in an area with a design wind speed above 90 mph, however, you should verify that the your furring attachment method is adequate for your local design wind pressure.
Siding attachment requirements
Okay, now your furring strips are firmly secured to your building. The next question is: how do you attach your siding to the furring strips? After all, once you have installed vertical 1x4 strapping over foam sheathing, your siding nails are embedded in only ¾ in. of wood. Is that enough?
Table R703.4 in the 2006 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. (“Weather-Resistant Siding Attachment and Minimum Thickness”) specifies the required minimum fastener lengths for attaching siding. Moreover, in Footnotes n, p, and y — footnotes that apply to some, but not all, types of siding — the IRC notes that fasteners must “penetrate framing 1 1/2 inches.”
Confusingly, however, Table R703.4 provides no guidance to builders installing siding on furring strips over foam sheathing. The table anticipates several scenarios, including “fiberboard sheathing into stud,” “gypsum sheathing into stud,” “foam plastic [presumably without furring strips] into stud,” and “direct to studs.” Each of these scenarios deserves its own column in the table. However, there is no column for “furring strips over foam sheathing.”
Until recently, many siding manufacturers recommended that siding nails penetrate 1 in. or 1¼ in. into wood. That’s beginning to change, however. The Vinyl Siding Institute requires only ¾ in. of fastener penetration for vinyl siding; James Hardie Corp. accepts only 7/16 in. of penetration for fiber-cement lap siding.
According to building scientist Joseph Lstiburek, if you have any doubts arising from the fact that your siding nails penetrate into only ¾ in. of wood, just switch from smooth-shank nails to ring-shank nails. Unless you’re building near the coast in south Florida, ring-shank nails will be more than adequate, even when penetration into wood is only ¾ in.
If you have more questions about attaching furring strips to foam sheathing, or to request a copy of “Attaching Exterior Wall Covering Assemblies with Foam Sheathing to Wood Wall Framing,” call the FastenMaster Technical Support team at 800-518-3569.
For more information on this topic, see two useful documents:
- How to Install Rigid Foam Sheathing
- “Cladding Attachment Over Insulating Sheathing,” on the Building Science Corporation’s Web site.
- “REMOTE: A Manual,” a document published by the Cold Climate Housing Research Center in Fairbanks, Alaska.
- “Securing Rainscreen Siding” by Zeno Martin, JLC, February 2012.
Last week’s blog: “Makeup Air for Range Hoods.”
- Rob Wotzak
- Cold Climate Housing Research Center
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