What’s the Building Code for Housewrap Installation?
Code? What code? Well the IRC is pretty straight-forward: Prescriptive code measures + Manufacturers' Installation Instructions.
It’s common in my area to see housewrap installed improperly. Most city and town building inspectors only make four building inspections during the building process: (1) Footings/foundation before placing concrete, (2) Rough framing, after the utilities are roughed in, (3) Insulation / fireblocking / draftstopping, and (4) Final. Friends of mine in parts of Canada tell me that they are required to get a WRB/Window/Door installation inspection done. If we had that here, housewrap installation quality would probably rise significantly.
“It’s just housewrap. Who cares anyway? My siding doesn’t leak.”
Siding jobs do leak so when the water-resistive barrier leaks, water can get into the wall and lead to mold and rot. Often the leak can go on for many years inside the wall before anyone notices. And by the time it is discovered the damage can be extensive.
It’s all outlined in the code
Though many builders think of the IRC as a low bar / bare minimum, when it comes to WRB installation the code is actually ‘best practice.’ It’s all in section R703 – Exterior Covering of the 2018 IRC.
R703.1.1 “The exterior wall envelope shall be designed and constructed in a manner that prevents the accumulation of water within the wall assembly by providing a water-resistant barrier behind the exterior cladding … and a means of draining to the exterior water that penetrates the exterior cladding.”
R703.2 Water-resistive barrier
This section gives us two options for materials: #15 lb tar paper meeting ASTM D226 requirements OR “other approved water-resistive barrier” (Meaning: approved by the local building official.) The manufacturers of plastic housewraps and other WRB materials generally obtain an ICC Evaluation Service Report that code officials rely on when determining whether to accept an alternative material to #15 tar paper or not. Though I’ve never heard of a local code official denying approval of a housewrap but it is within his authority.
On the installation side, when tar paper is used it must be “free from holes and breaks,” “applied horizontally,” with laps “not less than 2 inches.” Vertical and other joints must be lapped 6 in. Fastening requirements for the WRB are not mentioned in the IRC. Figuring that tarpaper installation must be “free from holes and breaks,” it’s important to consider the effect of common installation practices like fine wire hammer-tacker staplers. Often the contact rim on a stapler rips a small hole each time the tacker is whacked. Another problem with fine wire staples is they frequently pull-through the WRB on windy days leaving holes that often don’t get patched.
And there are other WRB installation requirements that often gets overlooked “The No. 15 asphalt felt or other approved water-resistive barrier material shall be continuous to the top of walls and terminated at penetrations and building appendages in a manner to meet the requirements of the exterior wall envelope as described in Section R703.1”
What about housewrap installation?
When “Other approved materials… are used as a WRB they shall be installed in accordance with the water-resistive barrier manufacturer’s installation instructions.” There it is — the manufacturer’s installation instructions are referenced in the code and are enforceable by the local official.
Housewrap manufacturers list their installation requirements in the instructions. I read through several major housewrap manufacturer installation instructions and most have minimum horizontal overlaps of 4 in. or 6 in. and minimum vertical overlaps of 6 in. to 12 in. Most also require or recommend 1 in. plastic or metal cap fasteners. The caps protect the housewrap from damage during installation and spread the surface area of the fastener over a broader area of the WRB so the fastener is less likely to tear through. The fastener spacing patterns vary widely between manufacturers. Some call for fasteners spaced as wide as 32 in. on center vertically and horizontally, others are as close as 12 in. oc in some circumstances.