Most of Foard Panel’s experience and customer base is in the northeast. The best practices outlined here are based on a combination of our experience and our best understanding of building science. Since our geographical range of experience is regional, these recommendations should be taken as guidelines and only applied in areas with roughly similar climate and weather.
We’ve learned that some amount of liquid water will get everywhere in any building’s envelope/weather-protection assembly. The sources of water vary from extreme wind driven rain, imperfections in flashing details, and many other sources. (We’ve seen cases where there was liquid water behind the synthetic building-wrap high up on a gable wall, above all ROs, and under an overhang with no sign of air exfiltration.)
Wet wooden sheathing dominantly dries toward outside. In conventional/pre-IECC construction, there is plenty of air movement within stud bays and through sheathing to encourage the water to move from the sheathing to the outdoors. Additionally, low- and moderate-performance buildings have a meaningful amount of heat heat loss, from inside-to-outside, during the winter. this provides energy to evaporate the liquid water in the sheathing into a vapor, allowing it to pass through vapor-open building-wraps easily.
The Conventional Building Wraps Do NOT work with High Performance Systems Like SIPs.
When the envelope is much higher performance, like SIPs, the “drying dynamics” change dramatically. SIPs have no stud-bays nor in-wall or in-roof caveties so there there is no air circulation to help the drying process. Also, the huge reduction in thermal bridging and full-thickness “continuous insulation” integral to SIP envelopes reduces the heat flow (BTU/sqft/hr) through the wall/roof assembly. This removes most of the energy required to evaporate water in or near the sheathing, rendering “liquid-tight/vapor-open” membranes much more likely to retain water within wood components.
For these reasons, Foard recommends traditional #15 or #30 tar paper over any synthetic building-wraps or roofing underlayments. We have evaluated the properties of each house wrap and roofing underlayment suggested by our customers. Comparing these properties with our on-site investigations and other evidence, we expect the best of the synthetic building-wraps may approach the protection and durability provided to wood-based/low-heat-loss envelopes by tar paper.
We’ve yet to document a tar paper covered SIP envelope with a building-wrap-driven durability problem. On the other hand, we get a tragically high number of calls and emails each year from owners of synthetic/hydrophobic building-wrapped buildings, SIP or otherwise.
From our analysis of all of the site visit information and consultations with well respected building science engineers, we’ve been able to amass the following list of recommendations:
• All sheathing eventually gets wet. Encouraging it to dry rapidly is the key to durability.
• Liquid water can reach the sheathing either from the outside or the inside, but outdoor water penetration is most common by far.
• Liquid water management is at least as important as water vapor management.
• To reduce wetting the sheathing from the inside, the most effective air- and vapor-control layers must be on the warm side of the envelope. Everything to the cold side of these retarders should be of increasing permeance.
• To reduce wetting the sheathing from the outside, use the most proven and durable flashing details, use “rain-screen” siding and vented or “cold” roof designs, reduce water concentration as much as possible, and understand that large overhangs are cheap insurance.
• Use house wrap and roofing underlayments that allow water vapor AND liquid water to slowly pass through.
• Provide a 1/4” or greater vent-space/capillary-break between the roofing or siding and the sheathing. This is among the most effective durability enhancements we’ve found. Capillary breaks can be made with strapping or furring strips or with a super-porous mesh product like Home Slicker or Cedar Breather.
The following picture illustrates an example of the long term effects of hydrophobic roof underlayments direclty applied to the exterior sheathing. The roof on this 18 year old house was covered with Grace Ice & Water Shield, a material that is hydrophobic and has a maximum permeance rating of 0.05. The permeability rating of SIPs is about 0.2.
Because of the difference in partial pressure of water vapor between indoors and outside, water vapor always diffuses outward during New England winters. The “peel & stick” membrane stopped the outward diffusion of water vapor and, being on the cold-side of the assembly, condensed into liqued water at the wood sheathing. This diffusion rate ws slow but, over time, the moisture content of the wood rose to the level where it supported fungal growth. The water had no ability to migrate anywhere else so it stayed in wood sheathing and rotted it. The rate of water accumulation is relatively slow as it is limited by the cumulative permeance of the envelope assembly interior of the Ice & Water Shield.
The following is a list of siding and roofing layups that have proven to be among the most durable:
• Clapboard siding:
SIP, #15 tar paper, 1×4 or 2×4 vertical strapping, clapboard -or-
SIP, #15 tar paper, Home Slicker, clapboard.
• Traditional shingle, shake, or vertical board & batten or ship-lap:
SIP, #15 tar paper, 1×4 or 2×4 horizontal strapping, shingle -or-
SIP, #15 tar paper, Home Slicker, shingle.
• Stucco, brick, stone, or fake-stone veneer siding:
SIP, #15 tar paper, 2×4 strapping/furring, PT plywood or fiber-cement panel, masonry finish, functioning top and bottom vents.
• Asphalt shingle roofing:
SIP, #30 tar paper, 1×4 or 2×4 strapping/furring, T&G sheathing, any roof underlayment, asphalt shingles
• Standing seam metal roofing:
SIP, tar paper, standing seam metal roofing -or-
SIP, tar paper, standing seam metal roofing w/ vertical spacers between each seam to create a vent space
• Wood shingle roofing:
SIP, tar paper, Cedar Breather, wood shingles -or-
SIP, tar paper, horizontal wood furring strips, wood shingles
• Pre-formed, ribbed metal roofing panels:
SIP, tar paper, steel roofing panels
• For other siding and roofing types, please contact your Foard Panel project manager or Foard Panel engineering.
Our Current Project at Hilltop Montessori School is now being sided. They are using our recommended layup for clapboard siding. Here are some pictures of SIPs with #30 Tar Paper, Homeslicker and Clapboards.