Insulating on the Outside
Wrapping a home with insulating sheathing saves energy and can be cost effective, but the design must consider a host of structural concerns including moisture, shear loads, insects and fire.
The use of insulating sheathing is becoming popular as consumers and energy codes demand energy-smart details. Obviously, wrapping a home with foam insulation saves energy, but it affects overall performance and cost. Foam-sheathed walls are more complicated to build and most insulating sheathings are non-structural. Extra building components must be added to take up the slack. Also, the exterior skin of a house protects the structure and its inhabitants from the environmental influence of moisture, fire, and insects. Wrapping a home with insulating sheathing may be a great idea, but this plan requires a holistic sensitivity.
Adding a layer of rigid foam to the walls of any building is somewhat labor intensive. When properly constructed, it provides a tight, dry, warm, and durable structure. Re-siding an existing home presents an opportunity to upgrade the home’s energy performance. But adding foam requires fussy detailing for trim and flashing. Windows, doors and trim must be built out and the flashing sealed to the foam sheathing.
Polyisocyanurate, molded expanded polystyrene (MEPS), and extruded expanded polystyrene (XEPS) are common sheathing choices. Foam sheathing is fastened to the structural sheathing or framing with broad-head nails, wide plastic washers, and/or adhesive caulk that is compatible with the foam. Check with the manufacturer before using any adhesive because solvents found in some adhesives eat foam. The layer of foam sheathing should be continuous and tight. Tape all the joints with a high quality construction tape, not duct tape. Good choices include 3M contractor’s tape, TU-TUF 4 Tape, and Insultape III. Installing a layer of foam will improve the R-value and improve air tightness when sealed.
Fastening of siding can be tricky. You can install vinyl siding directly over foam as long as your nails extend through it and attach firmly to a solid nail base. The Vinyl Siding Institute recommends 3/4-inch nail penetration, but the International Residential Code requires 1 1/2 inch nail penetration for vinyl siding. This means you must target the underlying studs even when foam overlays structural sheathing. Check your local code on this issue. When it comes to installing wood siding over foam be careful. I don’t think builders should install wood siding directly over plastic foam insulation.
Wood siding applied directly to foam has a history of failures. Nailing wood siding directly to foam doesn’t work because nails have to be extra long to reach through the siding and foam to a solid nail base. Long nails have a larger diameter that can split the siding unless every nail hole is predrilled. Plastic foam traps heat and moisture beneath the siding. Hot sun can overheat wood siding causing it to dry excessively and crack. Foam is less permeable to water vapor and the back of the siding stays wet as the face of the siding dries. As a result, the siding cups, cracks and sheds paint. Wood siding needs an air space between its backside and the face of foam sheathing.
Vented rain screens are correctly touted as a superior weather barrier system. Here vertical strips of furring are attached over the layer of foam sheathing. The furring strips are fastened directly over stud locations and then siding is nailed to the furring strips. This creates an air space between the back of the siding and the face of the foam that drains and dries readily. It is highly effective, but here are 2 issues to consider. Furring that is only 3/4-inch thick does not provide the 1 1/2-inch nail penetration required by manufacturers and building codes. This does not mean siding fastened in this way will fail, but it may not satisfy your local code or manufacturer’s warranty. And then there is the fire-safety issue.
Building codes may not allow you to leave an air space behind the siding. Section R602.8 in the International Residential Code (and sections of other building codes) requires fire blocking at the bottom, between stories, every 10 feet, and at the top near the roof in concealed stud spaces, including furred spaces. Many builders and building code officials think the spirit and intent of this code provision is not directed at vented rain screens, but rather at furred interior spaces. However, other inspectors disagree and think the code section holds as written. Inspectors have ordered completed rain screens stripped and others have stopped jobs in progress demanding the addition of fire blocking. The code is not clear and the decision will be your local building inspector’s. So run your rain screen design past the inspector before building the walls.
Structural sheathing like plywood or osb provides substantial racking resistance when nailed carefully to the face of a wall. Structural sheathing and diagonal braces installed in walls, parallel to the wind flow, transmit lateral loads safely to the foundation. Any switch away from plywood to non-structural sheathing must be evaluated carefully. Walls sheathed exclusively with rigid foam panels need additional lateral bracing to resist wind and seismic loads.
Wall bracing regulations are based, in large part, on the Federal Housing Administration (FHA) Technical Circular number 12, published as an interim standard in 1949. This document established 5,200 pounds as the acceptable base level of racking resistance for wood-framed walls. The FHA minimum value reflects the wind resistance provided by wood-framed walls, sheathed with horizontal boards and braced by 1″x4″ let-in bracing, a common construction practice in 1949. The conversion of regional wind or seismic exposures into an effective wall design is complicated and should be left to engineers. However, building codes do prescribe specific materials and applications for builders. Prescriptive codes allow: 1″x4″ diagonal bracing, metal strap bracing, and plywood/osb sheathing as wall-bracing options. But amazingly, 1″x4″ material used as let-in braces is not structurally graded. It is graded for appearance. Certainly 1″x4″ let-in braces have worked well in the past. Homes built to code have a good track record. But in the past, these braces were used in conjunction with board sheathing. Will foam-sheathed homes perform as well? There are no standards controlling the method of installation. And research shows that the structural contribution of let-in braces is negligible when used as isolated members.
In 1977, Roger Tuomi and David Gromala, engineers with the Forest Products Laboratory (FPL) in Madison Wisconsin studied let-in bracing. Tuomi and Gromala learned that much of a braced wall’s racking strength is owed to the interaction of board sheathing and let-in bracing. No such interaction occurs with non-structural foam sheathing. Later, in 1983, FPL researcher Ronald Wolf studied the contribution made by off-the-shelf No. 2, 1”x4” let-in braces in unsheathed walls and found they provided only 600 pounds of resistance to horizontal loads like wind. Tests conducted by Simpson Strong-tie a leading manufacturer of metal bracing and fastener systems yield similar results. So using 1×4 let-in braces may not be an automatic solution when building foam-sheathed walls
Metal bracing may not be the answer either. Several manufacturers of rigid foam insulation recommend metal strap bracing as a structural solution for foam-sheathed walls. Building codes allow them as substitutes for 1”x4” let in braces. This prescription makes be nervous. Tests conducted by the Forest Products Lab (FPL) and Simpson Strong-Tie show that metal bracing provides little in the way of lateral resistance. In fact, the allowable design value issued by Simpson Strong-tie for its T-shaped metal braces is only 195 pounds. Simpson’s product manual clearly states that metal wall braces are temporary braces, but are allowed to replace let-in bracing where codes allow. They prevent walls from racking during construction, but are not designed to replace important shear wall load-carrying components. Failure in let-in braces and metal braces generally occurs as a result of nail slippage. The braces are only as strong as the nails used at the ends of the brace! So size and the number of nails in brace ends limit design values.
The use of let-in or metal bracing systems in foam-sheathed walls probably will not result in catastrophic failure. But their use may provide a steady diet of callbacks related to structural movement. A competent engineer should carefully review designs that exclude structural sheathing.
Plywood or osb corners provide better lateral support. You can brace foam-sheathed homes with plywood or osb corners. Half-inch thick sheets can be installed vertically at the corners and overlaid with half-inch thick rigid foam. One-inch thick foam can be used to sheath the remainder of the house, leaving the exterior wall surface flush. Each corner panel will resist an ultimate load of 3,120 pounds when nailed with 8d nails, spaced 6-inches at the edges and 12-inches in the field of the panel. So a wall (2 ends) will resist 6,240 pounds. Tighten the nailing schedule and you can improve the rating. It’s fast and easy, but you should still have an engineer review the plans to establish working design values. Some builders use 2 layers of sheathing: a layer of plywood or OSB for structural support plus a seam-staggered layer of rigid foam insulation.
The most destructive force influencing structural durability is moisture. If used, foam sheathing must function as an integral part of an effective weather barrier system. All seams should be taped. Windows, doors, and other penetrating details must be flashed and sealed to shunt water from the sheathing. The top edge of the flashing must be sealed with sheathing tape and/or be protected by an overlapping layer of house wrap. While liquid water is the greatest concern, the movement of water vapor must be controlled.
Warm air leaking from a heated home into a colder wall cavity raises the relative humidity of the cavity. If the exterior wall sheathing is below the dew point temperature, condensation will form on the sheathing. Condensation and elevated moisture levels cause rot, mold and mildew. A continuous layer of insulating sheathing applied to the exterior surface of a wall will minimize condensation and moisture levels. It’s like slipping on a winter coat in a cold climate. It keeps the underlying structure warm and as a result drier. The reverse is true in a hot humid climate. Here warm moist air is on the outside and the vapor drive is inward. In hot humid climates, exterior insulating sheathing should be continuous and vapor impermeable to retard vapor migration into the wall. Impermeable foil-faced sheathings like Dow Tuff-R or Rmax R-Matte® Plus are good choices. Tightly sealed exterior sheathing functions as an important air barrier detail in any climate, but can be critical in a hot humid one.
Some people worry that using impermeable sheathing in a cold climate will trap moisture within wall cavities. Building scientists have learned that as airborne moisture leaks into wall cavities, so does heat that warms walls to safe levels. While studies show it might be ok to use impermeable foil-faced sheathings in cold climates, you have to ask: why take the chance? It’s simply safer to use more permeable sheathings. Extruded polystyrene (XEPS) and molded polystyrene (MEPS) products give thermal protection with perms of 1+/inch and 2+/inch of thickness respectively. Polyisocyanurate products like Rmax Durasheath® (perm 1+/ inch) and Dow Sturdy-R (perm 3+/inch) are available too. Vapor permeable sheathings allow the wall cavities to dry faster if they get wet.
I clearly remember my first experience with ants in rigid foam insulation. A crewmember arrived in my office with a bag of what looked like the foam “peanuts” used for packing fragile items. It was ant-chewed rigid foam insulation removed from the sidewall of a remodel job he was working on. Certainly not an epidemic, but since that time I have discovered several ant infestations in stress skin panels and rigid foam sheathing.
Carpenter ants thrive in damp climates like those found in densely populated coastal regions. They are accidental tourists in the dryer Southwestern states. Unlike termites, ants don’t eat wood or foam for its nutritional value. They merely use it for shelter. And foam makes a nice shelter! Ants like to nest in foam because it’s soft and easy to chew. Stress skin panel manufacturers have taken notice. In fact they advertise stress skin panels treated with boric acid promising reduced likelihood of ant infestation. Although I have heard that borate-treated foam sheathing is available, I have not seen any sold at lumberyards. In fact, since ants chew foam in a small minority of homes, it seems impractical to treat foam sheathing with insecticide. The best way to minimize ant damage is to follow good ant prevention practices on the building site.
Ants may be a nuisance, but termites pose a structural threat. As a nation, we spend two-thirds of our annual pest control budget on termites. The greatest threat exists where the average annual outdoor temperature exceeds 50 degrees. However, termites readily venture north into the comfort of centrally heated homes. When insulating foam is used to wrap the exterior of a home, it provides an undetected pathway connecting soil to structure. Termites can tunnel from the soil up behind the foam panels and attack the structure of a house without being noticed. Building codes are clear. Section R324.4 in the International Residential Code (IRC) requires that in areas where the probability of termite infestation is very heavy, including California, Texas, Louisiana, Mississippi, Alabama, Georgia, Florida, and South Carolina; foam plastic can not be installed on the exterior face or under foundation walls or slabs located below grade. The clearance between foam plastics installed above grade and exposed earth must be at least 6 inches. This requirement allows you to track termite activity. If you live in area with even moderate termite probability (anything south of a line drawn from southern Maine to southern Oregon) it is a good idea to use termite shields and provide a vision strip between soil and the foam used on the exterior of the house.
Framing the Energy Issue
So far we have talked a lot about the insulating skin, but the choice of framing material can make a big difference in a wall’s energy performance. Steel framing is becoming more popular. Steel studs are strong, straight, and stable. They make flat, straight walls. The price of steel is more predictable than wood, allowing you to develop better cost-estimates. But these benefits come at a price. Steel is much more thermally conductive than wood. Thermal bridging compromises performance and can cause condensation, discolored wall surfaces, mold, and occupant discomfort.
Studies conducted by Oak Ridge National Laboratory (ORNL) shows that thermal bridging through the framing elements of a structure degrades the overall energy performance of a wall system. Research shows that steel studs are much worse than wood studs. You can’t simply substitute off-the-shelf steel framing components for wood components. You loose too much energy through the steel frame. Installing a layer of insulating sheathing helps steel frames perform better.
Clear-wall values developed by ORNL researchers help us compare the performance of various wall systems. Clear-wall values are composite R-values of a wall section including the thermal bridging effect of studs. This is good to know. These values are more useful than simple “center-of-cavity” R-values that consumers read on the bags of insulation they install. But clear-wall values are limited. They do not include the effect of all design elements normally used.
ORNL scientists have developed a method to account for the thermal bridging of all framing and connections used in a typical wall. Headers; jack studs; corner posts; and partition intersections at floors, wall, and roofs are weighted in this “whole wall” evaluation. Whole-wall modeling provides a more meaningful prediction of performance. ORNL provides a free on-line calculator to predict the performance of various wall systems at http://www.ornl.gov/roofs+walls/calculators/wholewall/index.html. This calculator shows that a 2×4 wood-frame wall really delivers 76% (R-10.21) of the center-of-cavity R-value (R-13.5) that so many builders think they are getting. A 3.5-inch steel-frame wall delivers only 45% (R-6.1) of the center-of-cavity value, just 59% of a wood-frame wall value.
Thermal Resistance of Various Wall Configurations1
|Center of Cavity2||Clear Wall||Whole Wall|
|2×4 wood 16” o.c. R-12 cavity insulation||R-13.5||11.43||10.21|
|2×4 wood 16” o.c. R-12 cavity + R-4 foam||R-17.5||15.47||13.5|
|2×4 wood 24” o.c. R-12 cavity insulation||R-13.5||12.08||10.62|
|2×4 wood 24” o.c. R-12 cavity + R-4 foam||R-17.5||16.12||13.91|
|3.5” steel 16” o.c. R-12 cavity insulation||R-13.5||7.44||6.10|
|3.5” steel 16” o.c. R-12 cavity + R-4 foam||R-17.5||12.10||9.45|
|3.5” steel 24” o.c. R-12 cavity insulation||R-13.5||9.43||6.96|
|3.5” steel 24” o.c. R-12 cavity + R-4 foam||R-17.5||13.96||10.25|
- Values from Oak Ridge National Laboratory’s on-line calculator http://www.ornl.gov/roofs+walls/calculators/wholewall/index.html
Note: this calculator is part of the material database used by DOE’s EnergyPlus.
- Center of Cavity value includes R-value of drywall, osb sheathing and wood siding.
This level of performance hardly meets the expectations builders have as they fill walls with fluffy batts of insulation. Some professionals argue this level of performance fails to meet at least the spirit of performance dictated by many building codes. The results of this research tell us that if we build with off-the-shelf steel studs, we should use exterior insulating sheathing to achieve an acceptable level of energy performance. Insulating sheathing provides a thermal break between the conductive steel studs and the outdoor environment. Depending on your climate, you might consider increasing stud depth and on-center spacing.
Budget controls building design. Sure, everyone wants to conserve energy, but at what cost? Substitute insulating sheathing for plywood or osb in new construction and there is virtually no added cost. You break even on installation and capture an immediate energy savings. However, figuring a payback for foam sheathing on a residing project is tricky. The list of variables is long. Climate, air tightness, level of insulation, present and future fuel costs, size and shape of house, and radiant heat gain all affect payback. Adding R-5 insulated sheathing while residing an existing average-sized Boston home should cost about $1200. Payback for electric heat can take a measly 5 or 6 years. It will take more than 15 years to recoup the cost of oil or natural gas at today’s prices. Philosophically, there is no question; saving energy is a good, healthy, enlightened practice. Wrapping a house with foam insulation and building a vented rain screen is perhaps the Mercedes of wall options. Weigh the following pros and cons on a case-by-case basis and execute any plan with thoughtful details.
- improves comfort
- improves air tightness
- reduces conductive heat loss and thermal bridging
- reduces condensation in walls by raising cavity temperature
- reduces rain penetration into wall cavities through pressure equalization and drainage.
- helps block unwanted sound
- cost effective in some cases
- upgrade appreciates in value as energy costs rise
- complicated detailing
- labor-intensive process
- added cost of materials and labor
- potentially long payback period
- insect problems
- fire code issues
Further Information and Product Resources
Florida Solar Energy Association http://www.fsec.ucf.edu/
1679 Clearlake Road, Cocoa, Florida 32922 Phone (321) 638-1000
Oak Ridge National Laboratory, Building Envelope Programs (BEP) http://www.ornl.gov/roofs+walls/research/
P.O. Box 2008, Mail Stop 6070, Oak Ridge, Tennessee 37831-6070
The Polyisocyanurate Insulation Manufacturers Association (PIMA)
515 King St., Suite 420, Alexandria, VA 22314
US Department of Energy, Building Technologies Program
Vinyl Siding Institute (VSI)
Western Wood Products Association (WWPA)
522 SW Fifth Ave. Suite 500, Portland, Oregon 97204-2122
Polystyrene Insulating Sheathing
Dow, Building and Construction http://www.dow.com/products_services/industry/build.htm
Owens Corning, http://www.owenscorning.com/
Polyisocyanurate Insulating Sheathing
Dow, Building and Construction http://www.dow.com/products_services/industry/build.htm
Construction Sheathing Tape
3M Contractor Sheathing Tape, 3M Construction Markets Division, St. Paul, MN 800-480-1704
TU-TUF 4 Tape, Sto-Cote Products, Inc., Richmond, IL 800-435-2621
INSULTAPE III, Conserv Products, Inc., Oregon, WI 608-835-7299
Last updated: May 7, 2009