Insulation Repair Allston MA
Designer / Architect, Remodeler
2007 Guildmaster, 2008 Guildmaster with Distinction, 2009 Guildmaster, 2010 Guildmaster with Distinction
Custom Builder, Designer / Architect, Remodeler
Boston Society of Architects, National Association of the Remodeling Industry, National Kitchen and Bath Association
CSA Home Services
Custom Builder, Designer / Architect, Remodeler, Specialty Contractor
2009 CotY Awards, Build It Green, Eastern Massachusetts Chaper of NARI, NARI Certified Remodeler, NARI Green Certified Professional (GCP), National Association of the Remodeling Industry
Designer / Architect, Remodeler
2009 Guildmaster, 2010 Guildmaster
Remodeler, Designer / Architect
2009 Guildmaster, 2010 Guildmaster with Distinction
2006 CotY Award, 2008 CotY Awards, Builders Association of Greater Boston, Eastern Massachusetts Chaper of NARI, NAHB Remodelers, National Association of Home Builders, National Association of the Remodeling Industry, National Kitchen and Bath Association, Rotary International
Designer / Architect, Remodeler
2007 Guildmaster with Distinction
Insulating on the Outside
Insulation helps us tolerate nature's extremes, lowering home heating and cooling bills, and keeping us comfortable. Rising energy costs and more stringent government recommendations on energy efficiency are prodding builders and homeowners to look at this basic building component more carefully and find ways to stretch insulation dollars. One promising trend that can work wonders: Adding a blanket of insulation to the outside of your house.
In conventional construction, an insulating material, such as fiberglass or cellulose, is packed into wall and roof cavities before the house is covered with structural sheathing, such as plywood or oriented strand board (OSB). Newer construction practices add a layer of insulating sheathing to the exterior of a home, either augmenting or virtually replacing structural sheathing. Benefits range from lower energy costs to healthier buildings. Insulating sheathing can be built into a new house or retrofitted to an old one. If there's a down side, it's the potential for more construction complexity and higher costs.
Polyiso and Other Tongue Twisters
Available in 4-foot-wide panels 8 or 9 feet long, insulating sheathing is a general term used to describe several types of rigid foam. Chances are good the foam panels will have a higher per-inch R-value than conventional insulation used in wall and roof cavities. (R-value is a measure of the insulating material's resistance to heat transfer.) While loose-fill fiberglass, for example, has an R-value of around 2.2 per inch (a higher-density form of chopped fiberglass blown into wall cavities under pressure achieves R-4.0 per inch), and fiberglass batting has an R-value of around 3.1 per inch, foam panels range from R-3.8 to R-7.0 per inch. Just as important, that final layer of insulation on the outside of a house reduces something called thermal bridging, the short-circuiting of heat and cold across poor insulators like wood or steel framing.
There are three common types of insulating sheathing: molded expanded polystyrene (MEPS), extruded expanded polystyrene (XEPS) and polyisocyanurate, also known simply as polyiso. Manufacturers sell panels under a variety of brand names. Styrofoam, for instance, is a kind of XEPS made by Dow. R-values for the three types of foam insulation range from about R-3.8 per inch of thickness for MEPS to R-6.5 for faced polyiso board.
Extruded panels once were made with the help of chlorofluorocarbon (CFC) blowing agents that, while effective, damaged earth's ozone layer. However, manufacturers have switched to hydrochlorofluorocarbons (HCFCs), which have a less harmful environmental effect. Foam panels that contain no HCFCs will be available soon; manufacturers will use hydrofluorocarbons (HFCs) as a blowing agent.
Expanded types of insulating sheathing contain no CFCs or HCFCs. Instead, they're made with a steam process using a non-CFC gas called pentane as a blowing agent. Expanded types have some postconsumer content made from recycled fast-food containers and cups. In addition, certain types of foam insulation, such as extruded polystyrene, are recyclable.
Solving the Structure Dilemma
Building codes may allow either metal or wood diagonal bracing to substitute for a full layer of structural sheathing, although building experts are divided on how well bracing actually performs. Depending on local conditions and code requirements, plans may need the approval of an engineer.
Another approach is to combine structural sheathing and foam panels. The corners of the building are reinforced with 1/2-inch-thick, 4-by-8-foot structural panels attached vertically and wrapped with 1/2-inch-thick foam panels, while the remainder of the wall is covered with 1-inch-thick insulating sheathing alone, so the two surfaces meet evenly. A third approach, which offers even greater structural stability, is to sheath the house conventionally, then add a seam-staggered layer of foam to the outside.
Insulating sheathing is fastened with broad-head nails, wide plastic washers and/or foam-compatible adhesive caulk. The sheathing should be continuous and tight, with all joints taped with a high-quality construction tape, not duct tape. Good choices include 3M contractor's tape, Tu-Tuf 4 tape and Insultape III.
Attaching Siding Is More Complex
Because insulating sheathing is an inadequate nail base, attaching vinyl siding to the sheathing is more complex. Builders can attach vinyl siding directly over foam as long as the nails extend through the foam sheathing and attach firmly to a solid nail base such as a stud. The Vinyl Siding Institute, a trade association, recommends 3/4-inch nail penetration for vinyl siding, while the International Residential Code requires 1-1/2-inch nail penetration. Local codes will determine what's acceptable.
Wood siding applied directly to foam has a history of failure for two reasons. First, nailing wood siding directly to foam is difficult 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 and can split siding unless every nail hole is predrilled. This, of course, can be time-consuming and expensive.
The second reason involves the redistribution of moisture. As moisture in the wood is driven inward by the heat of the sun, it gets trapped beneath the siding. The back of the wood siding stays wet as the face of the siding dries, causing cupping, cracking and peeling paint.
A weather barrier system known as a vented rain screen is one possible solution. This construction technique leaves a narrow air space between the back of the siding and the face of the foam, so moisture can drain and dry. To build a vented rain screen, vertical strips of furring are attached over the layer of foam sheathing. The furring strips are fastened directly over stud locations. Siding is then nailed to the furring strips.
There is also a fire-safety issue. Building codes may require fire blocks even in furred spaces. Although many builders and building code officials think the spirit and intent of this code provision is not directed at exterior vented rain screens, other inspectors disagree. Inspectors have ordered completed rain screens stripped, while others have stopped jobs in progress, demanding the addition of fire blocking.
Controlling Moisture in the Walls
While rain may be the greatest water concern to homeowners, the movement of water vapor also 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, condensation will form on the sheathing. If water is trapped inside the wall, mold and rot may result.
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 dry. Permeable sheathings (panels that allow air and moisture to pass through) are probably the best choice in cold climates. Building scientists have learned that as airborne moisture leaks into wall cavities, so does heat, which warms walls to safe levels. As a result, vapor-permeable sheathings allow wall cavities to dry faster if they get wet.
XEPS and MEPS products provide thermal protection with perms of 1 or more per inch and 2 or more of thickness, respectively. (Anything with a perm rating of 1 or less is considered impermeable.) Polyiso products like Rmax Durasheath (perm of 1+ per inch) and Dow's Sturdy-R (perm 3+ per inch) are available, too.
In hot and humid climates, warm, moist air is on the outside, and the vapor drive is inward. To retard vapor migration into the wall, exterior insulating sheathing should be continuous and vapor-impermeable. Impermeable foil-faced sheathings like Dow's Tuff-R or Rmax's R-Matte Plus are good choices for buildings in these climates.
When Ants Go Marching
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 foam packing peanuts. 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 rigid foam sheathing and stress skin panels, which are panels made of a foam core sandwiched by two layers of structural material like OSB.
Stress skin panel manufacturers have taken notice. In fact, they advertise panels treated with boric acid to reduce the likelihood of insect infestation. Although I have heard that borate-treated foam sheathing is available, I have not seen any at lumberyards. In fact, since ants chew foam in a small fraction 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.
The International Residential Code requires that in states where the probability of termite infestation is heavy including California, Texas, Louisiana, Mississippi, Alabama, Georgia, Florida and South Carolina foam plastic cannot 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.
If you live in an 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.
How Much Money Will It Save?
Does such an approach pay off? It all depends. Adding insulating sheathing during new construction is simpler than during a renovation, where adding a layer of foam requires fussy detailing for trim and windows.
Present and future fuel costs, the size and shape of the house and radiant heat gain all affect payback. Adding R-5 insulating sheathing when re-siding an existing average-sized home in Boston, for example, costs about $1,200. Payback for electric heat can take merely five or six years, but it will take more than 15 years to recoup the cost of oil or natural gas at today's prices.
As you can see, there are a number of issues to consider when you're deciding whether to wrap your home in insulating sheathing. Structural, installation, fire-code, water and pest issues all must be evaluated carefully. However, the benefits of insulating sheathing are great, since it improves the R-value, saves energy and provides a tight, dry, warm and durable structure. Before you decide how to proceed, analyze the pros and cons on a case-by-case basis, and execute any plan with thoughtful attention to details.