Category THE ENERGY-SMART HOUSETHE

■ BY DANIEL S. MORRISON

inished basements are a great way to add living space to a house without adding on. You often can add almost as much living space as the main floor offers. Before thinking about flooring choices and paint colors, though, think about the basics. Moisture, insulation, and air infiltration must be tackled before any finish materials are installed. In new construction, these issues are addressed from the outside before the basement is backfilled. Retrofits mean that you have to work from the inside. In either case, it is important to consider the climate before work begins.

Start with Water Management

Because basements are mostly buried in the ground, they are sometimes wet, are usually damp, and are seldom dry. Rarely do old houses have perimeter-drainage systems, in­sulation, or capillary breaks. When convert­ing a basement to living space, the basement must manage moisture better than it did before the insulating and air-sealing, because

a tighter basement is less able 2 3: R-5 to dry out when it becomes wet.

You can use grading to manage bulk groundwater on the outside, but founda­tions also have to disrupt capillarity. Water in the soil can and will wick up to the roof framing if you let it. Capillary breaks such as brush-on damp-proofing, sill sealer, and rigid insulation block this process.

Air-Sealing Saves Energy and Stops Moisture

The connection between concrete founda­tions and wood framing is almost always

Insulation Amount Depends on Location

The International Residential Code (IRC) specifies particular R-values for each climate zone; how you get there is up to you.

For very cold climates, you may need to add extra thick rigid insulation or fill the stud cavities. Don’t, however, treat a below-grade wall like a regular wall. Expect bulk-water problems, and choose insulation that can handle it. Never include a plastic vapor barrier when insulating a basement wall, because it will trap moisture.

You can’t count on a footing drain to exist (or work properly) in an old house, so use grading to push away bulk water.

leaky because wood is often warped and concrete is rarely flat. Air leaks waste energy and cause moisture problems. Most base­ment air leaks occur between the top of the concrete wall and the bottom of the sub­floor, where there are many joints and con­
nections. The easiest way to seal and insulate the rim-joist area is with spray foam, but blocks of rigid foam sealed in place can work well, too.

Which Rigid Insulation Should I Use?

Expanded polystyrene

The least-expensive choice,

EPS is manufactured in different densities. EPS (typically white in color) is not as strong as XPS, and it’s susceptible to crumbling at the edges. EPS is the most vapor-permeable type of rigid foam.

R-value: About 4 per in.

Perm rating: 2.0 to 5.8 for 1 in., depending on density

Extruded polystyrene

Because of its high strength and low permeance, XPS (often blue or pink in color) is the most commonly used type of rigid foam for basement walls R-value: About 5.0 per in.

Perm rating: 0.4 to 1.6 for 1 in., depending on density

Polyisocyanurate

Polyiso has a higher R-value per inch than EPS or XPS. Many building officials allow foil-faced polyiso to be installed in base­ments without any protective drywall, making polyiso the preferred foam for basements without stud walls.

R-value: Up to 6.5 per in.

Perm rating: 0.03 for 1 in.

(with foil facing)

Mineral wool

Although many energy experts advise against using fibrous materials to insulate basement walls, some builders may want to consider using mineral-wool batts because they are less susceptible to water damage. Manufacturers include Thermafiber® and Roxul®. r-value: 3.7 per in.

Perm rating: Hasn’t been tested, but highly permeable

Adding exterior foam to walls works well for new construction. As long as you meet local wind and earthquake codes, it’s usually pos­sible to build a foam-sheathed wall without structural oriented strand board (OSB) or plywood sheathing. Foam-sheathed walls are braced using one of four methods:

• Diagonal 1×4 let-in braces.

• Diagonal T-profile steel strapping, such as Simpson TWB.

• Inset shear panels.

• A few strategically placed pieces of OSB (they are usually installed at corners).

Of course, you should check with an engineer and your local building official before finalizing your wall-bracing plan.

Exterior foam sheathing is often installed on existing homes as an energy-saving

retrofit when new roofing or siding is need­ed. You can install rigid foam on the inside of a wall as well, but adding exterior foam increases the R-value of a wall or roof with­out eating up interior living space.

Installed on the roof, exterior foam makes the living space below more comfortable and reduces the likelihood of ice dams.

The illustration on the facing page shows two layers of OSB or plywood roof sheath­ing: a lower layer conventionally nailed to the rafters and an upper layer installed as a nailing surface for the roofing material on top of the rigid foam. The type of fasteners used and the way they need to be spaced for the top layer of sheathing depend on the pitch of the roof and roof loads, particularly wind and snow loads. It’s fairly easy to find screws with a pullout-resistance rating ex­ceeding 400 lb., even when they are secured just to plywood or OSB sheathing. The fas­tener rating increases if the screws are driven into the rafters. (Fastener sources include Wind-lock® and FastenMaster®, which man­ufactures HeadLok® and OlyLog® screws.)

Most foam-sheathed walls include a rain – screen gap between the foam and the siding. After the foam is tacked in place temporarily with a few nails, it is secured in place with vertical 1×3 or 1×4 strapping that is screwed through the foam to the underlying studs. Some siding types, including cedar shingles, may require a drainage mat, kerfed horizon­tal furring, or an OSB or plywood nailer over the foam.

Does Exterior Foam Create a Wrong-Side Vapor Barrier?

Some builders worry that exterior foam sheathing is a "wrong-side vapor barrier" that can trap moisture in walls. In new construction without interior polyethylene vapor barriers, the worry is baseless. As long as the foam sheathing is thick enough, it will reduce the chance that moisture will accumulate in a wall. By warming the wall cavity, exterior foam eliminates cold surfaces where moisture can condense. Here are the minimum R-values for exterior foam for 2×6 walls:

• R-15 in climate zones 7 and 8

• R-11.25 in climate zone 6

• R-7.5 in climate zone 5

• R-3.75 in climate zone 4

When exterior rigid foam is used as an energy-saving detail in warmer climates, condensation is not a concern, so no mini­mum R-values apply.

Since exterior foam reduces a wall’s abil­ity to dry to the exterior, foam-sheathed walls should be able to dry to the interior. That means that foam-sheathed walls should never include interior polyethylene or vinyl wallpaper. Painted drywall has a high enough permeance to allow any incidental moisture that enters a wall cavity in the win­ter to evaporate through the drywall during the summer.

If you are considering installing exterior foam on a house with interior 6-mil poly­ethylene, proceed with caution. If water ever enters a wall with foam sheathing and inte­rior poly, the wall has a very limited ability to dry. This raises the stakes, and water – management details must be impeccable.

After all the siding has been removed, inspect the existing wall sheathing for stains or moisture damage. If you find either, you’ll need to diagnose the cause and implement remedies. If the existing sheathing is clean, dry, and sound, it’s safe to install exterior wall foam, as long as the new siding is installed with a rain-screen gap and meticu­lous flashing.

Martin Holladay is a contributing editor to Fine Homebuilding.

Using Rigid Foam for an Efficient and Dry House 81

■ BY MARTIN HOLLADAY

emodelers who open up fiberglass – insulated walls in the middle of the winter are often surprised to see a thin layer of frost on the interior side of the wall sheathing. The frost indicates that warm, humid interior air is leaking through the wall penetrations, leading to condensation on the cold surface of the sheathing.

One way to limit this phenomenon is to keep the sheathing warm by adding a layer of rigid-foam insulation on the exterior side of the sheathing. If there are no cold sur­faces in the wall cavities, condensation is unlikely. A layer of exterior foam also makes a house more energy efficient by increasing insulating performance, reducing thermal bridging, and minimizing air leakage.

All three types of rigid-foam insulation— expanded polystyrene (EPS), extruded poly­styrene (XPS), and polyisocyanurate—are suitable for use on the exterior of walls and
roofs, though they do not perform equally. EPS is the most vapor permeable of the three types; at R-4, it also has the lowest R-value per inch. Foil-faced polyiso is the least va­por permeable and has the highest R-value per inch, at R-6.5. XPS (R-5 per in.) and the denser types of EPS can extend below grade, but polyiso absorbs water and therefore should not be installed in contact with soil.

Every wall needs a water-resistive barrier such as asphalt felt or housewrap. It’s also possible to use rigid foam as a barrier, as long as foam seams are sealed with a suitable tape or installed with Z-flashing. Regardless of your choice of barrier, all penetrations, including windows and doors, need to be flashed properly; these flashings need to be integrated with the barrier using adequate overlaps or durable tapes.

The benefits of adding a layer of rigid-foam insula­tion to the exterior of walls and roofs are twofold. First, the foam will increase ther­mal performance by adding R-value and minimizing thermal bridging. Second, the foam will keep the sheathing warm, so mois­ture passing through the wall or roof will find no cold surfaces for condensation to occur. For this reason, the roof does not need to be vented. That’s why exte­rior roof foam makes a lot of sense on difficult-to-vent hipped roofs or on roofs with multiple dormers.

It never ceases to amaze me how many builders omit seam tape from housewrap installations. Although proper lapping is enough to create a watershed, all seams must be sealed to stop air infil­tration. Taping the seams also helps to preserve the housewrap’s integrity throughout construction and makes the membrane less likely to catch the wind and tear.

Seam tape also provides a means to repair cuts, but every cut or penetration should always be treated like a horizontal or vertical seam. Seam tape is never used to make up for improper lapping. In fact, assume that the tape adhesive will fail eventually, allowing water to penetrate the drain­age plane and wet the framing. In contrast, a proper lap can last forever.

Almost every housewrap manufacturer provides a seam tape for their product. Generic building tapes such as duct tape should be avoided because they might fail sooner.

Housewrap can be attached with plastic cap nails (see the photo, near right), l-in.-wide crown staples, or large-head roofing nails. Many builders use a hammer tacker to fasten the wrap with staples, but this type of fastener is much more likely to pull through the housewrap before the siding is installed.

For a better installation, manufacturers recom­mend the use of plastic cap nails or cap screws, which are available for manual fastening or collated for use in pneu­matic nailers (see the photo, far right). Cap nails also act as a gasket to keep water from leaking through the nail holes.

Whatever fastener is used, the manufacturer’s recommenda­tions for spacing are important. The most common spacing is 8 in. to 18 in. vertically, and 16 in. to 24 in. horizontally. Fasteners should be driven into studs or sheathing such as plywood or OSB.

Housewrap installation starts from the bottom and works its way up. All horizontal joints should overlap at least 6 in., and all vertical joints 12 in. If housewrap is applied to the sheathing before the wall is raised, there needs to be enough material left to cover the band joist. Horizontal laps are as important as vertical laps because wind­blown rain can travel sideways.

Housewrap should always be installed with the same care and attention devoted to siding. Although no one will see good work underneath the siding, correctly installed housewrap still pays off in the long run.

Fernando Pag6s Ruiz operates Brighton Construction Co. in Lincoln, Neb. He is also the author of Building an Affordable House: Trade Secrets for High-Value, Low-Cost Construction (The Taunton Press, 2005).

Builders have debated the chemical compat­ibility of housewrap for years. Studies have found that certain types of wood siding, like cedar and redwood, leach surfactants (surface-active contaminants) that can affect the water resistance of housewraps. The sur­factants reduce the surface tension of water, easing its ability to pass through micro­scopic openings in the membrane. To com­bat the problem, manufacturers recommend back-priming potentially troublesome wood siding with a water-repellent primer. In a
limited study, Fisette found that the newest version of Typar had superior resistance to surfactants compared to the performance of similar products.

In addition to the water-soluble extractives found in wood siding, the water resistance of housewrap also can be compromised by soaps, power-washing chemicals, and even some types of latex paints. The perforated variety is most susceptible, so consider choosing a high – quality, nonperforated housewrap.

It’s also important not to leave housewrap exposed for longer than necessary. House- wrap left uncovered for longer than its intended UV-rating will deteriorate and decline in performance, and should be

Housewrap Choices continued

Rain screen and housewrap combined

o matter how tight the joints, how thorough the flashing installation, or how far the roof over­hangs the walls, water always finds a way behind the siding of a house.

Housewrap or felt paper is a good safeguard for protecting sheathing and framing, but many builders also add a Уд-іп. to 3/8-in. drainage plane between the housewrap and the siding by tacking up vertical furring strips. This vented space allows moisture to dissipate naturally so that paint won’t peel prema­turely, surfactants from the siding won’t be in contact with the housewrap, and bulk water won’t be trapped behind the siding with nowhere to drain.

Several manufacturers have started combin­ing the water-shedding benefits of rain-screen-wall construction with the ease of installation and the added benefits found in typical housewrap, creat­ing a separate category sometimes referred to as “drainscreen.”

To the right are a few different designs that aim to accomplish the same basic task.

covered with a fresh layer before siding is installed.

Weatherproofing Comes with Workmanship

Here’s the bottom line: Installation is more important than material choice. No matter what brand of housewrap you choose, you will be wasting money unless the stuff is in­stalled carefully.

Poorly installed housewrap will cause more problems than it solves. Getting the installation right is not hard, but it requires a basic understanding of how housewrap
works. Detailed installation instructions can be found on manufacturers’ websites and often at the lumberyard or home center where housewrap is purchased.

The basic installation premise is to think like a raindrop. Imagine a drop of water hitting the side of your house at the top of the wall. Gravity pulls the drop down along the face of the wall, and as long as all the courses, joints, tears, and penetrations are sealed and lapped in shingle fashion, the drop eventually will reach the ground. The moment that raindrop finds a puncture, a reverse lap, or an unflashed component, it will seep behind the housewrap and into the framing.

Weather Trek®

(Berry Plastics Corporation)

Type: Perforated polyethylene Perm rating: 6.5 Notes: A clever nondi­rectional surface pattern (reminiscent of the texture of a basketball) ensures that water drains easily, regard­less of orientation to the sheathing. 877-832-0333; www. berryplastics. com

GreenGuard

Raindrop®

(Pactiv Corporation) Type: Woven polyolefin Perm rating: 10.0 Notes: Relies on drainage channels woven into the surface to direct water down and out. Channels must run vertically to be effective. 800-241-4402; www. green-guard. com

When you’re using vinyl siding, which comes with built-in drainage holes and fits on the wall loosely, an ordinary smooth­faced housewrap provides good drainage. But with tightly fastened board siding, any water trapped between the siding and a smooth housewrap will sit and eventually could make its way through the housewrap and into the framing. Remember, although many housewraps are good at resisting bulk water, they should not be considered waterproof.

In these cases, it’s a good idea to choose a furrowed rain-screen housewrap. Its embossed texture provides just enough air­space for liquid water to drain away before it has a chance to penetrate the membrane (see "Rain screen and housewrap com­bined," p. 76).

When applying stucco, choose a house – wrap designed specifically for stucco and masonry, such as DuPont’s StuccoWrap or Benjamin Obdyke’s Mortairvent®, which not only provide a corrugated surface but also are compatible with the chemicals in stucco.

Housewrap Choices

(Dow)

Type: Nonwoven polyolefin Perm rating: 6.7 Notes: Membrane has a more substantial, foamlike texture compared to other housewraps. 866-583-2583; www. dow. com/styrofoam

Weathermate (Dow) Type: Perforated, woven polyolefin Perm rating: >5.0 Notes: Translucent; perforated products are less resistant to water intrusion; does not meet the air-barrier requirement of the National Building Code of Canada. 866-583-2583; www. dow. com/styrofoam

Barricade®

(Berry Plastics Corporation)

Type: Perforated, woven polyolefin Perm rating: 9.0 Notes: Translucent; perforated products are less resistant to water intrusion; resists UV- degradation for 12 months. 877-832-0333; www. berryplastics. com

I Still Prefer Felt Paper

Based on my lab testing at the Univer­sity of Massachusetts, if I were buying housewrap today, I likely would choose a nonperforated product because it displays the best water resistance.

As it happens, I have felt paper on my own home. If I could do it over again and choose between felt and housewrap, I’d still choose felt. That’s because I believe that under cer­tain circumstances, felt paper outperforms housewrap.

For example, an ice dam or a roof leak might allow liquid water to get behind the

felt or housewrap. It’s also possible for the sun’s heat to drive water vapor through the housewrap from the outside, where it can condense on the sheathing. In either case, you have liquid water on the wrong side of the wrap. Under these conditions, the liquid water is trapped by the housewrap, which is permeable only to diffusion of water vapor. Felt, on the other hand, absorbs water and dries more quickly to the outside.

Paul Fisette is director of building materials and wood technology at the University of Massachusetts.

R-Wrap (Berry Plastics Corporation)

Type: Nonwoven polyolefin Perm rating: 56.0 Notes: Membrane can be installed with printed logo in or out without change in performance; manufac­turer will replace product if damaged by wind. 877-832-0333; www. berryplastics. com

GreenGuard® Value (Pactiv® Corporation) Type: Perforated, woven polyolefin Perm rating: 15.0 Notes: Low-cost house – wrap for the value­conscious builder. Resists UV-degradation for 12 months; translucent membrane makes it easy to see where to nail siding. 800-241-4402; www. green-guard. com

GreenGuard Classic (Pactiv Corporation) Type: Perforated, woven polyolefin Perm rating: 15.0 Notes: Resists UV – degradation for 12 months. Highly tear – resistant; translucent, glare-reducing green color. 800-241-4402; www. green-guard. com

GreenGuard Ultra (Pactiv Corporation) Type: Nonwoven polyolefin Perm rating: 48.0 Notes: Uses a reinforcing scrim that makes it highly tear-resistant. Translucent membrane makes it easy to see where to nail siding. 800-241-4402; www. green-guard. com

Fisette conducted independent testing of housewrap not to establish quantifiable data that mimicked real-world performance, but rather to subject the products to a set of sim­ple laboratory conditions to see how they compared. For more on Fisette’s testing, see www. umass. edu/bmatwt/publications.

According to Fisette’s research, the best housewraps (those that resist water infiltra­tion and also permit water vapor to evapo­rate) include Tyvek HomeWrap, R-Wrap by Berry Plastics™ Corporation, Typar® (manu­factured in 2003 or later), and—believe it or not—traditional #15 felt paper (see the side­bar on p. 75).

I prefer Tyvek, which scored well for resisting water penetration in the Massachu­setts study while also having one of the industry’s highest perm ratings for water- vapor diffusion. Although #15 felt paper costs less and scores well in all categories,

I like housewrap products because the vari­ety of sizes available (3-ft. to 10-ft. widths) really speeds up the installation process.

Also, the compatible sealing tapes and acces­sories make housewrap a superior air barrier compared to felt paper.

Nowadays, any approved weather-resistive barrier, from #15 felt to high-tech house – wrap, touts the dual benefit of being a weather-resistive drainage plane that also allows the passage of water vapor. But not every product balances these two features equally. To add to this confusion, house – wraps are now available in dozens of variet­ies, so how do you choose? Unfortunately, there’s no easy answer.

The American Society for Testing and Materials (ASTM) is working to standardize the tests used to evaluate weather-resistive barriers. For now, when trying to gain code approval, manufacturers can choose from at least two dozen different tests. Even if two manufacturers choose the same test, though, there is nothing to regulate the way in which the test materials are set up. This vari­ability makes it nearly impossible to com­pare one product’s performance to another’s.

According to Paul Fisette, director of building materials and wood technology at the University of Massachusetts, one tested value that usually stands up to side-by-side comparison is a material’s permeance rating, but sometimes even that can be misleading.

1. Create a secondary weather barrier behind the siding, preventing wind-driven rain and other water from reaching the sheathing.

2. Serve as an air barrier

to prevent air infiltration, helping to reduce heating and cooling costs.

3. Provide a vapor – permeable membrane

that allows moisture in framing lumber or insula­tion to escape.

Perm Ratings Tell Part of the Story

Permeance ratings, or perms, reflect the measure of a material’s ability to transfer water vapor; the higher the perm number, the more permeable the material. For instance, 6-mil polyethylene sheeting has a very low perm rating of 0.06, which means that it prevents the passage of nearly all water vapor. Current building codes require a weather-resistive barrier to match or exceed grade-D building paper, which has a perm rating of about 5.0. To meet this requirement, perm ratings for commonly available brands of housewrap range from about 5.0 for Dow®’s Weathermate™ to

58.0 for Tyvek®’s HomeWrap®.

Materials with higher perm ratings speed the escape of trapped moisture. But higher ratings do not necessarily equal better housewraps, because the methods of achiev­ing a high perm rating can be different.

For instance, some low-tech housewraps achieve their high perm ratings with mechanically punched perforations in the membrane. These perforations increase the passage of water vapor, but they also make the housewrap more susceptible to bulk – water leakage.

On the other hand, more-advanced non­perforated housewraps, such as HomeWrap and R-Wrap®, offer even greater moisture – vapor transmission (higher perms) than their perforated counterparts. They are also more effective at preventing the movement of bulk water.

■ BY FERNANDO PAGES RUIZ

hen I started building houses nearly 30 years ago, we lapped lightweight #15 asphalt – or rosin-impregnated building paper directly over the stud framing before installing the siding. Nowadays, concerns with energy-efficient construction and moisture infiltration have focused a great deal of attention and no small amount of high-tech chemistry on this thin layer of paper. Although some builders still advocate the felt-paper barriers of yesteryear, most have switched to plastic-based housewraps, products designed to stop air infiltration and wind-driven rain while allowing water vapor to evaporate—a great concept.

However, like everything high-tech, new solutions come with new problems. The range of choices and the precise installation requirements of modern housewraps challenge builders with terms like spun – bonded, polyolefin-based moisture, and air-infiltration fabric. Even if the technical terminology is hard to remember, learning how to install these products correctly is im­portant. Yet a quick look around a construc­tion site reveals that most builders, with thirty years or with three behind the ham­
mer, are having a hard time handling this new technology.

There’s plenty of confusion surrounding weather-resistive barriers. Many home­owners and builders don’t know which prod­uct to choose, some builders never learned how to install it correctly, and many have no idea what housewrap does in the first place.

What Does a Housewrap Do?

Placed beneath the siding, housewrap is a second layer of defense for your home. When installed properly, it performs three basic functions (see the drawing on the facing page).

First and foremost, housewrap acts as a backup barrier that keeps water off the struc­tural sheathing and framing. Properly in­stalled siding is the first line of defense, but sometimes wind-driven rain and snow still find a way through. Housewrap also func­tions as an air barrier that stops hot – and cold-air movement through the wall cavity. As long as joints are sealed properly, house – wrap is designed to cut utility costs and

increase comfort by reducing air infiltration and potential drafts.

The real magic of housewrap lies in its third function: allowing the free passage of water vapor so that wall cavities and framing lumber can dry to the outside of the build­ing, reducing the threat of mold and rot. Without this feature, installing housewrap would be like putting a thick raincoat over your house: great for keeping out the rain, but terrible at releasing water vapor from within. Instead, housewraps are designed to act like a Gore-Tex® jacket, allowing water vapor to pass through the building envelope in case moisture problems arise.

In most closed-cell foams, such as those made by Corbond®, an HFC blowing agent is captured in the foam’s cell structure. This gas has a better thermal performance than the air-filled open-cell foam and gives it a higher overall R-value. However, while HFC – blown closed-cell foam might initially have an R-value as high as R-8 per in., its R-value diminishes as the blowing agent evaporates through cell walls and is replaced by air. Closed-cell foam’s "aged" R-value is roughly R-6 per in. Some manufacturers produce water-blown closed-cell foams. These foams have the same performance properties as HFC-blown foam, but slightly lower R-values, at around R-5.5 per in.

Closed-cell foam’s greater density, 2 lb. per cu. ft. compared with open cell’s 1/2 lb. per cu. ft., also increases its R-value and offers it the rigidity that open-cell foam lacks. Tests at the National Association of Home Builders research center confirmed that closed-cell foam can actually increase the shear strength of conventionally framed walls by 30%. Closed-cell foam also has a low vapor-permeability rating (roughly 0.5 perms at a thickness of 3 in.) and is con­sidered a class-II vapor retarder, meaning it’s semi-impermeable.

OPEN CELL

Density: Vi lb. per cu. ft. R-value: 3.5 per in.

Vapor permeability: Permeable Air barrier: Yes Blowing agent: Water

Open-cell foam, made by companies such as Icynene® and Demilec®, has a greater expansion rate than closed-cell foam. It expands 100 times its initial volume (closed­cell foam expands only 30 times its initial volume), so less of the foam is needed to insulate a house.

Open cell’s one major weakness is its lower R-value, roughly R-3.5 per in. This means that when used in a 2×4 exterior wall, it will create an assembly that’s approxi­mately only R-12, which won’t meet code in most parts of the country.

Installing Lots of Foam Isn’t as Effective as You Think

A lot of energy-conscious architects and builders shoot for the highest R-values they can possibly attain: R-40 walls and an R-60 roof. However, R-values aren’t necessarily an accurate reflection of overall thermal perfor­mance. For example, you would think that an R-40 wall full of spray foam would per­form twice as well as a wall sprayed to R-20 with the same foam, but that’s not the case.

Chris Porter, the building-science and code manager for BioBased Insulation®, explains that "open-cell foam reaches a point of diminishing returns at around
5 in. That threshold is even lower for closed­cell foam, which experiences diminishing returns at around 3 in. or 4 in." Those thick­nesses create assemblies between R-20 and R-24, which by the numbers seem a little weak. Each additional inch of spray foam

More Than One Way to Use Spray Foam: Two Experts Weigh In

Most experts agree that spray polyurethane foam is a revolutionary product. What they don’t always agree on is the way it’s installed and integrated into a building assembly. To shed some light on this debate, energy-efficient building expert Bruce Harley (Westborough, Mass.) and architect Peter Pfeiffer (Austin, Texas) explain how they use spray polyurethane foam to insulate the homes they build.

PETER PFEIFFER No other insula­tion system I am familiar with provides the real R-value that spray foam does, ac­complishes the air-sealing it does, or thwarts vapor flow as well. Closed-cell spray foam greatly re­duces the chance for condensation within the framing of a home. I think it is critical that houses be built to thwart vapor flow correctly. I insulate all homes pretty much the same way. However, in colder climates, I use 2×6 exterior walls and insulate the basement or crawlspace. Peter Pfeiffer of Barley & Pfeiffer Architects is a LEED-accredited architect and building scientist who has spent the past 30 years developing high – performance building-design strategies.

R-17 WALLS

R-19 BASEMENT

Bruce Harley of Conservation Services Group is an energy-efficient construction expert and author of Cut Your Energy Bills Now (The Taunton Press, 2008).

Fiber-cement siding Vapor barrier extends 12 in. up wall and is secured with mastic.

Damp proofing

Consuming fossil fuels to make products intended to conserve fossil fuels makes little sense to a lot of people. All spray foams contain a certain level of petroleum in their A component and in their B component. Manufacturers such as BioBased Insulation, Demilec, and Icynene have created more environmentally benign spray-foam products by reducing the amount of petroleum used in their B component. They replace a portion of the polyol resin, which makes up 20% to 30% of the B component, with a renewable resource such as soybean or castor-bean oil. Apex even has a sucrose-based polyol. Manufacturers say that the transition to bean oil or sucrose doesn’t alter the look or the performance of open – or closed-cell foam in any way.

The amount of soybean, castor bean, or sucrose found in foam varies by manufac­turer, so identifying the “greenest” foam might not be so easy. According to the U. S. Department of Agriculture, only 7% of a spray-foam product needs to be made of a renewable resource to be labeled as a bio­based foam. This, of course, doesn’t factor in the petroleum fueling the crop-cultivation process.

yields little performance. In fact, while the cost of an R-40 wall is indeed double that of an R-20 wall (not factoring in the construc­tion materials used to create deeper cavities for the extra foam), it reduces the heat flow through a wall by only an additional 2%.

For this reason, Porter says that in most parts of the country, 6 in. of foam—be it open or closed cell—is perfectly adequate.

Spray foam is priced based on board feet. Manufacturers don’t price their product. Instead, cost is determined by installers. The spray-foam market is extremely competi­tive, and spray-foam prices can be astonish-