[Editor’s Note: The new book, Pretty Good House: A Guide to Creating Better Homes (Taunton Press) provides guidance on water and moisture management in residential home construction. The following article is an excerpt that discusses rain control layers, rain screens, vapor control layers, wall assemblies, roof assemblies and more.]
A house is always under pressure, from gravity, wind, and air, but also from water either as a liquid trying to get in or as vapor trying to get out. A Pretty Good House is designed to manage moisture migration through the building assemblies. Failure to do so could mean condensation. And where there’s water,air, and organic material (such as the wood that comprises most of your building materials), conditions are ripe for mold, mildew, and decay.
When designing a Pretty Good House, keep in mind the four control layers of a building envelope (as listed by the Building Science Corporation and shown in the diagram), know exactly where those layers are in the building assembly, and ensure their continuity.
The thermal control layer mainly refers to insulation and the insulative values of windows and doors. A continuous air control layer seals the building envelope as tightly as possible, and it should be readily visible on construction drawings.
In this article, we’ll focus on the other two critical control layers: the rain (or weather) control layer and the vapor control layer.
RAIN CONTROL LAYER If you can’t keep the rain outside the building envelope, none of the other layers really matter. Therefore, making sure your house will shed water (whether as rain, snow, sleet, or hail) is more important than worrying about other control layers. This seems obvious, and any builder or architect worth a damn is going to be familiar with the traditional methods of shedding rain: pitched roofs, ice and water shield in the trouble areas, good flashing details, roof overhangs, proper lapping of membranes at window connections, and so on. A Pretty Good House takes this a little further.
Our mantra when drawing up construction details is this: “Be the water drop.” Water doesn’t just fall straight down from the sky. Here in Maine, it exists in all three different states of matter and blows from every direction. And water has its own behavior when moving on surfaces: it clings, it spreads, it can be absorbed. A PGH detail will always shed the water away, will always work with gravity, and will always minimize or eliminate points of entry.
A leak needs both water and a driving force. Usually, this force is gravity. Sometimes it’s wind. But what not-so-good detailers often fail to realize is that the pressure differential between inside and outside can be a substantial driving force. As a water drop passes a small air leak, let’s say at a window connection, wind can cause the house to suck that water drop into the wall assembly. We need to either stop that entry point or give the drop a way to escape (such as a rainscreen cavity behind the siding or trim).
A PGH controls water below ground as well as above. Gone are the days when you would just pour a concrete foundation, backfill, and call it good. It’s a recipe for disaster. Concrete is porous and absorbs moisture even when fully cured. While concrete is,by and large, unaffected by the presence of water, it will allow water to permeate through it, leading to the wetting of critical building elements such as sill plates, flooring, and posts. We’ve all been in basements that have a musty smell. This odor is an indicator of higher relative humidity, and even though it may be the norm in basements, high humidity is a major contributor to both poor air quality and mold and rot.
The first line of defense is the application of a good waterproofing membrane on the exterior of the foundation (this isn’t necessary if it is just a frost wall for a slab on grade because there’s no occupied space inside the walls). Available products include membranes and fluid-applied waterproofing. This is the equivalent of a weather control barrier, only below grade.
Just as air pressure can drive water into a house, hydrostatic pressure (the pressure that ground water exerts on the foundation) can push water through any flaws in the foundation or waterproofing membrane.Creating a drainage plane on the exterior of the foundation wall is a fantastic second line of defense. This can mean backfilling with a fast-draining material(like crushed stone or gravel) or installing a dimpled membrane against the foundation wall. Water drops to the footing where it can be collected and drained to daylight. If this is impossible, then of course an old-fashioned sump pump can be used in the basement. But if you’re installing a sump pump, you’re already assuming some failures and ruling out the use of the basement for living space.
Because concrete is thick, porous, and usually below grade, it almost always contains some moisture.Therefore, untreated wood (or any other organic material) should never come in direct contact with concrete. Through capillary action, concrete can lift water against the pull of gravity, sometimes many feet up a foundation wall. A PGH will prevent damage from this natural phenomenon by protecting vulnerable house parts with a capillary break.
There are two common locations for a capillary break: across the top of the footing before the concrete wall gets poured, and at the top of the concrete wall where the wood framing begins.
The footing is a wide band of concrete that is poured before the foundation walls. It is wider than the walls(typically 16 in. to 24 in. wide), and it helps distribute the weight of the foundation (and thus the house that sits on top of it). Sealing the top of the footing helps prevent water from wicking up into the wall from the ground below.
RAINSCREENS
One of the easiest and most important things you can do for a wall assembly is to include a rainscreen. A rainscreen is essentially an air gap behind the siding, vented at the top and bottom to allow airflow behind the siding. Rainscreens are usually made by installing strapping (vertical wood strips) between the water-resistant barrier (WRB) and the siding. Wood siding, if allowed to dry properly, will be far more dimensionally stable, hang onto its finish coatings much longer, and be far more durable. This goes for other painted products that can also absorb water, such as fiber cement siding, as well.
When dealing with siding that needs more frequent nailing, such as shingles, a rainscreen matrix can be used. This material is a plastic mesh that creates an even surface for the installation of shingles while still maintaining an air gap behind them.
VAPOR CONTROL LAYER We need to get a little technical to discuss water vapor in building assemblies. You can rely on physics to be the same at any given time in any climate zone in the country. If you want to make tea in Florida or in Alaska, you can put water in a tea kettle, put it on a heat source, wait for the water to heat up, and pour it in a cup. The tea kettle whistles because the water is changing state, from a liquid to a vapor. Why are we talking about tea in a building-science chapter?It’s because these simple principles of temperature,pressure, and the phases of water, will be discussed quite a bit and this little tea kettle will be a good reference point.
BASIC PHYSICS OF PRESSURE AND THERMODYNAMICS
Heat flows from hot to cold, or more accurately from high energy to low energy. If you set that hot teakettle on a trivet in a room that is 70ºF, eventually the high energy heat of the kettle will radiate out into the room. Eventually the tea kettle and the water inside will become the same temperature as the room.
What is a little less intuitive is that vapor density acts in much the same way. While the water was piping hot in the kettle, you would expect the steam to escape as it moved from high pressure to low pressure. But what about when the kettle is at the same temperature as the air in the room? The whistling stopped, the steam stopped. Everything is equal, right? Well, no. There’s water in that kettle in a slightly contained environment. The vapor pressure in the kettle is slightly higher. That vapor seeks equilibrium with its surroundings, just like the heat.
Water vapor is often the cause of moisture-related problems in buildings. Think of a PGH as a giant tea kettle. Imagine a cold winter day. Inside, there’s a family gathering. It’s warm. They’re making pasta downstairs, while Grandpa takes a long, hot shower upstairs. The humidity level is on the rise. It’s not uncomfortable for the occupants, but there is now substantial outward vapor pressure on the building envelope (like the pressure building in the tea kettle) and that pressure is from the warm higher humidity inside toward the colder lower humidity outside.When warm moist air finds a cold surface, it will condense, causing water to accumulate in some of the worst places for your building. A Pretty Good House will have a very well-defined vapor control layer that is continuous around the entire building envelope. It can be comprised of many different materials and membranes.
Prior to the 1950s, no one really worried about vapor control, because houses were built from vapor-open materials and were so leaky and draughty that they had excellent drying potential. With the invention of plywood and drywall, houses started to get tighter and less permeable, and problems with moisture became more common.
CONDENSATION: MOLD AND MILDEW’S IRRIGATION SYSTEM
For mold or mildew to grow it needs two things: organic materials (food) and high relative humidity(water). It is nearly impossible to eliminate the food source, since we build our buildings mostly from organic materials (wood and cellulose) and the dust in the air is comprised mostly of organic material. So, we must manage the relative humidity or moisture content of our building and the building assemblies to minimize the risk of mold or mildew.
The building envelopes of our homes are far less perfect, and more permeable, than the walls of the tea kettle. Each little leak is a place where vapor is allowed to escape and force its way into our building assemblies. This is why air sealing is far more important than vapor sealing. The amount of water and water vapor that can travel through a leak in the building envelope is far greater than the water vapor that can permeate through different materials. Once all the air leaks are sealed, however, we still have the problem of water vapor passing through permeable materials such as wood, insulation, or drywall. Therefore, it is important to understand exactly where the vapor control layer is located.
For example, if there is no vapor barrier on the interior of a 2x6 stud wall, then the most vapor-impermeable material in the assembly is most likely the exterior sheathing. On a cold winter day this sheathing also will be cold, and if water vapor makes its way to this surface it will condense, form water droplets, be absorbed, and ultimately promote the growth of mold and mildew.
In the 1970s, ‘80s, and ‘90s, installing polyethylene (“poly”) vapor barriers on the interior (just behind the drywall) was common practice to mitigate this problem. It had little success in all but the coldest climates and varying degrees of failures in all climates. Theoretically it was a good idea, but in practice the continuous membrane was usually flawed with uncountable staple and nail holes, tears, and other failures that allowed the moisture from the interior of the house to pass into the wall assemblies. Adding insult to injury, the poly greatly reduced the drying capacity of the wall and roof assemblies to the interior, trapping the moisture in what’s called a “vapor sandwich.”
A PGH needs a better vapor control layer than that. Before we dig into methodology, let’s get some terminology out of the way. We’re going to be talking about vapor retarders, which are classified by how open they are to water vapor movement (as classified by the International Code Council and ASHRAE):
•Class I vapor retarder, 0.1 perm or less (may also be called vapor barrier); vapor impermeable.
•Class II vapor retarder, more than 0.1 perms and up to 1.0 perm; vapor semi-impermeable. •Class III vapor retarder, more than 1.0 perm and up to 10 perms; vapor semi-permeable.
•Vapor permeable (or vapor open) is a material greater than 10 perms.
WALL ASSEMBLIES There are four basic methods that a PGH designer can take to establish a vapor control layer within a wall assembly. •Vapor-variable membranes. These specialized plastics have pores that open and close based on relative humidity and are capable of regulating the transmission of water vapor. •Insulation as a control layer. Relying on the vapor-closed properties of certain insulation types(usually spray foam) to keep water vapor from reaching and condensing on the sheathing or other layers in the wall assembly. •Vapor-open assemblies. Ordering the layers of the wall assembly so that the most impermeable is in a warm part of a wall and safe from condensation. •Warm sheathing as vapor control layer. This is an “outsulation” approach, where the sheathing is used as the vapor control layer with the majority of the wall assembly’s insulation on the exterior of the sheathing. This approach keeps the sheathing(vapor control layer) warm, thereby greatly reducing the risk of condensation.
VAPOR-VARIABLE MEMBRANES
When you finished reading the previous chapter on building envelope basics, you may have come away thinking that double-stud construction is probably one of the best ways to create a high-performance wall assembly. But where is the vapor control? Everything you’ve read in this article would lead you to think that this is a dangerous assembly with the Class II sheathing on the extreme cold side of the assembly.If water vapor were to migrate through from the interior to that sheathing, it could condense and become active water within the wall. To reduce the mold risk of a wall assembly like this, we need a way to stop that vapor from entering the wall—a vapor control layer on the warm side of the wall.
In the earlier days of PGH construction, some builders and architects addressed this by following the airtight drywall approach as prescribed by the Building Science Corporation at the time. Essentially,this means using the painted drywall in combination with air-sealing gaskets at penetrations as the vapor control layer. However, with the advent of more modern materials, that method is rarely used today.Instead, builders opt for vapor-variable membranes.These membranes are applied to the interior of the assembly (just behind the finish material). But unlike poly used in previous decades, these membranes are composed of materials that become less permeable as the humidity increases on its interior side and more permeable as the air dries. This creates a membrane that slowly opens up to allow assemblies to dry to the interior when that option is possible while still inhibiting vapor entry from the interior.
There are a number of membranes like this on the market today, including offerings from Pro Clima,CertainTeed, Siga, Delta, and Rothoblaas. Hopefully there will be even more in the future. They’re all a little different, but in general they are designed to have low permeability when the relative humidity is low and higher permeability when the humidity increases. The idea is that the membrane will block vapor from migrating into wall and roof cavities in the winter, when indoor humidity is low, and open up in the summer to allow any accumulated moisture to dry to the interior. Siga’s Majrex is a little different. Rather than changing permeability to any significant extent, it is designed so that it’s always harder for water vapor to get into the wall cavity than it is for trapped moisture to get out.
All of the membranes have their merits, and arguments abound as to which is better. Regardless of which you go with, make sure that a vapor-variable membrane is installed correctly and in accordance with the manufacturer’s instructions. One benefit to the newer membranes on the market is that unlike polyethylene they are made from more rugged materials. But they still need to be installed in combination with tapes and other sealants to maintain the integrity and continuity of the membrane. In a hot and humid climate, remember that the vapor drive is reversed from that of a typical cold-climate building. In these climates, we are more worried about water vapor from the exterior of the building condensing on the cold innermost layer (usually drywall). Therefore, installing a membrane on the interior is unnecessary.
INSULATION AS A VAPOR CONTROL LAYER
This approach uses the insulation itself as a vapor control layer, which means it is comprised of a vapor-impermeable material such as closed-cell spray foam. (You could also use polystyrene on the interior in this manner, but the cost, level of detailing, and level of performance of rigid EPS or XPS make them poor choices for this method.)
For a variety of reasons, most PGH builders avoid the use of spray foams whenever possible. First, spray foams are made of toxic materials that can offgas into a home if not installed correctly, and they are unhealthy for the installers. Second, many spray foams use blowing agents with a high global-warming potential (GWP), making their ecological payback multiple decades away (and from a climate standpoint, we don’t have that kind of time). But spray foams are getting much better as time progresses, with different chemicals and blowing agents. Some even have blowing agents with a GWP of 1, which helps their overall climate impact, making them similar to EPS insulation in this regard. If the spray foam is a closed-cell spray foam (and qualifies as a Class I or II vapor retarder) and can be continuous across the surface of the interior of the assembly, it is an effective vapor retarder. But be warned that spray foams have been known to shrink and pull away from the surfaces upon which they have been sprayed. This has the potential to open small fractures and cracks for both air and moisture infiltration. Therefore, the classic technique of only doing spray foam in between framing members and not completely encasing them can add up to an enormous number of tiny failures in the vapor control layer (as well as the air control layer).
VAPOR-OPEN ASSEMBLIES
Another approach is to forgo the use of a membrane and reorder the components of an assembly based on their permeability so that the assembly is vapor open.
The Larsen truss wall method is a good example (diagram below, left). In this assembly, the sheathing, usually a Class II vapor retarder and the most vapor-impermeable material in the assembly, is on the warm side of the wall. From there moving out, there are vertical trusses (I-joists) filled with dense-packed cellulose, and a water-resistant barrier adjacent to a vented rainscreen cavity. From the sheathing outward, the wall is composed of highly permeable(vapor-open) materials, so that the wall can always dry to the exterior, minimizing the risk of mold or mildew.
This vapor-open construction can also be paired with an “outsulation” approach as described in the previous chapter: that is, installing layers of rigid insulation outside of the sheathing, essentially wrapping the outside of the building in a continuous layer of insulation (see the diagram above, right). If that outsulation is a material such as mineral wool or rigid wood-fiber insulation, then it, too, is a vapor-open assembly and has a very low risk of mold or mildew.
WARM SHEATHING AS VAPOR CONTROL LAYER
You might be thinking to yourself, “if the sheathing is a Class II vapor retarder and on the warm side of the assembly, isn’t that enough to have an effective vapor control layer?” The answer is yes. If you construct an assembly with outsulation as described above, then that insulation doesn’t necessarily need to be vapor open. It can be vapor closed (a Class I vapor-retarding material such as foil-faced rigid polyisocyanurate or XPS insulation). The strategy is to place enough of a building’s R-value on the cold side of your vaporcontrol layer, so that vapor control layer is safe from condensation.
So why, you may ask, do we even care about the vapor-open assembly if this warm sheathing method works? A PGH will always opt for drying mechanisms over wetting prevention. In other words, having an impermeable vapor control layer that keeps interior moisture out is pretty good, but having the ability to dry any moisture that does make it through is even better.
ROOF ASSEMBLIES Just like walls, roof assemblies need a vapor control layer. All the general strategies mentioned previously for walls are very similar for roofs. The major differences are that they exist at the top of the building envelope where the vapor pressure from the interior is greatest, and that to stand up to the elements their outer layer is almost always completely impermeable (asphalt shingles, standing-seam metal, EPDM roof membranes, etc.). Thus, it is even more important to get your detailing right.
•Vapor-variable membranes with vented roofs. Using a vapor-variable membrane as the control layer and under-sheathing venting as a rainscreen.This relieves any vapor pressure that might buildup in the assembly and allows drying to the underside of the sheathing in case there are any minor leaks that develop over time. •Insulation as a control layer with unvented (or “hot”) roofs. Similar to the same approach for walls, this approach relies on the vapor- impermeable properties of certain insulation types to keep water vapor from reaching and condensing on the roof sheathing. •Outsulation roof assemblies. As with walls, this method puts vapor-closed insulation on the exterior of the sheathing, which is used as the vapor control layer. •Vapor-open roof assemblies. Similar to the same approach for walls, layers of the roof assembly are ordered so that the most impermeable layer is in a warm part of the roof and so the assembly can dry to the exterior freely.
*Editor’s Note: Learn more about Best Practices in home construction by picking up Pretty Good House at your favorite a book retailer. Pretty Good House provides a framework and set of guidelines for building or renovating a high-performance home that focuses on its inhabitants and the environment―but keeps in mind that few people have pockets deep enough to achieve a “perfect” solution. The essential idea is for homeowners to work within their financial and practical constraints both to meet their own needs and do as much for the planet as possible. Pretty Good House is authored by two architects (Chris Briley and Emily Mottram), a designer (Michael Maines), and a building contractor (Dan Kolbert), all living and working in the southern half of Maine. Visit www.prettygoodhouse.org.