Building logo


When asked to describe characteristics of an energy-efficient building, most might list the following: well-insulated walls, a ventilated roof with a thick layer of insulation over the ceiling, quality windows with low-E glass, and a high-efficiency heating and cooling system. Surprisingly, many buildings with these features experience higher than anticipated utility bills, elevated levels of moisture or indoor air pollutants, and premature deterioration caused by moisture accumulation in walls and roofs. Why is this happening and what can be done to avoid these problems?


RULE #1: Seal all joints in the building shell. It doesn’t make sense to invest in well-insulated walls and ceilings yet do little to block air flow around and through the insulated cavities. Wood-to-wood and drywall-to-wood joints in the exterior shell of a building are not airtight and should be sealed with gaskets, foams, caulks, or air barrier films. The illustration Air Leakage Pathways shows where these sealing products are needed in conventional residential wood-frame construction. Other types of residential and commercial building systems have similar pathways.

Since buildings are always moving in response to changing wind, moisture, and temperature conditions, it is critical that air-sealing products last as long as the structure and be capable of withstanding movement. Building gaskets are preferable to foams and caulks because they last longer, respond better to movement, and don’t rely on adhesion to maintain an air seal (see Building Gaskets). Most exterior air-control films (“housewrap”) are of questionable value because they are difficult to install properly under field conditions and have a relatively short lifetime. Stabilized polyethylene films (see Air-Vapor Films) can provide reliable interior air-control in cold or temperate climates provided the wall or roof is designed to prevent vapor condensation.

RULE #2: Eliminate unnecessary holes and seal all those that are unavoidable. Locate electrical outlets and switch boxes on interior walls wherever possible, and try to use surface-mounted lights instead of recessed fixtures on insulated ceilings. Where electrical work on exterior walls and ceilings is unavoidable, tightly seal all box edges and wiring holes. Seal plumbing stacks with sheet-rubber gaskets slid over the pipes and mechanically clamped around the edges. Seal fire chases around chimneys with sheet metal flanges and high-temperature sealants. Insulate and gasket attic access doors or replace them with air-sealed versions (see Attic Access). Build removable gasketed lids for whole-house fans, or better yet, don’t install this type of fan in an unheated space. Avoid air leakage through unsealed spaces behind bathtubs, showers, staircases, and other construction adjacent to exterior walls by pre-installing plywood or drywall air barriers.

RULE #3: Block heat conduction pathways through framing lumber (see Heat Conduction Pathways). Since wood is not a great insulator and conventionally framed walls and ceilings are often 20% lumber, heat loss through the wood is a major component of the total conduction heat loss of a building. As an example, a 2x6 wall with R-19 fiberglass batts and plywood sheathing often has an effective R-value of less than R-15. An effective way to reduce framing heat loss is to apply insulating sheathing: 1” is the minimum required for mild climates and 1-1/2” or more should be used in hot or cold climates. As a side benefit, exterior insulating sheathing will greatly reduce the potential for water condensation on the back side of wood sheathing. As an alternative to foam sheathing, consider “strapping” the interior with horizontal 2x2’s and insulating in between the strapping.

Heat loss through window headers, exterior corners, and rim joists can be significantly reduced with simple framing modifications. Install rigid insulation between the inside and outside window and door headers, frame corners so that they are open on the interior and can be filled with insulation, and recess rim joists to create a space for several inches of insulation. Use raised-heel trusses or other roof framing modifications that create space for more insulation at the corners where walls and ceilings meet, thereby reducing heat loss and lessening the chance for mold and mildew at these points.

RULE #4: Completely fill all cavities with insulation. Even the smallest gaps between insulation and framing can cause a significant loss of insulation performance, especially where then is a space between the insulation and the drywall that allows air to move freely. Insulation should completely fill all wall and ceiling cavities and should be installed flush with the interior surfaces. When using batt insulation such as fiberglass, always use unfaced batts: if cut properly, they will stay in place in both walls and ceilings without any support. Avoid kraft-faced or foil-faced fiberglass because the paper facing hides the insulation from view, making it difficult to determine if it has been fit properly. If you must use faced insulation, staple the facing to the edges of the joists or studs, not to their sides, to avoid creating any cavity behind the drywall. As an alternative to batt insulation, use a spray-applied cellulose or fiberglass with an adhesive binder, or use spray applied foam.

RULE #5: Insulate foundations adequately, preferably on the exterior. Basements are rarely insulated to the same standards as upper floors, even though basement walls often have a similar exterior exposure. When an uninsulated or poorly insulated basement is later converted to living space, it is often difficult to retrofit sufficient insulation, and many basement insulation techniques can create condensation problems that can lead to mold and wood rot. Basements should be properly insulated during the initial construction process, preferably on the exterior. Exterior insulation reduces the chance of condensation and eliminates the extreme thermal cycling that causes foundations to crack.


Buildings cannot be designed solely for energy-efficiency: they must also be designed for durability, comfort, and health. Failure to consider the impact of air-tightening on indoor comfort and air quality has led to serious moisture and air-quality problems. Consider some of the changes in the way we have built and lived in houses over the past few decades:

• In the past, building lumber was usually air dried over a period of many months, and the building process was so slow that even wet lumber could dry fully before the walls were closed in. Today, buildings often use lumber that is inadequately kiln dried, and the walls are closed in so quickly that the wood cannot dry on site.

• In the past, buildings had little insulation, so there was plenty of air circulation in stud and joist cavities to promote drying. Today, buildings have walls and roofs filled with insulation that blocks most air circulation in the cavities.

• In the past, buildings had board sheathing that was quite leaky and had a high moisture-absorption capacity. Today, buildings have plywood, chipboard, or foam sheathings that are air and moisture impermeable and have little or no moisture storage capacity.

• In the past, buildings were heated with combustion furnaces that consumed interior air and created negative interior air pressure. As air flowed from the outside in, the air temperature rose and its humidity fell, drying the wood framing. Today, buildings have heat-pumps and high-efficiency sealed combustion furnaces that do not require inside air, so buildings often operate under neutral or even positive pressure.

• In the past, buildings had leaky single-pane glass windows that let in plenty of dry outside air and caused excess interior moisture to condense as water in cold climates, dehumidifying the air. Today, buldings have tight insulated glass units that let in little air and cannot provide winter dehumidification by condensation because their interior surfaces are too warm.

• In the past, foundations could be economically backfilled with gravel and good soil. Today, buildings are usually backfilled with the same material that was excavated which often drains poorly and allows water pressure to build against the foundation.

• In the past, buildings had small, simple bathrooms. Today's buildings have elaborate baths equipped with multiple sinks, whirlpools, hot tubs, and saunas, all of which generate moisture.

• In the past, the average building contained a few potted houseplants. Today, many buildings contain a rainforest of plants, and some have integral sunspaces, greenhouses, waterfalls, pools, and other moisture generators.

• In the past buildings were made of mostly natural materials such as plaster, wood, clay tile, and natural fibers. Today, most building products and furnishings are made of plastics and composition materials emitting a bewildering assortment of chemicals. Combined with the dozens of new chemical-laden cleansers, paints, glues, and personal hygiene products, the emission of indoor air pollutants has risen enormously.

All this is a formula for disaster, so it should not be surprising that problems with uncomfortably high moisture levels, interior mold and mildew, rotting in sheathing and framing, and high levels of indoor air pollution have appeared. The issue is how to prevent these problems without reducing the level of interior thermal comfort we have come to expect.

Some builders have proposed that we leave framing joints a little leaky, omit air and vapor barrier films, and generally build less airtight. Unfortunately, while these actions may sometimes reduce visible interior moisture problems, they can significantly worsen invisible moisture problems within walls and roofs where the potential for lasting damage is much greater. In addition, leaving framing joints leaky or omitting vapor barriers may have little or no effect on indoor air pollution levels, since there is considerable evidence that very tight buildings do not have higher levels of indoor air pollution than other new houses and may, in fact, have lower levels. A better way to avoid moisture and air-quality problems is to follow these five additional construction rules that directly address the problems:

RULE #6: Block all pathways for moisture to enter walls and ceilings from the exterior. The illustration Moisture Transmission Pathways shows the many ways in which moisture can get into a wall or ceiling from the building exterior. One common problem is the direct flow of water though roof and walls, such as when wind-driven rain manages to pass between roof shingles or wall siding boards. Ventilating under the roof or behind the siding generally solves this problem by equalizing air pressures on both sides of the shingles or siding, removing the driving force. A simple way to vent siding is to install it on furring strips: in the case of wood siding, this will have the added benefit of reducing the chance for warping and paint-peeling due to uneven wetting and drying. Rigid roof units such as wood shingles, slates, and tiles can similarly be installed on furring.

Below ground, water enters through floor slabs and foundation walls by capillary wicking, moisture diffusion, and by direct flow through cracks. Wicking and diffusion through floor slabs can be eliminated by using polyethylene and gravel under floor slabs. Wicking and diffusion through foundation walls can be eliminated by exterior dampproofing or waterproofing. Gravel or synthetic drain materials can relieve hydrostatic pressure against both floor slabs and foundation walls if a drain line carries water to daylight or to a sump. It is also important to provide capillary breaks at footings and between the foundation wall (polyethylene sheet or rubber) and sill plate (rubber sill-plate gasket) to prevent moisture from rising up the foundation wall and into the wood framing.

RULE #7: Block all pathways for moisture to enter walls and ceilings from the interior. The illustration Moisture Transmission Pathways also shows the many ways in which moisture can get into a wall or ceiling from the building interior either by transmission in air that flows through holes and cracks or by diffusion through drywall and other building materials. Of these two mechanisms, air-carried moisture is the most significant. In cold or moderate climates, air-carried moisture can be stopped by an airtight seal at the inner surface of the wall. An exterior seal would not be as effective in preventing the entry of warm, moisture-laden air into the wall where the water might condense. The methods for creating interior air seals have already been identified (seal all building joints and holes with rubber gaskets, continuous sheets of air-impermeable materials, molded wiring enclosures, etc.).

Moisture movement through diffusion can be easily stopped with vapor barrier films, paints, or other materials with low moisture permeability. It is generally not necessary to have a perfect diffusion barrier unless, as can be done with a quality polyethylene film (such as our Tenoarm film), it is desirable to have one product serve as both an air transmission and vapor-diffusion barrier simultaneously.

RULE #8: Design walls and roofs so that water and moisture that enters can’t do any damage. Regardless of how effective a defense is built to keep water and moisture out of walls and roofs, some will always get in. Good design provides a way for this water to drain or diffuse outward without jeopardizing the integrity of the wall or roof system. Using a thick layer of insulating sheathing is one of the best precautions: if the insulation is thick enough, its interior surface will not be cold enough for the moisture to condense as water. In cold climates where the required sheathing thickness might be unmanageable, floor-to-floor transitions and flashings can be designed so that any water that does condense will drain out of the wall harmlessly.

RULE #9: Ventilate adequately with a continuously running mechanical ventilation system. All homes built today – regardless of whether they are designed to be energy-efficient – are too airtight and should have mechanical ventilation. Intermittent bathroom and kitchen exhaust fans do not constitute an effective ventilation system because they may not be used enough: water vapor and indoor air pollutants are produced on a continuous basis, not just when we cook or take a shower. Every building should have a continuously operating mechanical ventilation system that draws a small controlled air flow from bathrooms and kitchens (see Ventilation).

A major benefit of this type of system is that it helps create negative indoor air pressure, just as inefficient gas and oil furnaces have always done (unlike modern sealed-combustion furnaces and heat pumps that do not use interior combustion air and have no effect on air pressures). Negative pressure encourages dry outdoor air to move inward to dry out walls and ceilings and prevents indoor air from entering the walls and ceilings where moisture can condense and do damage.

RULE #10: Control air pollution and moisture at the source. It’s often very easy to reduce the amount of moisture and air pollution that is emitted into the interior of a building at a very low cost. Buildings built in areas with known radon problems can be inexpensively radon-proofed at the time of construction, whereas to do this later can cost thousands of dollars. Major moisture sources such as sunrooms containing hot-tubs or large numbers of plants or indoor swimming pools can be carefully isolated from the remainder of the house. Laundry and bathroom exhausts should always be vented outdoors, never into attics.

Since building materials such as concrete and drywall contain enormous amounts of water and can take a year or more to dry, it should not be surprising that the worst indoor moisture problems such as mold and mildew are often seen in the first year. Wise practice would dictate that every new building should have a dehumidification system for at least the first year.

Air Leakage Pathways

This illustration shows the cross-section of a building that many would consider to be energy-efficient: 2x6 studs, “housewrap” on the exterior, thick roof insulation, and insulated windows. As the arrows show, there can still be many places where there can be substantial air leakage. Some arrows indicate how air can flow from the interior to the exterior or from the exterior to the interior; other arrows indicate how exterior air blowing past the drywall can cause energy loss through conduction without entering or leaving the interior of the building; still other arrows indicate pathways for radon and other soil gases.

A – between wall top plates and drywall
B – through cracks in recessed fixtures
C – short circuits through attic insulation
D – between wall top plates and drywall
E – through gaps in siding and sheathing
F – through holes in electrical boxes
G – between bottom plate and drywall
H – between bottom plate and subfloor
I – between rim joist and subfloor
J – between rim joist and top plate
K – between top plates and drywall
L – around window and door jambs
M – leaky windows and doors
N – between window framing and drywall
O – between bottom plate and drywall
P – between bottom plate and subfloor
Q – between rim joist and subfloor
R – between rim joist and sill plate
S – between sill plate and foundation wall
T – through cracks in foundation wall
U – between floor and foundation wall
V – through cracks in floor slab

air leakage pathways


Heat Conduction Pathways

This illustration shows the cross-section of a building that many would consider to be energy-efficient: 2x6 studs, “housewrap” on the exterior, thick roof insulation, and insulated windows. As the arrows show, there can still be many places where substantial heat loss can occur through conduction.

A – uninsulated joists
B – minimal insulation above fixtures
C – insufficient ceiling insulation
D – insufficient insulation at corners
E – uninsulated double top plates
F – uninsulated wall studs
G – poorly fitted insulation
H – uninsulated electrical boxes
I – uninsulated bottom plate
J – inadequately insulated rim joist
K – uninsulated double top plates
L – uninsulated window header
M – improperly insulated window gaps
N – uninsulated bottom plate
O – inadequate rim joist insulation
P – uninsulated sill plate
Q – inadequate wall insulation
R – uninsulated slab

heat conduction pathways


Moisture Transmission Pathways

There are four basic ways moisture moves in and out of buildings: it can flow as liquid water (since this is usually a consequence of poor flashing and drainage details, it is not covered here), it can can be sucked as liquid water by capillary forces acting in narrow spaces, it can be carried in as a vapor in air, and it can diffuse as a vapor through building materials.

A – in air, between top plates and drywall
B – in air, through cracks in fixtures
C – by diffusion, upward in winter
D – by diffusion, downward in summer
E – in air, through ventilation inlets
F – by diffusion, outward in winter
G – by diffusion, inward in summer
H – by capillarity, through siding spaces
I – in air, through cracks in electrical boxes
J – in air, between bottom plate and drywall
K – in air, through insulation materials
L – in air, between drywall and top plates
M – in air, condensation on cold glass
N – in air, between framing and drywall
O – in air, between bottom plate and drywall
P – in air, through insulation materials
Q – in air, behind basement walls
R – by capillarity, from foundation to sill
S – by diffusion, through basement drywall
T – by diffusion and capillarity
U – by water flow, through cracks
V – by diffusion, from foundation to drywall
W – in air, between plate and drywall
X – in air, between bottom plate and slab
Y – by diffusion or capillarity through slab
Z – by diffusion, through footer and up wall

moisture transmission pathways


return to top of page