Preventing Water Entry Into a Home

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Preventing Water Entry Into a Home

To lower your home’s indoor relative humidity, you need to address all sources of water entry

Posted on Jan 15 2016 by Martin Holladay

If it is designed well, the thermal envelope of your home should control the flow of heat, air, and moisture. Unfortunately, the floors, walls, and ceilings of older buildings are often leaky: they leak heat, they leak air, and they leak moisture.

If you are building a new house, you have the opportunity to control the flow of heat, air, and moisture through your home’s building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.. The result will be a durable, comfortable building that doesn’t cost much to heat and cool.

Of course, we use insulation to control the flow of heat and we use air barriers to control the flow of air. In almost all cases, it’s a good idea to limit the flow of heat and air through a home’s thermal envelope.

Controlling the flow of moisture is a little more complicated than controlling the flow of heat and air, however.

When your indoor air is too humid…

There are lots of ways that moisture can cause problems in a home — almost too many to list. Moisture entry can cause basement puddles, sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. rot, and drywall mold. Because the list of problems is so long, I’m going to limit the focus of this article to just one issue: indoor air with elevated relative humidity (RH).

Signs that the RH of your indoor air is too high include mold, condensation on windows, and clammy-feeling air. If you’re not sure whether your indoor air is too humid, you can measure RH with a hygrometerA device that measures relative humidity of air. Mechanical hygrometers that rely on a coil of thin metal are not terribly accurate; electronic hygrometers available at most electronic or hardware stores are usually accurate to about plus or minus 2 - 3%.. In general, indoor RH above 40% in winter or above 60% in summer is considered high.

The moisture that causes elevated indoor RH might be generated indoors — for example, it might come from a plumbing leak — or it might be exterior moisture that has managed to penetrate a building’s thermal envelope.

A strategy to control indoor RH will include one or more of the following measures:

  • You need to build a thermal envelope that prevents outdoor moisture from entering your house.
  • You need to limit indoor activities that tend to generate moisture.
  • In some weather conditions, you may need to operate a ventilation system to reduce indoor humidity levels.
  • In some weather conditions, you may need to operate an air conditioner or a dehumidifier.

Preventing exterior moisture from entering your house

There are many types of exterior moisture, including snow, rain, fogTo fog a room or building is to use a fog machine during a blower door test, revealing locations of air leaks where the fog escapes. The fogging material is usually a glycol-based solution, completely non-toxic., and rising ground water. You don’t want water of any type to be able to penetrate your home’s thermal envelope.

Roofs. It goes without saying that roofs shouldn’t leak, but they occasionally do. That said, roof leaks are almost never the cause of elevated indoor RH.

Above-grade walls. There are at least two ways that water can enter a wall from the exterior: as liquid water or as vapor. Here’s an example of how liquid water can enter a wall assembly: wind-driven rain can get past defective flashing. Here’s an example of how water vapor from the exterior can enter a wall assembly: the sun can shine on damp siding, driving water vapor towards the interior of the wall assembly.

It’s possible to create a wall assembly that strictly limits the ability of liquid water or water vapor to travel from the exterior side of the wall toward the interior. One example of such an assembly: a wall with a layer of rigid foam on the exterior side of the wall sheathing.

Walls that are vapor-permeable — for example, a straw-bale wall or a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. insulated with cellulose — can work just fine, as long as they include good flashing details and details that prevent moisture accumulation or condensation problems.

Although bad wall details can cause wall rot, these problems are rarely responsible for elevated indoor RH.

Basement walls and crawl space walls. When it comes to moisture entry into a home, the weak links in most thermal envelopes are basement walls (or crawl space walls) and basement floors (or crawl space floors). In older homes, these areas are often responsible for a significant amount of water entry. Even when the foundation walls and the slab feel dry, lots of moisture may be evaporating from these surfaces.

In a new home, it’s relatively easy to include details that stop almost all water entry at these locations. If you want to limit water entry through your basement walls or crawl space walls, you should specify:

  • Wide roof overhangs to keep rain away from the foundation;
  • Gutters at the roof eaves; these gutters should be connected to conductor pipes that convey the roof water far from the house (either to daylight or a dry wellUnderground structure that captures, then slowly releases storm-water runoff so that it can be absorbed by the soil.);
  • A capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. break (for example, an asphaltic dampproofing compound, UGL DryLok, or elastomeric paint) between the top of the concrete footing and the foundation wall;
  • A ring of perforated drain pipe on the outside of the footing, surrounded by crushed stone and wrapped with filter fabric to make a “burrito,” drained to daylight, to a distant drywell, or to an interior sump;
  • An application of dampproofing compound or waterproofing compound on the exterior side of the concrete foundation walls;
  • A layer of dimple-mat drainage board installed on the exterior side of the foundation walls; failing that, the foundation should be backfilled with coarse, free-draining material like crushed stone, topped with an 8-inch layer of dirt (ideally, dirt with a high clay content);
  • Careful backfilling to ensure that the grade slopes away from the foundation on all four sides of the house;
  • Closed-cell foam sill seal between the top of the foundation walls and the mudsill, to reduce air leakage and to act as a capillary break.
  • A layer of insulation on the exterior or interior side of the foundation wall. Exterior insulation can be closed-cell spray foam, XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation., EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest., or mineral wool; interior insulation can be closed-cell spray foam, XPS, EPS, or polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. .

Although these details are relatively easy to include during new construction, they are hard to retrofit. Do it right the first time.

If you have an older home where some of the above-listed details are missing, you'll need to approach your moisture problems like a detective. For more information on this issue, see Fixing a Wet Basement.

Basement floors and crawl space floors. Older homes often lack a layer of polyethylene under the basement slab. As with foundation walls, a dry-feeling slab is no guarantee that moisture isn’t entering through the slab.

If you’re building a new home with a basement, you should specify:

  • A 4-inch-thick layer of crushed stone under the basement slab as a capillary break; the crushed stone layer needs to be vented through the roof to help control radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles.;
  • In cold climates, a layer of horizontal rigid foam on top of the crushed stone to insulate the slab from the cold soil below;
  • A layer of polyethylene above the rigid foam (directly under the concrete slab) to act as a vapor barrier;
  • At least one 4-inch-diameter drain pipe running through the footing, to connect the crushed stone layer under your basement slab with the exterior footing drain;
  • A bead of sealant to seal the crack between the basement slab and the basement walls.

The fix for an older home that lacks a layer of polyethylene under the basement slab is to install a layer of polyethylene above the slab, followed by a layer of rigid foam and a layer of plywood. The plywood can be secured with long TapCon fasteners that pass through the foam to the concrete below.

The fix for dirt-floored crawl spaces is to install a layer of polyethylene over the dirt, either secured with a few scattered bricks or protected by a rat slab.

Moisture piggybacking on infiltrating air

When outdoor air is cold, infiltration tends to lower the indoor RH. (Cold air is dry.) When outdoor air is hot and humid, however, infiltration tends to raise the indoor RH.

If you live in a hot, humid climate, air sealing work is an essential part of any effort to control indoor humidity levels. Air leaks introduce exterior moisture, and this added exterior moisture increases the latent loadCooling load that results when moisture in the air changes from a vapor to a liquid (condensation). Latent load puts additional demand on cooling systems in hot-humid climates. that the cooling system must handle. When exterior moisture enters your home, your air conditioner has to work harder than it would if your house were tight.

So seal those air leaks, starting in the attic, crawl space, and basement.

Limiting indoor activities that generate moisture

Human activities generate moisture, and that’s not a bad thing. We breathe, we sweat, we cook pasta, we take showers. All of these activities are good, and we’re not going to stop doing them.

However, if your house has elevated indoor RH, you need to take a close look at possible indoor sources of moisture. Some of these sources can be addressed in a way that limits moisture generation; others may be hard to change. Here are a few items to look at:

  • Dripping faucets or plumbing leaks (especially plumbing leaks in a basement or crawl space);
  • Clothes dryers that are vented indoors;
  • Drying laundry on indoor racks;
  • Drying firewood indoors;
  • Frequent floor mopping;
  • Having too many houseplants;
  • Having too many tanks of tropical fish;
  • Cooking large stockpots full of soup or stew;
  • Brewing beer;
  • Making maple syrup;
  • Long showers taken by family members who forget to turn on the exhaust fan.

When should I operate a ventilation fan?

When damp activities happen in the bathroom, it’s important to operate the bathroom exhaust fan while the activity is occurring (and in many cases, for five or ten minutes after the activity has stopped). Moisture arising from cooking can be addressed by operating a range hood fan — assuming, of course, that the fan is ducted to the exterior rather than a recirculating model.

During the winter, when outdoor air is cold and dry, operating ventilation fans — either exhaust fans or the fans in HRVs or ERVs — tends to lower indoor RH. During hot, humid weather, however, ventilation rates should be reduced, since operating a ventilation fan under these conditions tends to increase indoor RH.

Air conditioners and dehumidifiers

If your home has an impeccable thermal envelope — one that prevents exterior moisture from entering — and your family doesn’t engage in egregious moisture-generating behavior, congratulations. Your house is in good shape.

However, your humidity-lowering work may not be done. If you want your indoor RH to be within a reasonable range during the summer, you may need to operate an air conditioner or a dehumidifier.
Either one of these appliances will reduce the indoor RH.

If the interior of your home is humid, but you don’t want to lower the indoor temperature, use a dehumidifier. If the interior of your home is hot and humid, use an air conditioner. If your air conditioner manages to keep your house cool, but still can’t lower the indoor RH to the range you want, you may need to operate a dehumidifier as well as an air conditioner. (For more information on this topic, see All About Dehumidifiers.)

Remember: if you’re using an air conditioner or a dehumidifier, keep your windows and doors closed.

Martin Holladay’s previous blog: “Air-to-Water Heat Pumps.”

Click here to follow Martin Holladay on Twitter.

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  1. Martin Holladay

Jan 15, 2016 10:06 AM ET

Indoor Relative Humidity Limit
by Jerome Lisuzzo

Martin - Thanks for another clear, informative article. This one triggered a question that I've had for some time. Why is 60% an acceptable indoor relative humidity limit in the summer, but not in the winter? Intuitively, it seems that if the indoor temperature remains relatively consistent throughout the year, the acceptable indoor relative humidity limit should not change.

Jan 15, 2016 10:59 AM ET

Response to Jerome Lisuzzo
by Martin Holladay

There are several reasons:

1. Lowering the indoor RH to 40% during the summer would be very energy-intensive in humid climates (for example, in the U.S. east of the Rocky Mountains). In the summer, 60% is usually achievable; 40%, not so much.

2. While 60% indoor RH would cause condensation problems on windows during the winter, 60% indoor RH doesn't cause the same problem during the summer.

There are more reasons, I'm sure. Readers: chime in with your reasons.

Jan 15, 2016 1:51 PM ET

Edited Jan 15, 2016 1:53 PM ET.

Why not 60% in winter?
by Allison A. Bailes III, PhD

Jerome, that's a great question and Martin gave a good answer. I'll just add a little bit and say that you want lower indoor RH in winter because there are a lot more colder surfaces available in winter that could become condensing surfaces if that humid air comes into contact with them. Martin mentioned windows, but it can happen inside walls, on ceilings, or other places.

I wrote about Building Science Corporation's study on double-stud walls last year, and Kohta Ueno, the author told me, "I have *very* seldom seen houses running 40% RH through the entire winter around here." Most homes, he said, run at about 20-30% RH through the winter, and he considers 40-50% RH to be "crazy-dangerous levels in a New England/Zone 5 type of climate."

Here's that article:

Is Cold Sheathing in Double-Wall Construction at Risk?

Another side of this is that unless you're generating a lot of water vapor indoors, it'll probably be hard to get to 60% RH in the winter in a mixed or cold climate. There's not a lot of water vapor in cold outdoor air, so when it comes into a house through infiltration or ventilation, you end up with lower RH. Cold air is dry air. (

Jan 15, 2016 8:11 PM ET

Indoor Relative Humidity
by Jerome Lisuzzo

Thanks, guys! Another mystery (of mine) solved. And Allison, thanks for the articles. The second one, in particular, helped me sort out another mystery; this one about how to reconcile the outdoor RH (which often reads at 90+ percent on winter mornings) with our indoor humidity, which seems much lower (according to our hydrometer), but could actually still be higher in absolute terms. (Hopefully I've got it right!)

Jan 16, 2016 11:41 AM ET

Summer Condensation
by Armando Cobo

Great article Martin. Almost all conversation about condensation is during winter and cold climates, but its a normal happening in the south (Dallas) and southeast where houses are exposed constantly to 68°F conditioned air, developing condensation in windows and walls, for the entire summer. Maybe someone would care to talk about it...? Dr. Joe told me that he'll come up with an article on it "one of these days", but I think is worth addressing it as part of this blog as well.

Jan 16, 2016 12:14 PM ET

Response to Armando Cobo
by Martin Holladay

In Dallas and South Florida, the classic moisture problems happen behind vinyl wallpaper (formerly common in hotel rooms) and mirrors that are glued to the wall. As you can imagine, there is a lot of moisture behind these materials if the room is air conditioned.

Jan 16, 2016 1:57 PM ET

Another RH article for Jerome
by Allison A. Bailes III, PhD

Jerome, a lot of people get confused by relative humidity. Even people who should know better sometimes say things that just don't make sense because of the relativity part of relative humidity. Here's another article that will help you sort through it:

Relative Humidity Doesn't Tell You How Humid the Air Is

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