Improving Humidity Tolerance  by John F. Robbins

We in the Ohio Valley live in what building scientists call a "mixed climate", meaning that most of us have substantial need for heating as well as cooling and dehumidification in our conditioned structures.  Believe it or not, it is much simpler to design and build an optimum building for just one of these needs than it is to design for all three!  This writing focuses on our high humidity, especially how that affects us differently in winter than in summer.

Airborne humidity is naturally higher outside during the warmer seasons, while it is naturally lower outside during the cooler seasons.  This is partially because warmer air can hold more humidity than cooler air.  It is also because the main air currents we get in the Ohio River valley come from the southwest, or in other words, the Gulf of Mexico.  So we spend lots of time and money removing humidity with our air conditioners and dehumidifiers during summer and spring, while we sometimes spend lots of time and money adding humidity with humidifiers during winter.  This is one of the characteristics of our "mixed climate". 

Sometimes I think water vapor simply wants to condense into liquid water, as if this was a simple tendency toward laziness.  After all, in the vapor state, water molecules are moving around at enormous speeds, while in the liquid state, those molecules are able to slow way down.  Doesn't this sound like laziness? The two classic examples of water condensing out of humid air during summer are (1) dew on the lawn when a humid day is followed by an overnight cool-down and (2) water quickly covering that cold pop can or ice tea glass soon after exposure to humid air.  A classic example of water condensing out of humid air in winter is the wet windows many of us put up with inside our living spaces, especially those with inefficient windows. 

Water vapor in humid air is driven from where it’s warm enough to be vapor to where it’s cool enough to condense.  Difference in temperature is the main driving force.  It’s important to realize that our insulated building assemblies commonly experience substantial differences in humidity and temperature between their inside and outside surfaces, so they must contend with this humidity-driving force.  And since many of our assemblies are made of materials like wood and metal which can rot, rust or grow mold when exposed to wetness over time, it’s best to design them to tolerate humidity and to stay as free of liquid wetness as possible.  It’s also important to learn that we can sometimes adjust our indoor temperature and humidity to minimize what building scientists call "wetting potential".

In winter when it’s generally colder and dryer outside, warmer and more humid inside, the humidity in our indoor air is being driven by differential temperature toward the outside.  If our home has non-airtight walls and ceilings, we may notice that our indoor air gets too dry for our comfort during winter, because air leaks carry our precious humidity to the outside air, which is both cold and dry.  Many of us with more airtight homes don’t need humidifiers because we’ve tightened up those air leaks and are keeping in the humidity, but this introduces another problem of indoor air with too much humidity.  As a designer of energy efficient homes, I neither spec nor approve any humidifiers in any of my customers’ homes, because I don’t want the indoor humidity to build up to the point where it’s harmful to the structure or sustains mold.  It’s generally accepted that mold growth is possible when relative humidity reaches 60%.  It’s usually advisable to keep indoor humidity closer to 40% in winter to make sure there are not sustained humid pockets in the home with levels higher than 60%.

Since insulated walls and ceilings stay cooler in winter because their insulation is holding the heat in the conditioned spaces they enclose, any humidity that escapes into their unventilated or poorly ventilated cavities during winter can condense, causing rot, rust and mold problems down the road.  Wintertime condensation inside stud and rafter cavities usually happens at the outermost surfaces, like inside the exterior OSB sheath.  The building code requires a vapor barrier on or behind the interior finish surface to slow or stop the interior moisture which tries to "diffuse" through solid materials like drywall, paneling, wood trim and paint into unventilated insulated assemblies.  But even if the vapor barriers are installed correctly, which is often not the case, reports have shown that the biggest injections of humidity into walls and ceilings occur where there are breaks and penetrations in the barrier, like unsealed electrical outlets and pipe chases which punch holes in the wall or ceiling barrier, like structural ceiling beams which prevent easy installation of a continuous vapor barrier.

Since the warmest air wants to rise to the highest point in a space and since warm air can hold more humidity than cooler air, unbarriered ridge beams may be most vulnerable to allowing humidity to escape into rafter cavities.  Only if there is excellent ventilation to permit this humidity to escape to the outside will such diffusion not risk eventual rot or other degradation.  And since ridge vents can get clogged after a significant ice or snowstorm, the interior vapor barrier at these high locations is still critical even if there is passive ventilation designed into the roof assembly.  Yet if you have a home built merely to comply with the local minimum energy codes, it’s possible that you don’t have either ceiling vapor barriers or carefully sealed seams at these kinds of locations.

What can you do?  First, you can keep indoor humidity not higher than 40-50% RH during winter.  When it’s really cold outside, keep indoor humidity as low as you can tolerate it.  Some say as low as 30% RH.  Second, make sure there’s an airtight seal between your conditioned living spaces and their insulated surrounding assemblies, especially sealing cracks and penetrations in and around vaulted ceiling beams and other features higher in the living space.  Get into your attic and seal any gaps in the ceiling plane, including around exhaust stacks, chimneys and wire chases.  There are also vapor barrier paint primers and sealants like silicone which can be applied to the inside surfaces and seams of exterior insulated building assemblies.

If you’re planning a new home, you could add rigid foam insulation over the outside of the wall studs or ceiling rafters, so that the cavity surfaces inside are kept warmer and less prone to condensation even if humid air leaks into the assembly later.  Another option is to insulate the wall cavities with foam, which, unlike fiberglass, seals the wall more tightly against air and vapor movement. Even cellulose can be better in this regard than fiberglass, since it is more effective at blocking air leaks and even absorbs moisture faster than wood does.  But damp cellulose in an enclosed insulated cavity not only loses some of its R-value, but also poses risk for wood rot.  So in the best of situations, you’d consider and apply some or all of these options to improve moisture tolerance.

During the summer, this whole process of humidification is reversed, because it’s hotter and more humid outside, cooler and dryer inside since we’re typically air conditioning our indoor air.  As air conditioning has grown in popularity, it has caused a different stress on our traditional insulated building assemblies for a couple reasons.  First, even though there’s the code-required vapor barrier on the inside of our insulated and unvented assemblies, there’s likely no such barrier on the outside.  This is because wrapping a simple plastic or paint vapor barrier on the outside could cause a typical fiberglass-insulated stud or rafter cavity to stop and trap moisture on the coldest surfaces inside the assembly. So it’s common to put permeable housewrap over or create ventilation and drip paths in the outermost sections of insulated ceiling and wall assemblies, so that moisture can get out.

However, if humid air in a ventilated attic or wall vent space cools down below its dew point, it can condense inside an intentional vent space, especially when the air is not moving.  If humidity leaks in through the breathable exterior housewrap or unsealed seams in the framing of the outside wall or ceiling, it can condense inside the stud or rafter cavity, usually on the backside of your drywall, paneling or trim in summer. As long as this condensate can get into the conditioned space to be dried by a dehumidifier or air conditioner or out through weep holes or other intended paths to the outside to dry, it may be okay.  But if it can’t get out, resulting in long-term wetness, it poses a problem.

But remember that warm-to-cool driving force?  Well, in summer it’s actually pushing the humid vapor in toward the backsides of our interior finishes, because that’s the most air conditioned surface inside the wall or ceiling.  If there’s a vapor barrier there, as often required by code, that barrier keeps the moisture from being dehumidified by the air conditioner.

So what can you do?  Start by learning about "dew point" and trying to adjust your living space conditions according to whatever the dew point is outside. Dew point is the temperature at which airborne humidity will condense out of humid air, just like dew.   Our summer outside dew point often exceeds our AC-cooled indoor temperatures.  I’ve seen local outdoor dew points reach as high as 79 F, but more commonly the worst cases around here are in the mid-70s. By this I mean that the outside air gets so humid at times that if it comes up against a surface with a temperature in the low-70s, it’s possible to condense out some of the air’s humidity.  We most commonly see this in our basements during spring and summer after we’ve opened windows.  Since an uninsulated basement slab and the base of an uninsulated foundation wall are usually at their lowest temperatures during spring, because the ground is cooler than the air during early spring, it’s not uncommon to get dampness, sometimes even small puddles in our basements, especially where an uninsulated slab touches the base of an uninsulated concrete foundation wall.

By learning about and adjusting our living space conditions based on what the dew point is outside during spring and summer, we can minimize at least some of these problems.  For instance, if the outside dew point on an otherwise nice spring day is in the mid-60s, then you can avoid opening windows in the basement, since the temperature of an uninsulated slab or base of an uninsulated foundation wall could be in the upper 50s or low 60s then. Similarly, when the dew point in the heart of summer rises into the 70s, set your AC thermostat so it’s never below that dew point, so your house walls and vaulted ceilings aren’t attracting moisture out of the outside air as you’ve seen a cold ice tea glass do on any hot humid day.

Consider as well that the air coming out of a register during AC season is about 20 degrees below the thermostat set point.  Any wall or ceiling exposed directly to this lower temperature air stream can actually get substantially cooler than the thermostat setting, making it attract and want to condense out vapor on its inside more than other parts of your house.  I try to direct my floor registers to blow away from insulated assemblies.  I go out of my way to keep cold ducts from being exposed to hot humid air, but even if you find such in your home, you can either have them sealed and insulated, or else moved more into the conditioned space.  So if you like your living space in the low 70s during summer, consider that your register air temp from May through September is probably lower than the average outside dew point.  If AC is operated to maintain indoor temps at or above 77, the register air temp is lower than the average outdoor dew point only during June, July and August.  So simply by setting the AC thermostat at or above 77 F, we can assist in minimizing the potential wetting of our vulnerable building assemblies while lowering our AC bills at the same time!

As one moves farther south into southern Kentucky and Tennessee where heating and cooling loads become more equal, designers and builders can protect vulnerable insulated wood assemblies by putting vapor-barrier rigid foams like extruded polystyrene and foil-faced isocyanurate on the outside, and sometimes even the insides, of their frame assemblies.  I commonly spec foam sheaths on the outside of my stud wall designs for this reason.  This technique keeps the inside stud cavity surface temperatures more moderate, less prone to wetting in both winter and summer.  There are also foam cavity insulations, which have very low vapor permeability, unlike fiberglass, and block air and most vapor movement in both directions because they literally seal the cavity tightly.  There are also assemblies, like insulated concrete forms, which do not rely on woods, metals and other moisture-vulnerable materials in their structures where the difference in temperature occurs.  However, even seemingly airtight assemblies like SIPS (structural insulated panels) experience vapor vulnerability especially at their seams, so need vapor barriers and airtight sealants to resist potential damage over time from exposure to potential condensation inside those connection cavities.

Finally, to minimize the movement of absorbed water and dampness from exterior finish materials like masonry veneers, exterior synthetic stuccos and sidings (besides vinyl), make sure there is an adequate air space, a liquid drip or flashing plane, plus a drainage path behind these materials so that bulk water and condensing vapors can drip down and out properly and safely, instead of trying to enter the more vulnerable parts of the wall.

When I’m designing someone’s home, I prefer to discuss issue like these with my customers.  I want future occupants to understand what my specs are and why, so they can stand behind them with me and help make sure a contractor doesn’t miss or eliminate important details during construction.  For instance, consider my preference for no humidifier in a furnace installation.  Since most HVAC contractors assume a homeowner will want a humidifier, it’s sometimes important for the homeowner to understand and insist that none be installed. Even if one gets installed, I want the homeowner to operate that humidifier as infrequently as possible, to make sure humidity stays closer to 40% than 50% RH during winter.  If cost overruns force cost-cutting measures, I also want the homeowner to insist that vapor barriers and other important moisture-protecting details are not eliminated in hasty decisions.  Since I commonly spec foam below and at the edges of my slab floors (to minimize spring and summer condensation as much as winter coldness), I want the homeowner to insist that this foam not be cut for cost reasons.

I also discuss moisture-related lifestyle issues with my clients, to help them improve the healthiness and longevity of their homes.  For instance, when bathing and showering, I advise them to make sure the bathroom fan is operating prior to turning on the hot water, to turn on the kitchen fan before boiling liquid on the range.  These measures will minimize the steam which otherwise gets into the air and tries to leak into vulnerable building assemblies. I make sure my clients know that STEAM IS THE ENEMY OF MOST WOOD AND METAL BUILDING ASSEMBLIES during winter.  If they have a heated aquarium, an indoor luxury tub or pool, I make sure they understand that these need to be covered when not in use to minimize excess humidification of their indoor air. And whenever their windows steam up, I tell them that's a signal either to increase exhaust ventilation or decrease humidification.  Sometimes both are needed to get humidity down.  And sometimes it’s simply advisable to minimize or avoid such injections of humidity or steam altogether, at least during the coldest weather when the moisture-driving force is greatest.

In extremely airtight homes, I typically spec an air-to-air heat exchanger, which exhausts humid air from bathrooms, laundry and kitchen at the same rate that outside air is drawn into the home.  Because the outside air is so dry in winter, an air-to-air heat exchanging exhaust ventilator usually does a very effective job of controlling excess humidity.  Sometimes I’m asked why I would want to seal a home up to the point that it needs a blower to bring in outside air...  My usual answer is that I want the whole house to be dehumidified and supplied with a little fresh air now and then, not just the rooms with leaky windows and doors facing the wind-side of the house.  But it's also okay with me if someone prefers to open his or her windows regularly in the middle of winter to get rid of excess humidity instead of operating a blower.  Achieving the goal is what's important, not which method is used.

Finally, there are occasions when certain kinds of moisture-intolerant building assemblies are just not appropriate for the kind of use or interior conditioning desired by a customer.  Conversely, excessive moisture-production is sometimes not appropriate inside certain kinds of vulnerable structures.  My clients deserve frankness about these issues upfront, especially when it can avoid future rot, rust or mold problems.  There are many ways to build insulated walls and ceilings, floors and foundation walls.  Sometimes it's better to consider alternatives instead of just doctoring more traditional assemblies. And sometimes it's better to restrain from pushing less capable structures beyond the limits of their capable function and performance.  The most important challenge is learning and respecting those limits, whether thinking about a new design or a structure that already exists.