09. Hygro-Thermal

The Alchemy of Mass & Energy Flows

h-t-engineeringHygro-thermal engineering is the analysis and management of the flows of heat (energy), air and moisture (mass) around, inside of, and through a building envelope. As such, it deals with the dense material (earth), the diffuse material (air), the transformative energy (fire) and the fluid medium (water) – and is a form of alchemy, which was the precursor to modern science and philosophy as well as foundation of the hero’s journey from a leaden youth to a golden maturity.

periodictabledevGiven how complex we’ve made our modern residential structures, it takes a hero to delve sufficiently into the mysteries of physics to confront and overcome the adversaries of heat, air and moisture and to transmute them into allies. Or, to be more mundane, the task facing every designer and builder of a modern home is to incorporate nature’s inexorable laws into every stage of residential engineering – from conception, through design, by way of planning, toward final manifestation, occupancy and maintenance.

Water as Mystery and Magic

We live on a watery planet. Water is the most common “stuff” in the universe, the most abundant material on earth, and the sine qua non of life. Water is the only terrestrial substance that naturally occurs in solid, liquid and gaseous phases, it is essential for photosynthesis, metabolism and the thermo-regulation of our bodies and the earth’s climate. In its solid form, it is less dense than as a liquid (so ice floats and allows aquatic life to survive the winter). Water has an abnormally high melting point, which allows liquid water to cover the earth and create a breeding ground and habitat for life.

Water has the highest specific heat of all liquids except ammonia, which facilitates heat transfer in the atmosphere and oceans and moderates temperature extremes. Water has exceptionally high surface tension, which allows drop formation and rain as well as capillary movement within plants and trees as tall as 400 feet. Water absorbs infrared and ultraviolet light, which encourages photosynthesis and regulates atmospheric and oceanic temperatures. Water is an excellent solvent for ionic salts and polar molecules (the universal solvent), which facilitates the transfer of nutrients in metabolism and in hydrologic cycles from the mountains to the sea. Water has the highest heat of vaporization and the highest thermal conductivity of any liquid (except for liquid metals).

water-dipoleAs we might remember from school, a water molecule is composed of two hydrogen atoms in a covalent (electron-sharing) bond with one oxygen atom, thus creating an electrical dipole with positive hydrogen charges that bond strongly to solids and to electrically-charged (ionized) materials. The hydrogen bonds keep liquid water strongly cohesive, with high surface tension that creates a “skin” on top that spiders can walk upon and that forms spherical droplets when it falls. That surface tension – coupled with water’s strong bonding to solids – creates the capillary action that allows water to climb magically in narrow channels against gravity.

Water’s high heat of vaporization – the energy required to break the hydrogen bonds and overcome atmospheric pressure – gives it a great deal of latent (or enthalpic) heat which is given up to its environment upon condensation (which is why fog melts snow faster than warm air, rain or sunshine). This latent heat is lost when warm, moist air leaves a conditioned space in the winter and puts an enormous load on an air conditioner in the summer because cooling below the dew point requires that the additional heat of condensation be removed from the air as well.

Water is always doing its dance. It moves continuously from solid to liquid to vapor, and from absorbed bound states to desorbed free states. In other words, condensation is not something that happens only at the dew point – it is just more noticeable then.

Hygro-Thermal Alchemy

Humidity (or absolute humidity) is the mass of moisture in a volume of air (lbs per cubic foot), at a specific temperature and pressure. The mixing ratio (or specific humidity) is the mass of moisture in a given mass of air (lbs per lb), and it determines the partial pressure of the water vapor – what we call vapor pressure. And vapor pressure is what drives water vapor diffusion. Relative humidity (RH), is the ratio of water vapor in air compared to the maximum it can hold at that temperature at saturation (100% RH). If half of the saturation volume is present, then the RH is 50%.

However, the curve of the amount of water vapor that air can hold as it increases in temperature is superlinear, such that – at a fixed absolute humidity – the RH will decrease by half with every 20° rise in air temperature. This also means that warmer air can hold an ever-increasing amount of water vapor and has the potential of producing high vapor pressures even at relatively low RH.

rh-temperature-relationshipAbsorption is the bonding of liquid water in the pores of a hygroscopic or hydrophilic material. Adsorption is bonding of water as a thin film on the surface of a solid (internally or externally). The two processes are simply variants of generic sorption, and the reverse process is desorption. Sorption can occur with water in either the liquid or gaseous states, but once water vapor bonds to a solid it loses the energy and freedom of a gas molecule and bonds as a liquid. Once sorbed, energy input forces bulk or surface diffusion into or out of a porous solid – a process also driven by a concentration gradient. Thus moisture diffusion is moved by heat or relative humidity, since the higher the RH the more opportunity for water vapor to condense into a state of sorption. Hence the moisture content (MC) of a hygroscopic material depends largely on RH, and the MC determines the likelihood of mold or decay.

A family of four will contribute 4–5 gallons of moisture per day from the normal activities of breathing, washing, bathing and cooking. A new house can release hundreds of gallons of water in the first year. A tight house can easily be overwhelmed by this moisture load, requiring both spot and whole-house ventilation, which is also necessary to maintain indoor air quality. But, even with indoor humidity controlled, moisture can intrude into the thermal or structural envelope from either inside or outside or from the ground.

moisture-potentialsMoisture intrusion requires a moisture source (atmospheric, interior, ground, mechanical or stored), a moisture mechanism (bulk movement, air movement, vapor diffusion, capillary action or condensation), a route of entry (gaps and penetrations, air permeable layers, vapor permeable layers or porous materials), and a driving force (kinetic energy, gravity, air pressure, vapor pressure, temperature gradient or surface tension). Moisture accumulation occurs when the rate of wetting exceeds (or is lagged by) the rate of drying and the difference exceeds the safe storage (buffering) capacity of materials. When moisture accumulation coincides with sufficient temperature and time, then problems occur – including mold, decay, rust, swelling, warpage, delamination, truss uplift, efflorescence, freeze-thaw damage, loss of thermal resistance, and insect infestation.

Moisture Management Methods

We have known for some time that moist air movement contributes as much as 100 times as much moisture to a thermal envelope as vapor diffusion through solid materials, so the focus today is on air barriers – both to prevent unnecessary heat loss or gain and to maintain the integrity and durability of the structure. In a cold climate, the dominant moisture drive (by both temperature and pressure gradients) is from inside to out – so it makes the most sense to locate the air (and vapor diffusion) barrier on the interior. The opposite is true in warm, humid climates where the air (and vapor diffusion) barrier should be on the outside. In mixed climates, moisture can move in either direction (and sometimes both at once), so take your pick. Except in the most extreme cold climates (zone 7 or 8), building scientists no longer recommend true vapor barriers, since they are as likely to prevent drying as to reduce wetting (as the moisture drives reverse), but merely class II vapor retarders of about 1 perm (which can be accomplished with latex vapor retarder primer).

differntial-moisture-drivesOf the various moisture mechanisms, bulk liquid flow and capillarity are the most significant, so careful exterior detailing and ground vapor barriers, capillary breaks and foundation waterproofing are the most important water management strategies. In fact, the four keys to moisture management are the four D’s: deflection, drainage, drying and durable materials.


Deflection includes siting and landscaping to minimize exposure, roof overhangs and window sills, lapped and integrated weather barrier, flashing and siding, and rainscreen claddings.  Drainage encompasses site grading, sub-surface (French) drains, footing drains, roof gutters and downspouts, as well as rainscreens. Drying relies on vapor-open surfaces or rainscreens and sufficient moisture buffers to safely store daily or seasonal moisture until release. And durable materials include both moisture-tolerant elements as well as hygroscopic substances that can absorb and redistribute moisture, thereby reducing local concentrations (cellulose insulation is excellent at this).

wetting-drying-balanceWhat is not, in my natural-law abiding perspective, a sensible moisture management strategy is the use of impermeable materials, such as plastics, vinyl, bituthene and foams, to create what I call the Hermetic House (hermetically sealed like a picnic cooler). One thing that we can be sure of is that every house will get wet at some point, and so a drying strategy can be more important than a waterproof approach. Every study that deliberately introduced a leak into either a wall or roof assembly concluded that drying was so significantly delayed by impermeable materials that damage was a certainty.

Another penny-wise but pound-foolish approach is the use of highly vulnerable materials like OSB. For exterior sheathing or roof decking I would never use anything other than CDX plywood or sawn boards (both of which are reasonably vapor-open). Similarly, I prefer #15 felt (ASTM D226, if possible) as a weather-resistant barrier (WRB) rather than the polymeric housewraps that can trap liquid water behind them and lose their water resistance because of surfactants in wood and stucco.

Just as I refuse to go to the extreme of designing or building a vapor-impermeable house, I also believe that air-tightness can be taken to an unnecessary and potentially dangerous extreme. With Passive House standards becoming the ideal for some designers, it is valuable to ask “How tight is tight enough?” The respected building scientist, John Straube, who has had the opportunity to test and observe thousands of houses in Canada’s cold climates, has offered an answer that agrees with my own experience and wisdom. Homes with greater than 3 ACH50 (air changes per hour pressurized to 50 pascals with a blower door) tend to have a risk of interstitial condensation; those with greater than 5 or 6 ACH50 tend also to be too dry inside.

Those with about 2 ACH50 tend to perform quite well, while houses with as low as 1.5 ACH50 have problems with high winter indoor RH. This does not mean you cannot design and detail a 0.6 ACH50 Passive House to work well, but the more we go to extremes on any parameter, the more likely that unintended consequences will exact their revenge effects.

An ACH50 of 1.5 to 3 will also result in a natural air exchange rate of 0.1 to 0.2 ACH, which is just sufficient to maintain indoor air quality when the power is down or the mechanical systems are malfunctioning. To me, this allows a house to be fail-safe rather than allegedly fail-proof, and keeps it within the Golden Mean that Horace, Aristotle, Buddha, Confucius and Lao Tzu taught us to honor.

If a house is, indeed, our “third skin,” then it must protect us without disconnecting us from the four elements of earth, air, fire and water which comprise our earthly abode, and it must honor the inherent mystical laws of nature’s alchemy.


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by Robert Riversong: may be reproduced only with author attribution for non-commercial purposes and a link to this page

4 Responses to 09. Hygro-Thermal

  1. Robert, Your site has been a breath of fresh air for me this week! I live in the rain forest of Southeast Alaska (Wrangell) and am stuck in the middle of the efficiency/ breath-ability conundrum with a few of the local guys who build houses and a state energy auditor, Air quality inspector, etc… Your ideas seem like a perfect compromise between the extremes of my acquaintances.

    So, does your wall system work in a VERY humid cold climate?? (right now October 19th I am looking at my weather station and it is Outside 45 deg @ 92% humidity and Inside we are at 68DegF and 58%RH) None of our sinuses like lower than 50% RH indoor because we are so used to the humidity…

    Could you elaborate a little about your use of drywall? Paper backed drywall in a breathable wall in such a humid place as where I live seems like it could be a problem. Is there a local alternative that would work in its place? (I can harvest 10,000 board feet per year from the Tongass Nat forest and have been wanting to use local lumber for my next house as much as possible).

    Thank you very much for your articles and your dedication to the craft.

    • Riversong says:


      All cold climates have high relative humidity in winter – typically somewhat higher on average than in summer – because cold air with little absolute humidity will register a high relative humidity.

      Your indoor 58% RH is much too high, and will almost certainly result in mold if not eventual structural damage to the house. I aim to keep all houses at no more than 40% RH in winter (Canada uses 30% as the standard in extremely cold climates). The human body tolerates 40% quite well.

      You can certainly avoid the more moldable surfaces, such as paper-faced drywall (though the only place I would recommend fiber-glass-faced drywall is in bathrooms or basements). I’ve found that moisture-resistant (MR) drywall works fine everywhere as long as indoor humidity is properly controlled.

      There are two elements of indoor moisture control. One is to avoid putting excess humidity in the indoor environment (source control), by covering cooking pots and using the range hood (a quiet one pushing no more than 200 CFM is best) , using the (code-required) bath exhaust fan when showering, exhausting clothes dryers outdoors, and avoiding jetted Jacuzzis and indoor storage of undried cordwood,etc.

      Element two is whole-house ventilation that meets ASHRAE 62.2 standard. There is no need for a complex, centrally-ducted heat-recovery ventilator (HRV), as a programmable timer on one or more bath exhaust fans (quiet, efficient ones) coupled with strategically-located passive make-up air vents (if the house is tighter than 3ACH50) is sufficient.

      Since taped drywall is the gold-standard of the Air Barrier Association of America (ABAA) for home air barriers, it is generally the least expensive and most effective way to keep air-borne moisture out of the structural and thermal envelopes, particularly when coupled with the Air-Tight Drywall Approach (ADA).

      If you need more specific feedback and advice, you can contact me through the email address at the bottom of the Home page of this site, as I offer consulting and design services for a very reasonable fee.

      • aron hendrix says:

        hi, i am wanting to build an off grid, very secluded home, double envelope over a full basement,( that heats and cools itself), but totally fire proof!! what are your thoughts on aac blocks and panels? the only plant in north america is in central florida, i’m in north georgia. aercon (aerconfl.com) is the company. thanks for the good info on your site.

      • Riversong says:

        A double envelope is not the same thing as a double-wall, as the former is two distinct walls with a circulating air space between them and the latter is a unitary wall full of insulation. And no house “heats and cools itself” any more than there is such a thing as a perpetual motion machine.

        Aerated Autoclaved Concrete, also called cellular or lightweight concrete, is a useful building material more suitable to commercial and industrial applications than single-family residential buildings. It’s about 20%-33% the weight and density of regular concrete block, with a similar decrease in compressive strength, but requires the same masonry skills to assemble properly.

        As an insulating material it’s mediocre, with an R/inch of between 0.8 and 1.25 (compared to most fibrous insulations at R-3 per inch), and as a thermal mass it’s of limited value because of its relatively low density.

        It might be fine in a north Georgia climate, and it’s certainly insect-and fire-proof. But it would likely be prohibitively expensive to use it in a double wall or double envelope configuration.

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