GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Calculating the effects of humidity on winter time design heat load seems a bit elusive.

dickrussell | Posted in General Questions on

It isn’t hard to calculate the amount of water vapor that is flushed out of a house each hour, using inside and outside air moisture content, ACH, and house volume. If inside RH is to remain constant at say a healthy 30-40%, then a certain amount of moisture must be added to the air.

In a tight house, with HRV for controlling air makeup rate, normal human activity requires a certain rate to keep humidity down. That implies that use of water for cooking, cleaning, and showers, plants and people respiring and perspiring, and anything wet that is drying all contribute to the amount of water entering the air.

The amount of water getting flushed out of the house via the HRV (or leakage) in turn requires close to 1,000 BTU/lb to vaporize that water, and at first thought that heat ought to be added to the design heat load of the house, for sizing the heating system.

But, wait – what is the source of that heat of vaporization? Certainly anything drying out (eg. towels on a rack) takes heat from within the building envelope. But water vapor from someone taking a shower arguably comes from the hot water itself, water vapor from cooking comes from the stove, and water vapor from human respiration and sweating comes from the human’s internal furnace.

Are there any standard assumtions to make when calculating winter time heat effects of the various additions of moisture to ventilation air?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    The standard assumptions for moisture loads in buildings -- including internal moisture loads and external moisture loads -- can be found in ASHRAE Standard 160, "Design Criteria for Moisture Control in Buildings."

    Establishing this new standard has been a complicated, arduous process, shepherded by several people, including Anton TenWolde of the U.S. Forest Products Laboratory in Madison, WI.

    The issue is quite complex. Suffice it to say that internal loads vary widely -- from families with 7 people taking showers frequently, who water numerous houseplants daily, and who cook spaghetti every night, to a one-person household consisting of a retired gentleman who rarely bathes and eats his meals in restaurants.

    You are mistaken when you note that a tight new home will require moisture to be added to the air. If your house is tightly built, that shouldn't be true. Before you plan to install a humidifier, live in your house for several years and see how it operates.

  2. Riversong | | #2


    Richard didn't say that a tight house requires additional moisture - quite the opposite. Though he left it unspoken, his first paragraph obviously referred to a "normal" leaky house and the second paragraph dealt with a tight house in which ventilation is required to reduce humidity.

    And his question is not about how much moisture a family generates, but rather how to calculate its impact on heating load calculations.

  3. dickrussell | | #3

    Oh, I didn't mean to suggest that a tight new home would necessarily require humidification. With normal human activity, one would first expect the reverse, that ventilation would be required to keep the humidity down. Or, at a fixed level of ventilation, the inside RH would simply settle down to some level that results in a water balance, which likely would vary from one part of the day to another.

    I brought this up because I was trying to estimate the amount of water vapor being rejected by ventilation. This puts an upper bound on the contribution of non-deliberate humidification to the heat load for the house, if all that moisture were due to things drying out. The first set of numbers I ran gave me an equivalent of 9% of the total calculated heat load. That told me that I ought to look more closely at the humidification effect, since I was getting uncomfortably close to the capacity of the heat pump that would be proper if I ignored the effect. I knew that the heat effect typically would be less than that upper bound, but I was hoping for some information on the distribution of moisture contributions, however general, to indicate what fraction might be coming from sources that provided the heat of vaporization along with it, like stoves or hot water.

    If it's reasonable to assume that say only half of the moisture contributions are from sources that rob vaporization heat from within the building envelope, then I'm down into the realm of what heat might be added from lighting and appliances, like TV, and people just sitting there watching it, and I won't be terribly concerned.

  4. Riversong | | #4

    Now to the question:

    Richard, the source of the enthalpic heat is irrelevant, since all internal heat sources are considered in the heating design load of the building. The ASHRAE standard of 800 btu/hr per person takes into account the metabolic heat and utility heat of normal occupancy (and you can adjust that for different occupancy patterns).

    It's simple enough to calculate the enthalpic heat losses due to ventilation. As a rule of thumb, you could start with the assumption that a "typical" family of four contributes 4-5 gallons of water vapor per day and that this additional moisture must be removed through ventilation. That amounts to an additional enthalpic heat loss of 1400-1800 btu/hr, or about half of the occupancy contribution.

    If you actually calculate the hourly enthalpic loss in a 2000 SF house with 70°/40% indoor and 30°/75% outdoor at 0.35 ACH, you'll get the same numbers (that's how the ASHRAE 0.35 ACH standard was developed).

    Depending on the total heat load of the house, that amount may be significant or miniscule. In a very tight, well-insulated house, it could be justification for installing an ERV to recover some of that enthalpic loss. But where you're trying to get rid of excess moisture, it may not make sense to recycle it back into the house.

    Another question is whether that enthalpic heat is really available to the interior environment. It can be recovered only through condensation, which is what we try to avoid.

    Bottom line is: if you're building to PH standards, you might want to calculate enthalpic losses. Otherwise, it's not going to make much of a difference.

  5. Riversong | | #5


    After the first year release of construction moisture, the internal moisture load should come mostly from bodies, cooking and washing. All three contribute both sensible and enthalpic heat to the interior environment. Unless the occupants are hanging all their laundry indoors or storing cordwood in the basement or have a tropical forest of plants in the home, I doubt that the moisture volume from drying would be a significant factor (but that is precisely why we discourage such activities in a tight house).

  6. dickrussell | | #6

    Thanks, Robert (#5). That's the sort of general quantification I sought.

    My heat load calc leaves me at about 15% short of the tabulated capacity of the GSHP I have been planning to use, and that's enough margin for me to cover uncertainties in calcs and how the house actually performs. I didn't want to go up to the next size unit. By the way, we've broken ground on the new house, and hit ledge. We're having a blast! Literally.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |