No, this isn’t an article about global warming, carbon footprints or the next ice age. This is about how outside conditions can influence restoration drying strategies and equipment requirements.

To illustrate this relationship, we’ll look at structures and studies dealing with infiltration rates, apply some simple logic and offer some solutions to modify dehumidification equipment sizing to compensate for the climate impact.

The diversity encountered in restoration drying in different climates isn’t new. After the initial dehumidification sizing chart was developed for Applied Structural Drying courses, it became evident that drying times in humid climates lagged behind those in dry climates. A “Load Multiplier Chart,” developed by Todd DeMonte, was first published in 2005. The chart reflected the lineal relationship between drying needs in areas where the ambient conditions were at a baseline of 52 grains per pound, or gpp, and higher ambient specific humidity levels up to 170 gpp. It provided a “multiplier” that was applied to the initial equipment sizing amount.

In 2007, Chuck Dewald III with the American Drying Institute expanded on this concept by focusing on infiltration, creating a formula to calculate the additional load based on an infiltration rate of 0.6 air changes per hour. ADI has included this concept in their courses for the past 2 years.

Chart 1

These efforts went a long way to give restoration contractors operating in humid climates a method to compensate for the slower drying times they were experiencing using the standard sizing. The purpose here is to try and add to these earlier efforts.

It’s important to understand that structures aren’t airtight; they leak. This leakage is called air infiltration, defined as “the unintentional or accidental introduction of outside air into a building, typically through cracks in the building envelope and through use of doors for passage.”¹ Since water vapor is a component of air, moisture will be transported through air infiltration.

How much a structure leaks is determined by its purpose, geographical location, vintage and weather. A pharmaceutical manufacturing facility will be built extremely airtight because inside relative humidity levels may need to be kept in the single digits or specifically controlled to within a couple of grains per pound. A building materials warehouse will tend to be very open, in order for materials to be in equilibrium with the climate of the region.

The air infiltration rate of a home will fall somewhere in between, but a home built in Alaska will be tighter than a home built in Hawaii. A home built today will be tighter than a home built 20, 40 or 100 years ago. A house in Grand Rapids, Mich., will leak more air on a cold and windy winter night than it will on a calm spring day.

Finally, some states have building codes that require a mechanical system to provide a minimum ventilation rate of .35 ACH to occupied residential structures. All of these factors affect the air infiltration rate of a particular building on a particular day.

The amount of air infiltration is normally expressed as air changes per hour (ACH). For example: a 2,000-square-foot house with 8-foot ceilings (16,000 cubic feet) and a measured air change per hour (ACH) of 0.35 would leak 93.3 cubic feet per minute (CFM). This means that, in less than 3 hours, a quantity of air equal to the volume of air in the house will be exchanged with the outdoor air. It is easy to understand that if the outside air has a high specific humidity, it will undermine a conventional drying strategy.

In 2003 Lawrence Berkley National Laboratories published Analysis of U.S. Residential Air Leakage Database. The purpose of this work was to identify house characteristics that can be used to predict air leakage². The data was collected from existing blower door tests from a sample of 70,000 homes in the U.S.³ The basic results are shown in Chart 1.

This report has become the bible of infiltration rates for U.S. homes. Unfortunately for the restoration contractor, while it can and does supply some important information, it falls short of being a definitive guide for estimating the impact of outdoor conditions on a water-damage dry down.

The first line of the introduction to the LBL report illustrates one of these short falls: “Intentional or accidental large-scale airborne toxic release (e.g. terrorist attacks or industrial accidents) can cause severe harm to nearby communities. Under these circumstances, taking shelter in buildings can be an effective emergency response strategy.” 5

This report was developed in the wake of the 9/11 terrorist attacks when duct tape and plastic sheathing were being promoted as a first line of defense against a “dirty bomb” or biological terrorist attack. It is reasonable to believe a building dweller would take extraordinary steps to be certain their home was sealed off from an outdoor airborne menace.

These steps closely duplicate the measures taken when conducting the blower door tests that are the basis of the LBL study. Windows and doors are closed. Heating, cooling and ventilation systems are shut down. That means bath fans, kitchen range hoods and clothes driers are turned off. One window left open 2 inches could switch a “tight house” to a “leaky house.” These exceptional steps are not taken when water damage occurs. Leaving a bathroom window open is a pretty normal condition; especially in older homes without bath fans.

Another problem I see with the LBL study is, since it utilized existing voluntary data, the sample is not balanced and focuses primarily on data from weatherization programs on low-income homes and energy-efficient building programs that have a minimum leakage requirement.6 Fewer than 5,000 of the over 66,000 homes in the study were classified as “conventional homes” and, of these, 44 percent were from one cold-weather state, Wisconsin.7

Another study, published in the Journal of Exposure Analysis and Environmental Epidemiology in 2002, provides some valuable information on an occupied residence. The study, titled “Continuous measurements of air change rates in an occupied house for 1 year: The effect of temperature, wind, fans, and windows,” recorded the ACH of a 1,600-square-foot townhouse in Virginia every 5 minutes for a year.8

The ACH of this study ranged from 0.04 in the winter to 4.10 in the summer with the windows open. The monthly average ACH varied from 0.44 in October and November to 1.3 in July. This real-world data presents a much different picture than the “snapshot” blower door tests utilized in the LBL study. Since the townhouse was an end unit, only three surfaces were exposed to outdoor conditions, so the data for a free-standing home would definitely be higher.

The HVAC systems in most structures are going to promote infiltration. Ductwork that runs outside the building envelope (in the attic or crawlspace) has a high potential to cause infiltration. Any leaks in the supply ducts force air out of the building envelope, to be replaced by unconditioned outside air.

Leaks in return duct work will suck unconditioned air into the building envelope. In air-conditioning mode, this return air will travel through the cooling coils and the humidity load can be reduced but, if the outside specific humidity is higher than the indoor air, there will be an increase in the indoor specific humidity, and the overall infiltration impact remains the same.

Unless the home has been vacated, the restoration contractor does not have control over the entire structure, so he cannot govern the activities in the unaffected areas. Since the unaffected area is still inside the building envelope, there are no vapor barriers or weather-sealed doors separating the drying chamber from the unaffected areas. Because the central air conditioning or heating system is normally operated during the water damage dry down, the air between the affected and unaffected areas is usually mixed.

Chart 2

There are also several water damage restoration devices that contribute to the infiltration of outside air. Negative air machines, desiccant dehumidifiers and high-pressure floor- and wall-drying devices are normally exhausted outside.

In light of all this information, how much air infiltration should be expected? Restoration contractors are not building scientists, and insurance companies certainly wouldn’t approve charges for a professional assessment.

The consequences of underestimating the impact can slow or stall the drying schedule. Overestimating will rarely impede the drying schedule unless the associated heat drives the temperatures above the effective range of the dehumidifiers. The additional dehumidifiers should increase the equipment charges.

If we use the LBL study data, should we assume that the house is going to be “leaky” because the mechanical systems will be operating and some activity will be occurring in the building envelope? If so, our range would be 0.4 to 1.6 ACH, depending on the weather conditions.

If we simply average these numbers we get 1 ACH. When we average the high (1.3 ACH) and low (0.44 ACH) monthly data from the Virginia continuous test, we get 0.87 ACH.

If we average these two averages, we get 0.94 ACH. From this information it would be easy to assume an infiltration rate of 1.0 ACH on water-damage restoration projects. Although this would apply in many situations, I think a little more precision is desirable. There are situations when a lower infiltration rate would be present. To simply ignore this possibility would be negligent.

As a guideline, I recommend that the restoration contractor makes a basic evaluation of the structure, taking into consideration the age of the building, the prevailing weather conditions and whether or not the structure will be occupied during the drying process.

Of course, add to this any factors that the contractor views as impacting the infiltration rate. These could range from exhaust ventilation from restoration or containment equipment to the general condition of the structure such as storm damage, broken windows or obviously leaky duct work. It’s important to understand the criteria for different estimated infiltration rates of residential structures (Chart 2).

Commercial buildings span a wide variety of purposes and designs. The restoration contractor should make an evaluation of the structure and HVAC system to understand the infiltration potential. Many of these structures will have mechanical make-up air systems that can bring a substantial amount of outside air into the structure. Auditoriums, theatres and gymnasiums often have to supply up to 15 CFM per person of the seating capacity. Make-up air systems should be disabled during drying projects.

Chart 3

Now that we can estimate the infiltration rate, we need to determine the impact. The precise impact will be the difference between the outside conditions and the affected conditions. That means that, at the beginning of the drying project, the outside conditions may actually aid in drying, because the outside specific humidity may be lower than the affected area. As the drying progresses this situation will change to where the outside conditions will increasingly demand a larger portion of the dehumidifying capacity.

Rather than making constant adjustments to compensate for the outside moisture load, I recommend a baseline specific humidity be selected and used as the measurement against the outside conditions.

I believe the conditions existing in the house prior to the water loss are a reasonable baseline. Since that data isn’t available when the restoration contractor arrives, we need to determine the conditions that existed.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) have developed a comfort zone that covers the range conditions that are acceptable and the mechanical systems in the home are designed to provide. Rather than work off the different ranges of conditions I suggest a single center point be determined and used as the baseline condition (Chart 3).

The center of the humidity range is 45 percent. The center of the temperature range is 75°F The specific humidity of 75°F and 45% RH is 58 gpp. Using 58 gpp as the baseline means that anytime the outside conditions are above 58 grains, a load adjustment may need to be employed to counteract the climate impact. Now that we can determine an infiltration rate and a baseline specific humidity, we can calculate the amount of equipment that will be necessary to nullify high outdoor humidity.

A water loss in a newer occupied house in good condition would indicate an infiltration rate of 0.75 ACH. If the affected area was 1,000 square feet with 8-foot ceilings, the cubic footage of the affected area of the house would be 8,000 cubic feet. The calculated infiltration would be 100 CFM, or 7.1 pounds of air per minute.

8,000 cu.ft. x .75 ACH = 6,000 cu.ft. per hour
6,000 cu.ft. per hour ÷ 60 minutes = 100 CFM
6,000 cu.ft. ÷ 14 cu.ft. (cu.ft. ÷ 1 pound of air) = 428.6 lbs/hour
428.6 lbs/hour ÷ 60 = 7.1 lbs per minute


If the outside conditions are 90°F and 60 percent RH, the specific humidity is 125 gpp. The moisture added by this air (above the baseline) is 475.7 gpp per minute, or 28,542 gpp per hour. That equals about 4.1 pounds, or 3.9 pints, of water per hour. The calculation of the climate impact is below:

125 gpp – 58 gpp = 67 gpp
67 gpp x 7.1 lbs/min = 475.7 gpp/minute
475.7 gpp/minute x 60 min = 28,542 gpp/hour
28,542 gpp/hour ÷ 7000 grains = 4.1 lbs water/hour
4.1 lbs water/hour x .96 = 3.9 pints/hour


If we ignore the impact of the outside conditions, it is like dumping 0.5 gallons of water back into the affected area every hour. That means that, over a 24-hour period, nearly 12 gallons of water would be added back into the affected area by infiltration of outside air.

Why would you ever put yourself in this position?

Chart 4

If the restoration contractor determined the loss to be a Class 2 and followed the current IICRC dehumidifier sizing chart, he would have placed LGR dehumidifiers capable of removing 160 pints per day at AHAM (8,000 cu.ft. / 50 (class 2 divisor) = 160).

The impact of the infiltration would not be initially evident. But as the specific humidity in the affected area dropped below the 125 gpp outside, the effects would begin emerge.

After the affected area reached 58 gpp, the full impact would be reached, and as the specific humidity in the affected area dropped below the baseline, the actual impact would increase. In many cases the baseline condition of 58 grains may be difficult to achieve in the affected area if the infiltration load is ignored, because it is consuming nearly 60% of the 160 pints of dehumidification equipment capacity. This is why many drying jobs in humid climates stall on the second or third day.

If the structure was looser, with an infiltration of 1 ACH, the climate impact would be 5.25 pints per hour, or 15.75 gallons per day; almost 80 percent of the placed dehumidification. Even a tight building with 0.5 ACH would have a climate load of more than 2.5 pints per hour or almost 8 gallons per day or about 40% of the placed dehumidification. The formula for estimating the climate impact follows.

Cubic Footage of affected area x ACH ÷ 14 (cu.ft./lb air) x (Outside gpp – 58 gpp baseline) ÷ 7000 (gpp/lbs) x .96 (lbs/pint) x 24 hours

Rather than perform this calculation on every job, the chart shows the Climate Impact in five-grain increments and various infiltration rates per 1,000 cubic feet of affected area. When compared to the previous example of an 8,000-cubic-feet affected area with an infiltration of .75 ACH, the additional load is easily calculated: 8 x 11.8 = 94.4 pints.

This is identical to the total of the long calculation. The chart ranges from 70 grains to 180 grains. There will be little impact from infiltration below 70 gpp and levels above 180 gpp are rarely seen in the U.S.

Climate does impact every restoration drying project (Chart 4). In hot and humid regions it can cripple the restoration contractor’s drying strategy. In cold or dry conditions if offers a hidden boost, but drying too fast is, in most cases, not a problem.

Unfortunately, because the IICRC S500 does not specifically address climate impact, some may believe its effect to be minimal or inconsequential. It’s a mistake to make this assumption. The S500 is constantly reviewed and updated and as more information becomes available; it’s realistic to believe the topic will be included in the standard in the near future.

Many contractors who follow the ADI dehumidification sizing chart will encounter slightly less climate impact. This is because, after many years of testing, ADI found that the initial sizing factors for dehumidification should be increased by 20 percent to 30 percent. This will reduce the apparent impact compared to the IICRC sizing, but the overall performance will be reduced by the same amount of calculated moisture load.

The climate impact chart and calculations are based on neutral pressure drying strategies. In many cases, desiccants are used in positive pressure configurations. This will influence the amount of infiltration. Positive or negative pressure will reduce natural infiltration by approximately half of the positive or negative air flow. Simply stated, if the structure has an infiltration rate of 1 ACH, then positive or negative pressure of 2 ACH will nullify the natural infiltration. Of course, the contractor must still adjust the dehumidification requirements for the amount of moisture brought in by the pressurized system.

A heat only-based drying system uses outside air to flush away the evaporated water. Heating air does not change its specific humidity but it will change its capacity to hold water vapor, so outside air heated to a higher temperature will usually have a higher specific humidity. A structure dried with this method will never have an affected area specific humidity lower than the outdoor ambient level. This is why heat only-based systems are more effective in dry climates than humid climates. If dehumidifiers are used in conjunction with a heat-based system, then the structure would be closed up and there would be an infiltration-based moisture impact.

One unavoidable side effect of placing additional equipment to combat a humid climate is, since the amount of dehumidification is being increased, additional heat will be generated. A dehumidifier is really a latent heat (moisture) to sensible heat (temperature) converter. For every pint of water condensed out of the air by a dehumidifier, approximately 1,000 BTUs of heat is added to the air temperature.

In the previous example, the climate impact on the 8,000 square feet affected area was 3.9 pints per hour. Combined with the heat generated by the amp draw of the equipment, this would add about 4,000 BTUs of heat per hour. This would increase the temperature in the affected area about 2 degrees every hour.

It would require 1/3 ton of air conditioning to mitigate this temperature increase. This may require the use of dehumidifiers designed for high-temperature performance, attaching an LGR performance amplifier or supplemental air conditioning. Controlling the temperature in the affected area is essential in all dehumidification-based restoration drying projects, regardless of climate impact.

Climate impacts every water-damage restoration drying project. In dry regions it will be beneficial; in humid climates it can be crippling and destroy the drying strategy. Being able to evaluate and compensate for its effects is essential to an efficient and effective drying project.

It is possible to introduce even more precision into the calculation of the outside moisture load by using the exact specific humidity rather than the Climate Impact chart, or accounting for the precise volume of a pound of air at different temperatures and altitudes.

On the other hand, it is extremely difficult to accurately determine the exact infiltration rate. It will change with every breeze and temperature change. In order to create a practical guide, some assumptions have been made and averages have been used. The information provided on climate impact will give the restoration contractor the tools necessary to achieve successful results, regardless of the region, season or weather.



1 Wikipedia, the free encyclopedia© 2001-2006
2 LBNL Report Number 53367; pg.1
3 LBNL Report Number 53367; pg.1
4 LBNL-55575 Air Infiltration and Ventilation Centre Ventilation Information Paper Sheltering in Buildings from Large-Scale Outdoor Releases, W.R. Chan, P.N. Price, A.J. Gadgil; pg 2
5 LBNL-55575 Air Infiltration and Ventilation Centre Ventilation Information Paper Sheltering in Buildings from Large-Scale Outdoor Releases, W.R. Chan, P.N. Price, A.J. Gadgil; pg 1
6 LBNL Report Number 53367; pg.7
7 LBNL Report Number 53367; pg.8
8 Journal of Exposure Analysis and Environmental Epidemiology (2002) 12, 296–306 10.1038/sj.jea.7500229