Resistant Housing - Principles and Practice
Florida Solar Energy Center
1679 Clearlake Road
Cocoa, FL 32922
The importance of common residential allergens and irritants
(tobacco smoke, dust mites, pollens, pet danders, cockroaches,
fungi and molds, endotoxins, volatile organic compounds, odors,
fine dust and radon) is reviewed. A systems oriented solution
called DDAMPFIRM (Drainage and waterproofing, Dehumidification,
Air tight construction, Material selection, Positive pressure
ventilation, Insulation to avoid cold floors, Return air ducts
or transfers, Maintenance) is discussed. Construction details
and measured data on energy use and dust mite levels in allergy
resistant and conventional houses are presented.
This work was sponsored by the U.S. Department of Energy, Office
of Building Systems, Mr. George James, program manager whose
support and goodwill is very much appreciated. The authors are
grateful to the American Lung Association affiliates in Minneapolis,
Central Florida and Washington who asked us to participate in
the design and construction of demonstration Health (or Healthy)
Houses®. We thank the homeowners who allowed
us to monitor their homes. The dust mite allergen analysis was
conducted by Rosa Codina, Ph.D. at the laboratory located at
the James A. Haley VA Hospital in Tampa. The laboratory is directed
by Dr. Richard F. Lockey, MD, at the College of Medicine, University
of South Florida. Recommendations on carpets and carpet pads
come from Alfred T. Hodgson, Ph.D. of the Lawrence Berkeley
The increasing incidence of allergy and asthma, particularly
among children, is a world wide concern. Since most of us stay
indoors over 90% of the time, the indoor air quality in buildings
is of paramount importance in maintaining respiratory health.
A review of literature indicates that the common allergens and
lung irritants in the home environment are:
- Environmental Tobacco Smoke
- Dust Mite Allergens
- Pollen Allergens
- Pet Dander Allergens from cats and dogs
- Fungi and Molds
- Cockroach allergens
- Volatile Organic Compounds and Odors(VOC's)
- Fine Dust
Of course exceptions prove the rule. For the chemically hypersensitive
a certain VOC might be lethal. Poorly maintained water systems
can harbor deadly bacteria such as legionella. Excessive
rodents, pesticides and malfunctioning combustion devices can
be extremely hazardous. Recent studies point to the increasing
importance of cockroach allergens to asthma in inner city housing
causing asthma in up to 8.5% of the children (1), almost twice
the national rate. Because of genetic factors, Native Americans
have a low incidence of asthma and smoking related lung disease
(2). To them tobacco smoking may be a lesser concern.
The above list is in probable descending order of importance
in terms of health impacts on the general population. What is
interesting about the list is the relatively low importance
of VOCs and Radon as far as their adverse health effects are
concerned. Yet substantial sums of money and effort has been
expended in controlling these two irritants and not enough effort
is expended on controlling the more important allergens.
Energy efficient buildings are sometimes thought of as harbingers
of poor indoor air quality. Yet allergy resistant houses must
begin by being airtight and well insulated. Unless the house
is airtight, one cannot control and filter the ventilation air;
nor can one maintain a positive pressure. Unless the house is
uniformly well insulated, cold spots and cold floors may be
present. The resultant high local relative humidity hastens
the formation of molds on surfaces and is conducive to breeding
and proliferation of dust mites. Thus energy efficient construction
is a prerequisite for allergy resistant housing. However it
is not sufficient. One must employ a systems strategy involving
DDAMPFIRM (Drainage and waterproofing, Dehumidification, Air
tight construction, Material selection, Positive pressure ventilation,
Insulation to avoid cold floors, Return air ducts or transfers,
Maintenance) to obtain an allergen free house.
Allergens and Irritants
Environmental Tobacco Smoke (ETS). About 25% of North
American adults smoke today. In countries such as Japan and
Poland, above 50% of the adult males smoke. The sidestream and
exhaled smoke (i.e. ETS) is a major cause of respirable particles
indoors as well as various carcinogens, odors and other toxic
compounds (3). Some air filtering and purifying devices are
reported to clear up ETS somewhat. High levels of localized
ventilation may also help. However, the most practical solution
seems to be banishing the smokers to the outside of the house.
Building science is probably of limited help in alleviating
Dust Mite Allergens. About 20% of the general population suffer
from allergies. About 5% suffer from asthma. Among children
the percentages appear to be higher. The increasingly large
percentage of people suffering from allergy and asthma appears
to be a worldwide phenomena. In humid climates, dust mites appear
to be a leading cause of asthma (4). In the hot, humid central
Florida climate, approximately 60% of the patients complaining
of respiratory problems tested positive to dust mite skin prick
tests (5), rates two to three times greater than any other aero-allergen.
In a study of North Carolina school children, 30% tested positive
to house dust mites (6).
There are several dust mite species. Dust mites feed on human
skin flakes and are about 300 mm (micrometers or microns) in
size. There are two types of dust mite allergens -- type I and
II. The dust mite fecal particles contain large amounts of the
most common type I dust mite allergens, Der p I and Der f I.
The fecal particles are about 10 mm in size and heavier than
air. When airborne, they settle out in about five minutes. Dust
mites live and breed on carpets, sofas, beds and other soft
and fluffy furnishings. When a person lies down on a bed, they
may release puffs of dust containing dust mite fecal particles.
These are likely to be breathed in by the person before they
get a chance to be filtered out by a room or a whole house air
filter. Dust mites themselves also generate type II allergens
from their body parts.
clothes in hot (greater than 130°
F) water, frequent vacuuming with a central vacuum cleaner or
a good dust filter (pore size less than 1 micron) and covering
mattresses and pillows with impermeable covers can reduce exposure
to dust mite allergens. As will be discussed later, our research
has documented that weekly vacuuming with a central vacuum cleaner
may be an effective strategy as it removes the food source for
dust mites. Another way to dehydrate and kill dust mites is
through humidity control. Laboratory experiments (7) have shown
that limiting relative humidity (RH) to about 50% kills the
two most common species of dust mites. Our research indicates
that it is difficult to maintain RH below 50% at the floor level
in humid climates at all times, even if the average air RH level
is maintained at or below 50% with whole house central dehumidifiers.
However, we consider this is a very promising strategy and are
continuing research in this area. Some results are presented
later in this paper.
Pollen Allergens. Pollens from grasses, weeds and local
trees are another frequent cause of allergy. In a study of North
Carolina school children, 29% tested positive to grasses and
weeds. Measurements in the Tucson area (8) indicate that between
10 to 25% of the outdoor pollen can come indoors. Pollen can
come indoors by convective transport through open windows or
cracks or can be carried indoors on clothes and footwear. Since
most pollen grains are larger than 10 microns, efficient filtration
should be effective if the pollen can be trapped before they
are breathed in by humans. The practice of removing shoes and
outerwear outside the house may be helpful in reducing the pollen
inside homes. This could be encouraged by designing such a "mud
room" space in the garage just before the garage entry
door into the house as most North Americans enter the house
through their garages rather than their front doors.
Pet Dander Allergens from cats and dogs. This is another important
class of allergens. In the same North Carolina study, 25% of
the school children tested positive to animal danders. Pet dander
allergens are small, about 0.3 microns in size, and adhere to
walls after being airborne. Frequent bathing of pets can reduce
the allergens. However, like smokers, pets need to be outdoors
to eliminate this allergen from the house.
Molds. Mold allergy is also quite common. 20% of the
schoolchildren in the above study tested positive to molds.
Molds are ubiquitous in nature and not all molds are harmful.
Some molds such as Stachybotrus can be toxic and have
been implicated in several sick buildings and in the deaths
of children. If the surface relative humidity exceeds 65% to
70% on a continuous basis then molds can amplify and create
a problem, particularly in the absence of light and airflow.
Most molds in buildings arise from water leaks or mist carryover
from cooling coils and grow in porous materials. Proper moisture
control techniques, biocides and regular cleaning can control
molds in housing.
Cockroach allergens. In the North Carolina study only
5% of the children tested positive to cockroach allergens and
that's why we ranked the cockroach problem lower than most others.
Recent epidemiological studies (1) point to the increasing number
of asthmatic children (about 8.5%), especially in inner city
housing, who have developed asthma because of cockroaches. Proper
housecleaning and use of safe pesticides are needed for cockroach
Endotoxins. Airborne endotoxins are common outdoors
and most likely arise from gram negative bacteria on leaves.
Working with compost piles can cause asthma attacks in individuals.
Indoors, endotoxins have been found in house dust and in the
air in residences. Cool mist and ultrasonic humidifiers can
propagate endotoxins (9). Endotoxins have been linked to severe
lung problems. The sources and the prevalence of endotoxins
in residential and commercial buildings is not known with a
high degree of confidence. Likely sources appear to be standing
pools of water (e.g. humidifier reservoirs, refrigerant drain
pans which are not properly drained), large amounts of dead
leaves and decaying indoor plants, cat and dog excrement etc.)
Volatile Organic Compounds and Odors(VOC's). Many VOC's
and bioeffluents exist in houses, especially during the first
several months. While irritating or unpleasant, VOC's are probably
not unhealthy in concentrations found generally in houses, except
to the hypersensitive. Proper choice of materials for construction,
adhesives, finishes and furnishings can dramatically reduce
VOC exposures in houses. Adequate ventilation ( with dehumidification
in humid climates) should take care of the remaining VOC, odors
Fine Dust. Can be generated indoors during remodelling
projects or brought in from outdoors. In large quantities, fine
dust can trigger bouts of sneezing but is not generally toxic,
except to the hypersensitive. Positive pressure ventilation
and good air filtration can minimize the dust problem.
Radon. Recent large scale epidemiological studies using
10,000 households in Norway(10) seem to suggest that at levels
generally found in houses, Radon is not a significant health
risk. If it were, the strategies of positive pressure discussed
in this paper should be helpful in mitigating the Radon problem.
The DDAMPFIRM Systems Solution
From the above discussions it appears that a systems approach
to improving residential indoor air quality involves DDAMPFIRM
(Drainage and waterproofing, Dehumidification, Air tight construction,
Material selection, Positive pressure ventilation, Insulation
to avoid cold floors, Return air ducts or transfers, Maintenance):
Please note that energy efficient construction are integral
parts of the systems approach. Also, this is a work in progress,
not all the answers are known yet and considerable challenges
Drainage and Waterproofing. It is quite amazing that
given the importance of this topic, how often it is overlooked.
A partial checklist of things to look for in slab-on-grade houses
- The bottom of the slab or floor should be at least 8"
above the top of the backfilled dirt of the yard which should
be graded to drain away from the house.
- Install a tough polyethylene vapor barrier (minimum thickness
of 6 mm) under the slab and take special care to seal the
vapor barrier to all floor penetrations.
- Garage floor and driveway should be sloped to drain out.
- The roofs should have at least a 3 in 12 pitch and all
roof penetrations properly flashed.
- All windows should be under at least a 2 foot overhang.
- Roof gutters should be used and drain out on splash blocks
so water drains away from the house
- The wall system must be designed so that even if there
is water penetration, it is allowed to drain out. Consult
the manuals from Building Science Corporation (11) for excellent
detailed guidance on this subject.
- The drain pan of air-conditioners should leave no standing
water and if the drain clogs an A/C shut off switch should
Dehumidification. Whole house dehumidification in addition
to central heating and cooling is recommended to control mold,
mildew, endotoxins and dust mites. Room dehumidifiers are unlikely
to control humidity throughout the house, although it might
benefit a single room. Whole house dehumidifiers, as diagrammed
in Figure 1,
run independently of the central heating and cooling system
and control the RH in the entire house. This is especially important
in the months when the weather is humid and the temperature
is mild. During these months the cooling or heating system does
not operate frequently.
Air tight construction. Air tight construction is needed
to avoid uncontrolled entry of dust, pollen and humidity. Good
air distribution systems assure that the benefits of mechanical
ventilation and central dehumidification are distributed evenly
throughout the house. Several items need careful attention in
Figure 2. Applying mastic taped register outlet boxes.
1997 Orlando Health House.
- Air tight recessed can lights must be used. Code requirements
in several states have caused production by several manufacturers.
These lights should also be rated to allow direct placement
of insulation on the lights. An alternative is to build
a drywall box over a recessed can light and then foam seal
- A good way to airtighten frame walls, band joists, garage
ceilings and other hard to reach places is to use a combination
foam insulation and air sealer (Figure
5). A recent innovation is to construct the roof and
wall as a bubble i.e. foam insulate the bottom of the roof
deck and the soffitt areas so that the whole shell is air
tight. Figure 6 shows this construction. Note that there
is no vented attic space any more. This results in the ductwork
being in semi-conditioned space. Moreover the attic space
becomes more usable as a storage space as it won't be as
hot or cold as a vented attic. Until further research is
done, this construction method (suggested to us by Building
Science Corporation (11)) is not recommended in cold climates.
Figure 6. Insulated walls and roofline foam insulation
results in a bubble protecting the occupants. 1997 Orlando
Material Selection. Materials should be selected
to minimize dust production or collection, minimize VOC emissions
and with an understanding of their moisture control properties.
Easy maintenance and cleanibility are additional important factors.
- Figure 7 shows the duct system
used in the New Orleans Health House. The fiberglass duct
has a inner coating with a mildewcide and is fairly rigid.
This should prevent loose fibers in the airstream and the
hard surface should be easy to clean. The flex duct has
a heavy liner to permit future cleaning. The cut lines of
the ducts and the joint with the flex duct are sealed with
a UL listed adhesive and sealant.
- To minimize VOC emissions, carpets should be made out
of 100% Nylon fibers. Carpet cushions should be made of
synthetic fibers and not from rebond or urethane.
- Vinyl covered wall papers should be avoided, especially
on exterior walls.
- Choose low or no VOC paints, caulks and adhesives which
are easy to use and maintain.
- Kitchen cabinets and countertops should be sealed so
that there is no exposed particleboard.
- Minimize the use of oriented strand board and similar
products inside the conditioned space to create built
In addition there should be no unvented combustion appliances
inside the house. Correctly installed sealed combustion appliances,
including the fireplace, are necessary. In garages and basements
combustion appliances should be placed in permanently ventilated
spaces which cannot go to a severe negative pressure.
Positive pressure ventilation. In all climates positive
pressure ventilation is recommended. This is only possible if
the house is air tight. Positive pressure ventilation requires
only one intake and thus permits filtering and dehumidification
of the incoming air. Maintaining a positive pressure minimizes
infiltration of humidity and dust and improves combustion safety
(as opposed to negative pressures which may cause backdrafting).
In humid climates mechanical ventilation must be combined with
dehumidification. Otherwise the house may experience serious
moisture problems in mild and wet months when neither cooling
nor heating is required. With careful product selection it is
possible to construct houses with positive pressure ventilation
even in cold climates. A thorough understanding of moisture
control and building science is necessary before attempting
The kitchen and bathroom exhaust fans should vent outside of
the house, not into the attic, and should be installed for point
source exhausts in addition to the central whole house ventilation
system. The bathroom fans should be on timers to prevent excessive
To avoid excessive negative pressures from clothes dryers,
either locate them outside the conditioned envelope or have
a operable window in the utility room.
Filtration. Excellent filtration is a key to achieving
excellent indoor air quality. For many years studies have shown
that allergy sufferers feel more energetic and sometimes require
less medication when using room air cleaners or good air filters.
4" and 7" pleated filters as well as HEPA filters
are all good choices. Disposable filter elements are probably
better than washable types. Most whole house electronic air
filters produce some ozone and they should be avoided. If used,
the electronic air filters should be well maintained and regularly
cleaned per manufacturer instructions.
A frequent problem noted in many homes is the air bypass around
furnace filters. This should be avoided by tightly sealing around
the edges of the air filter.
Insulation to minimize thermal gradient between floor and
air in the house. In slab on grade houses the slab is often
poorly insulated or not insulated at all because of concern
with termites. This creates a RH (as well as temperature) gradient
between air in the middle of the room and that at the carpet,
where dust mites live and breed. In order to limit the RH to
50% at the floor, it may be necessary to set the dehumidifier
at 45% RH. Slab on grade houses are difficult to insulate around
the slab. A promising new foundation insulation material is
now on the market which is claimed to be termite resistant.
Return Air Ducts. Return air ducts or transfer ducts
should be used to assure good air distribution to bedrooms even
when interior doors are used. Air distribution can be augmented
by the patented FanRecycler (12). This system helps equalize
the ventilation, temperature, and humidity throughout the house.
Maintenance. Technology alone is not able to create
excellent indoor air quality in a house unless it is regularly
cleaned and maintained. The use of a outside vented central
vacuum cleaner is highly recommended as it exhausts the dust
laden inside air outside the home.
In this paper results are presented for two allergy resistant
homes in Orlando, FL and New Orleans, LA (labeled AR1 and AR2
respectively) and two conventional houses in the same cities
(labeled C1 for the Orlando house and C2 for New Orleans). All
except C2 are owned and occupied by a single family. C2 is a
two story bed and breakfast place, about 100 yrs. old whose
owners live upstairs and rent out the first floor rooms. AR1
is a conventionally built home where a whole house dehumidifier
and ventilation (DV) system was added during construction. AR2
is the New Orleans Health House sponsored by the American Lung
Association of New Orleans and Minneapolis. It features a energy
recovery ventilation (ERV) system and a central vacuum cleaner
which is used regularly. All houses except C2 have wall to wall
carpeting. C1 is a conventional home constructed in 1991 which
is more airtight than average.
Figure 8 shows
the measured temperature (T) and relative humidity (RH) data
at the carpet level in the Orlando area homes during 1996-7.
Note that in AR1 the RH is well controlled even during the winter
months whereas the RH in C1 exceeds 65% frequently in the winter
Figure 9 shows
the measured T and RH data at the carpet level in the New Orleans
area homes during 1996-7. In AR2 the RH levels are higher than
AR1 for the winter months. In C2 RH levels are even higher.
shows the measured dust mite allergen (Der f I) levels.
In each month several locations in each house were sampled and
analyzed. The maximum allergen level among the samples from
each house is plotted. The critical dust mite allergen level
is 2 units. As expected, the allergen levels of the conventional
houses are high. The allergen levels in the allergy resistant
houses are negligible even though the RH levels are higher than
50%. This is believed to be due to the thorough cleaning (weekly
to twice weekly vacuuming) practices of the AR1 and AR2 homeowners.
The energy consumption of the DV system in AR1 varies between
5.5 and 11 Kwh/day while the ERV system in AR2 uses about 3.5
It is encouraging that these allergy resistant houses have
negligible dust mite allergen levels. An earlier paper documents
the data from two other houses in the Orlando area. One of them
had high dust mite allergen levels despite having a DV system
(13). However, that house did not have a central vacuum cleaner.
The authors are continuing this research with these and other
1. D.Q. Haney, "Cockroaches are a Factor in Severe Urban
Asthma, Researchers Warn", Associated Press, July 14, 1996
as reported in many major newspapers including the LA Times,
2. A. M. Pope, R. Patterson and H. Burge (editors), Indoor
Allergens. National Academy Press, Washington, D. C. p. 67,
3. Thad Godish, Indoor Air Pollution Control. Lewis Publishers.
p. 29, 1989.
4. S. Chandra, R.F. Lockey, L.W. Alidina, D. Beal, R. Codina,
and K. Gehring, "Adverse Health Effects and Elimination
of Dust Mites in Humid Climate Housing," Proc. 7th International
Conference on Indoor Air Quality, Nagoya, Japan, July 21-26,
1996. Vol. 3., pp. 433-438, 1996.
5. E. Fernandez-Caldas, R. W. Fox, G. A. Bucholtz, W. L. Trudeau,
D. K. Ledford and R. F. Lockey. House dust mite allergy in Florida
- Mite survey in households of mite-sensitive individuals in
Tampa, Florida. Allergy Proceedings, Vol. 11/No. 6, pp 263-267,
6. A. M. Pope, R. Patterson and H. Burge (editors), Indoor
Allergens. National Academy Press, Washington, D. C. p. 52,
7. L. G. Arlian, Water balance and humidity requirements of
house dust mites. Experimental and Applied Acarology, Vol. 16,
pp 15-25, 1992.
8. M.K. O'Rourke and M.D. Lebowitz. "Environmental Allergens
and the Development of Chronic and Allergic Obstructive Lung
Diseases," Chapter 13 of Environmental Respiratory Diseases.
Eds- Cordasco, Demeter and Zenz. Van Nostrand Reinhold. p.303,
9. D. K. Milton "Bacterial Endotoxins: A Review of Health
Effects and Potential Impact in the Indoor Environment"
Ch. 11 of Indoor Air and Human Health, second edition, CRC Press,
10. K. Magnus, A. Engeland et al "Residential Radon Exposure
and Lung Cancer -- an Epidemiological study of Norwegian Municipalities,"
Int J Cancer vol. 58, pp. 1-7, Jul. 1, 1994.
11. Building Science Corporation, Westford, Massachusetts.
Phone - (508)-589 5100.
12. FanRecycler, U.S. patent #5,547,017 by Armin Rudd, Florida
Solar Energy Center.
13. Chandra, S., Beal, D., Lockey, R.F., Codina, R., Orr, G.W.,
Astry, D.W. and L.W. Alidina PERFORMANCE OF HEALTH HOUSES IN
WARM HUMID CLIMATES. Proc. Healthy Buildings/IAQ '97 Conference
, Washington DC, Sept. 27 - Oct. 2, 1997.