Designs That Work
Cold Climate: Denver Profile
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Cross Section
Construction Recommendations
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Foundation: Basement w/sub-crawl
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Above Grade Walls: Wood frame
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Cladding (1st floor): Brick veneer
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Cladding (2nd floor): Wood siding
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Attic: Unconditioned
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Roof: Asphalt shingles
Building Science Notes
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Ducts in conditioned space - This building
profile is designed to accommodate HVAC equipment and ducts in the
basement and living space, not in the unconditioned attic. HVAC ducts
should not be run in exterior walls.
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Air sealing details at transitions - Air sealing
can be particularly difficult, but no less important, at assembly
transitions such as band joists, and between attached garages and living
spaces. These are discussed below because they have proven to be a
consistent challenge for builders.
Figure 9 |
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Band joists - Continuity of an exterior air barrier can
be maintained at the band joist with sealed or taped housewrap or rigid
foam insulation. Continuity of an interior air barrier can be maintained
through a combination of cut foam blocks and sealant/caulk, rigid
draftstopping material (wood blocking) and sealant/caulk, or spray foam.
Note that neither cellulose nor fiberglass (batt or blown) can be used
for the air barrier. The air barrier detail on second-story band joists
is important because it is inaccessible (covered by structural/finish
floor and ceiling finish) after construction. Note that while fiberglass
batts fulfill the requirement for protection from ignition in the open
band joists, fiberglass batt material by itself cannot maintain the air
barrier.
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Attached garages - the building enclosure surfaces shared
between conditioned space and an unconditioned garage must have a
continuous air barrier. See Figure 9 for details in terms of using
sealants and rigid insulation to create a continuous air barrier between
the attached garage and living space. See
Air Sealing / Air
Drywall Approach Details.
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Drying mechanisms - In any climate, vapor control
is based on the relationships among the following: the permeability of
wall components, the type of cladding (reservoir or non-reservoir), the
presence/lack/nature of an air space, and the magnitude/duration of the
vapor drive (based on the relationship between the exterior and interior
moisture content and temperature differences). The type of sheathing and housewrap
used in any wall assembly must be based on an understanding of these
inter-relationships. See "Insulations, Sheathings, and Vapor
Diffusion Retarders" for more information).
In cold climates, the moisture load in the winter months is primarily
from the interior, so roof and wall assemblies are generally designed to
dry primarily to the exterior. Wintertime condensation control can be
facilitated by elevating the temperature of the first condensing surface
(the back side of the exterior sheathing) via the use of insulating
sheathing. When XPS (with relatively low permeability) is used, then
only slow drying is available to the exterior. Accordingly, the majority
of drying occurs to the interior during the summer months. Therefore,
interior vapor barriers should not be installed. Note that there is a
difference between an interior vapor barrier and an interior vapor
retarder (see "Insulations, Sheathings, and Vapor
Diffusion Retarders"). Particular care must be
taken to prevent the entry of bulk water (i.e., leaks) and control
interior relative humidity in the coldest month. See Material
Compatibility and Substitutions.
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Drainage plane, air barrier, vapor control - On
the second story, the drainage plane is the housewrap - it must be
weatherlapped at any horizontal joints. On the first story, the rigid
insulation becomes the drainage plane - all vertical joints must be
shiplapped, flashed, or sealed; all horizontal joints must be sealed or
taped. Note how flashing maintains the continuity of the drainage plane
at transitions.
This building profile has a continuous air barrier on the interior
(see Airtight Drywall Approach
on ceiling and walls
and on the exterior walls (the housewrap on the second story and sealed
rigid insulation on the first story).
In cold climates, walls are generally designed to dry to the exterior,
with the vapor permeability of the exterior of the wall being 5 times
more permeable than the interior; or, they are designed with insulating
sheathing in order to control the temperature of the condensing
surfaces. The thickness of the insulating sheathing is determined by
calculation based on the severity of the climate (see "Insulations, Sheathings, and Vapor
Diffusion Retarders"). Walls constructed with XPS
insulating sheathing are designed to dry to the interior during the
summer months. Latex paint or some other vapor retarder (i.e. the kraft
facing on fiberglass batts or CertainTeed's MemBrain® Smart Vapor
Retarder) acts to slow moisture entry in to the framed assembly from the
interior. Ideally, the more vapor permeable EPS rigid insulation works
well as the thickness of insulation goes beyond 1 inch, but see
Field
Experience Notes for more discussion.
Figure 10 |
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Rough opening flashing - See the Figure 10 for
the brick veneer flashing (for drainage plane continuity). For more
details see the EEBA Water Management Guide at the
EEBA Bookstore.
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Advanced framing - On the first story, the wall
assembly replaces structural sheathing with cross bracing or some
alternative for shear resistance; thermal performance and reduced
drywall cracking are additional benefits of a comprehensive approach.
See Advanced
Framing Details
for details.
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Framing on slabs - Installing a capillary break
between the sill plate and a concrete slab on all walls - exterior,
interior, partition - is good practice. A closed cell foam sill sealer or
gasket works well. Alternatively, a strip of sheet polyethylene can be
used. This isolates the framing from any source of moisture that may be
either in or on the concrete slab (and using sill sealer on all walls
maintains wall height exactly the same). In this profile, only garage
walls would be affected.
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Soil gas ventilation - Soil gas ventilation is
accomplished by the continuous exhaust ventilation system in the
structural sub-crawlspace. Radon, soil moisture, and any volatilized
organic substances that may be in the soil from previous land use
(agriculture) are all handled with this exhaust system. The continuous
polyethylene sheeting should still be installed on the floor of the
sub-crawl to reduce the soil gas load that the continuous exhaust
ventilation system handles. Alternatively, a passive system exiting
through the roof can be used.
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Thermal barrier - Cavity-warming exterior rigid
insulation is important in this climate where the average monthly
temperature for the coldest month of the year goes below 45°F (see
Unvented Roof Systems
for more information). The heat loss through the basement walls is
significant enough to warrant 1-1/2 to 2 inches of rigid insulation;
note that the insulation must either comply with local codes for
protection against ignition or be covered with material such as gypsum
wallboard.
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Vented attic - Soffit and ridge vents provide
more effective attic ventilation than gable-end vents. Gable exhaust
fans do not provide effective attic ventilation. They are generally
temperature-controlled, when relative humidity is often the condition
that requires higher ventilation rates. They can also depressurize the
house causing loss of conditioned air. Generally, the area of the gable
and soffit vents combined with the leakage of the attic ceiling is such
that the fan pulls air not just from the exterior vent, but from the
conditioned space below.
Climate Specific Details
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Below-grade insulation - Ground temperatures make
wall and slab insulation an important part of the thermal barrier.
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Above-grade insulation - Homes in this climate
benefit from exterior insulation that warms whatever structural material
is to its interior, protecting it from the moisture degradation that can
occur as the result of condensation.
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Ice dam protection - The combination of
sufficient roof pitch, adequate insulation just above the exterior wall,
and air sealing at the wall-roof assemblies transition are all essential
to prevent ice dams. But ice dams can occur even in properly detailed
roof assemblies from differential solar snow melt. A water protection
membrane at the eave is recommended on all roof assemblies in this
climate.
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Mechanical ventilation -
Figures 11a-d |
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Heating and Cooling - Sealed combustion hot air furnace; SEER 12 air
conditioning unit. A single return requires transfer grilles to provide
return path and avoid pressurizing bedrooms. See Figures 11a-d.
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Ventilation -
For homes with central forced air distribution system:
Intermittent central-fan-integrated supply, designed to ASHRAE 62.2P
rate, with fan cycling control set to operate the central air handler as
much as 33% of the time, but not less than 25% of the time, occurring
within at least every three hours to provide ventilation air
distribution and whole-house averaging of air quality and comfort
conditions ($125 to $150). Outside air fraction is designed to keep
mixed air temperature at furnace heat exchanger above 50°F, usually not
more than a 10% outside air fraction. Optionally include a normally
closed motorized damper in the outside air duct (+$50 to $60). See
Figures 12, 13 and 14.
In very cold climates ventilation can also be accomplished using a
continuous single-point exhaust system, designed to ASHRAE 62.2P rate,
pulling from the principal living area ($150).
Figure 12 |
Figure 13 |
Figure 14 - Basement Configuration |
Figure 14a - Supplemental Dehumidification |
For homes without central forced air distribution system:
Continuous multi-point exhaust, designed to ASHRAE 62.2P rate,
pulling from each bedroom, unless the bedroom has a bathroom. Then it
will pull from the bathroom and from at least one location in the
principal living area. Any combustion appliances must be direct-vented
sealed combustion.
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Supplemental dehumidification - Although high
performance homes in this climate rarely require supplemental
dehumidification, it is:
- one of three strategies appropriate for conditioned crawlspaces (see
Building Science Notes),
- may be desired in homes with full basement foundations, and,
- is strongly recommended when occupants require humidity control (and
high-efficiency air filtration) for asthma trigger control.
There are a number of different ways to accomplish supplemental
dehumidification with varying costs and performance advantages (for a
detailed discussion of supplemental dehumidification see
Conditioning Air). Described below is one low-cost yet effective
approach and one more costly but higher performance/systems-engineered
approach:
1. Ducted stand-alone dehumidifier: This
system is a "site-constructed" one consisting of an off-the-shelf
standard dehumidifier ducted in the attic and controlled by a
dehumidistat located in the living space. This arrangement of individual
components has proven to be an effective and economical system for the
production home building setting. The installed cost ranges from
approximately $350 to $550. The system is comprised of a GE dehumidifier
model AHG40FCG1 (dehumidifier located in attic in an insulated enclosure
and ducted to living space), Honeywell dehumidistat model H8808C located
in living space, and Honeywell switching relay (with transformer) model
RA89A 1074. See Figure 14a. The selection of the dehumidifier is based
largely on the fact that it has a blower wheel rather than paddle fan
that moves air more efficiently in its ducted box configuration.
Note: The following manufacturers make Energy Star-qualified
blower wheel stand-alone dehumidifiers:
2. Aprilaire 1700: This is a truly
engineered, coherently manufactured, supplemental dehumidification
system with built-in air filtration, ducted design, and a controls
package that integrates central blower cycling for distribution,
dehumidification and intermittent introduction of outside air
ventilation. The system is also designed for flexibility-it can be
connected to the conditioned space directly or to the central air
distribution system in a number of configurations. It's also compact and
lightweight enough to be set on or hung from most framing. The product
cost for this system is currently about $1,100. For more
information, see:
http://www.aprilaire.com/category.asp?id=F63D255EB0054BBF811DBB024BF068FA.
For more information on other high performance supplemental
dehumidification systems, see:
http://www.thermastor.com/DesktopDefault.aspx.
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Structural sub-crawlspace - This detail is
actually soil- rather than climate-specific. For areas with expansive
soils, the simplest solution is to build post-tensioned slab-on-grade
foundations that "float" over and in expansive soils. If homes must have
basements, the structural sub-crawlspace resists the lateral forces of
expanding soils, while deep piles carry the house structural load to
bear on deep stable material. This sub-structural crawlspace must have a
continuous vapor barrier ground cover and must be vented to prevent
moisture problems. A continuous 50 to 100 cfm exhaust fan accomplishes
this ventilation without any significant or real effect on the pressure
balance within the basement, the rest of the home, or with any
combustion sources that may be in the home (50 cfm for homes 2,000 sq.
ft. and less and 100 cfm for homes over 2,000 sq. ft.).
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Insect management - In cold climates, insect
pressure (termites and carpenter ants) is less pronounced than in warmer
climates, but important nonetheless. A three-pronged approach deals with
the three things insects need - cover, moisture, and food (wood or paper).
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Reduced cover - Keep plantings 3 feet away from the
building perimeter, thin the ground cover (wood mulch or pea stone) to
no more than two inches depth for the first 18 inches around the
building, and maintain any termite inspection zone on the foundation.
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Control moisture - Maintain slope away from building
as shown, carry roof load of water at least three feet away from
building, and make sure that irrigation is directed away from the
building.
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Chemical treatment - Use an
environmentally-appropriate building materials treatment (such as Bora-Care®)
for insect-prone, near-grade wood materials.
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Inter-relationship of first three points - Since a
builder and a homeowner's ability to employ or stick to each of the
three strategies above will vary, make sure that an inability to fully
employ one strategy is compensated for by complete rigor with others.
For example, if for some reason, chemical treatment of soil or
building materials is not an option, then complete rigor in moisture
control and ground cover is required.
Field Experience Notes
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Selection of rigid insulation - Most builders select
rigid insulation based on costs and handling properties. Although the
vapor permeability of both EPS and rigid fiberglass insulation can make
them particularly well suited to cold climate envelope assemblies, their
availability or their user-friendliness (or both) generally make XPS
insulation the builder choice. For these reasons, we recommend 1-inch or
thicker XPS in most wall assemblies. But remember, the type of sheathing
to use is always a question that should be asked in the context of:
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Joint treatment in rigid insulation - Shiplapped
rigid foam insulation has proven to be available in only very limited
areas. Mastic works as a water sealant but its long term performance is
not known but appears promising. The flexible flashing with polyethylene
film is straightforward and creates a natural weatherlap and therefore
is the preferred approach.
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Reservoir claddings - Solar-driven moisture can
be a significant problem with cladding such as wood clapboards and brick
if wind, rain, and sun work together to saturate the siding and then
drive this moisture into the wall assembly. The primary defense against
this is the continuous back-venting of the air space interior to the
cladding. The low permeability of the material just to the interior of
the air space is a secondary defense. A 3/8-inch space works well for
the wood clapboard (large enough for free flow of water and air and not
large enough to create reveal problems at windows, doors, cornerboards).
For more information on this topic, see
Solar
Driven Moisture in Brick Veneer.
A 1-inch air space and open head joints (top and bottom of wall) are
required for the brick veneer. Harking back to "old-timer"
practices - place a thin sand layer at the bottom of the 1-inch air space
(to act as a bond break for mortar droppings) and leave bricks out
intermittently in the first course. After the veneer is completed, the
sand and mortar droppings can be easily removed and the missing bottom
course brick mortared in.
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Brick ties - The two-piece adjustable ties are
strongly recommended, given the 1-inch air space and foam insulation.
See the Masonry Institute Technical Notes -
http://www.brickinfo.org/pdfs/44b.pdf.
Figure 15 |
Figure 16 |
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Brick shelf in foundation wall - The brick veneer
"seat" is readily accomplished by securing dimensional lumber of the
desired size to the inside top edge of the concrete form.
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Perimeter drainage in expansive soils - The
plastic trough in which the perforated drain pipe is placed in this
assembly prevents water collected in the pipe from passing into the soil
around the foundation pilings. This is an important foundation detail in
expansive soils.
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Advanced framing - For a technical resource that
may help with resistance to advanced framing methods from local code
officials, see the
Building Safety Journal article written by
Peter
Yost of BSC.
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Stepped foundation insulation detail -
Maintaining thermal barrier continuity in stepped foundations has proven
difficult, or easy, to neglect. Figure 15 shows how the rigid foam needs
to be cut and placed.
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Energy trusses - There are a number of different
truss configurations that yield greater depth at the heel, but they vary
quite a bit in cost. The truss shown Figure 16 (sometimes called a
"slider" truss) has proven to be among the most cost-competitive. And of
course, the pitch of the roof affects just how much insulation you can
get at this location, regardless of the type of truss.
Material Compatibility and Substitutions
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Exterior sheathing - On the second story of this
building, cavity-warming rigid insulation (usually strongly recommended
for cold climates) is lacking. It is therefore doubly important that the
exterior elements of this wall be vapor permeable or vapor
semi-permeable. A material such as fiberglass-faced gypsum board (DensGlass
Gold®, for example) is more permeable than plywood, which is more
permeable than OSB, although all are acceptable. A material such as a
faced high-density paperboard (Energy Brace® or Thermo-ply®, for
example) is vapor impermeable and is not recommended in this climate.
See the Building Materials Property Table for more
information.
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Rim joist insulation - Fiberglass cavity
insulation at the rim joist is only acceptable if the exterior sheathing
is cavity-warming rigid foam insulation that extends down over the
entire rim joist. If non-insulating exterior sheathing is used at the
rim joist then one of the following insulation details must be used at
the rim joist (see Figure 16a) to manage condensation potential:
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Drainage plane on rigid insulation - An
alternative to flashing, shiplapping, or sealing the XPS insulation for
continuity of the drainage plane is to apply a housewrap to the outside
of the insulation. The housewrap then becomes the continuous drainage
plane. It is also possible to install the housewrap under the XPS
insulation (this is a more common commercial wall system approach).
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Interior latex paint - The substitution of low permeability interior
finishes (vinyl wall paper, oil-based paints) for latex paint should be
considered in the context of severely limiting or eliminating any drying
potential that the wall assembly has to the interior. These interior
treatments are not recommended.
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Cavity insulation materials - Acceptable cavity
insulation includes any that have a relatively high vapor permeability - cellulose, fiberglass, foam (as long as air sealing is
accomplished by a separate component or system when cellulose or
fiberglass is used). User discretion can be based on properties other
than building science.
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Floor system between basement and sub-crawl - We
selected the cast concrete system as the most robust and moisture
resistant of the options - wood, steel, and concrete.
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Gypsum wallboard - Areas of potentially high
moisture, such as bathrooms, basements, kitchens, are excellent
candidates for non-paper faced wallboard systems (e.g. James Hardie's
Hardibacker®, GP's DensArmor®, USG's Fiberock®). In addition,
paper-faced gypsum board should never be used as interior sheathing or
backer for tub or shower surrounds where ceramic tile or marble (any
material with joints or grout lines) is used as the finish.
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