Designs That Work
Mixed-Humid Climate
The Basic House - Building Enclosure
A fundamental part of durable, energy efficient, and sustainable
construction is the design of the building enclosure. Water managed,
thermally efficient, and leak free building enclosures, while providing
for durable structures and reducing energy consumption, also allow us to
maintain better control of our interior environmental conditions. In order
to achieve this, the various components of the building enclosure (roofs,
walls, foundations, windows and doors) must be designed to fulfill their
individual requirements. However, these components must also be tied
together in such a way as to create a complete system to control rain
water, air leakage, vapor migration, and thermal transfer. In addition,
the systems should be economical while still being robust enough to handle
the various climate loads that are imposed on them.
Rain water infiltration is the largest source of material deterioration
in buildings. The control of rain water is best achieved if some simple
principles of drainage are followed. The fundamental design looks to
create a means to drain water off the building, out of the assemblies and
components, and away from the building. The design uses a strategy
referred to as an open rain screen approach. In an open rain screen
approach, the exterior primary layer of water shedding (cladding,
shingles, metal roofing, etc) is not relied upon to be completely
watertight. A secondary drainage layer (usually a housewrap or taped
insulating sheathing) is installed behind the main exterior water shedding
surface. This drainage layer, often referred to as a “drainage plane,” in
combination with flashing details allows water that may penetrate through
the exterior water shedding layer to drain back out to the exterior.
Figure 6: Diagram of Drainage
After liquid water intrusion, air
leakage is the second most common mechanism for depositing moisture in
wall assemblies. Air leakage occurs due to air pressure differentials
causing air to flow through or within the building assembly. In order to
control air leakage a continuous plane of airtightness should be created.
This plane of airtightness or air seal should be continuous not only for
each building assembly, but at the connection between adjoining building
assemblies. Uncontrolled air leakage can also impact the energy efficiency
of the building as infiltrating air will need to be conditioned or through
the loss of exfiltrating conditioned air. The Building America goal is to
achieve an infiltration rate equivalent to 2.5 square inches per 100
square feet of building enclosure area. Creating a continuous air seal is
possible with special attention at transition details between different
assemblies and systems.
Figure 7: Moisture transport comparison
Vapor transport through diffusion can be a benefit or a detriment. In
some circumstances, vapor diffusing into a wall assembly can condense and
accumulate resulting in problems with material deterioration. On the other
hand, vapor diffusion can also be used as a drying mechanism that will
allow assemblies to dry to either the exterior or the interior or both. In
general, the vapor control strategy used should maximize the drying
potential of the assembly while minimizing the potential for wetting. With
vapor diffusion being affected by both permeability of building components
and temperature gradients across assemblies, the vapor control strategy is
often related to, and integrated in, the insulation system design as well.
For mixed-humid climates, walls are generally designed to dry to both the
interior and the exterior (flow through assemblies) 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.
To control thermal transfer, the intention is to maximize the thermal
insulating value of all 6 sides of the building enclosure to levels that
are suited for the climate zone while not becoming cost prohibitive. The
thermal transfer if primarily managed by the insulation type, thickness,
and location; however other aspects such as framing design, and window
U-value and Solar Heat Gain Coefficient (SHGC) are important as well.
To keep the cost of the systems down, reducing material use in the
assemblies and material waste on the project is important. This can be
done by efficient layout of the house plan and efficient use of materials.
Reducing material use must be done in such a way however so as not to
affect the robustness or structural integrity of the building. Provisions
to maintain adequate lateral load resistance must always be incorporated
into the design.
The house is designed to be located in areas where termites are a
concern. Termites are best managed with a three-pronged approach that
deals with the three things termites need - cover from sunlight, moisture,
and food (wood or paper). Keeping plantings away from the building a
minimum of 3 feet and thin the ground cover (wood mulch or pea stone) to
no more than 2 inches for the first 18 inches around the building will
help to dissuade termites due to the lack of cover. A termite inspection
gap at the top of the foundation wall should be left as well. To reduce
the presence of water near the perimeter of the building, maintain a
positive slope away from the building to carry ground water away from the
foundation. It is also considered a good idea to make sure that irrigation
is directed away from the building. Finally the use of an environmentally
appropriate soil treatment (such as Termidor®) and treated wood in near
grade applications is often warranted.
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 by the others.
For example, if for some reason, chemical treatment of soil or building
materials is not an option, then complete rigor in controlling moisture
and ground cover must be maintained.
Roof Design
The roof is designed with asphalt shingles installed over a
predominantly cathedralized ceiling. While the shingles will ensure that
the vast majority of the liquid rain water and snow melt sheds off the
surface, an SBS roof membrane (similar to a W.R. Grace Ice and Water
Shield) fully adhered to the roof sheathing is installed at the eave
locations and completely over the low slope roof areas to protect the roof
from ice damming and potential water penetration from wind driven rain. A
primer may be required to facilitate the adhesion of the membrane to the
OSB. The overhangs from the roof are designed to extend a minimum of 12
inches from the exterior wall. This amount of overhang will provide some
protection for the wall elements such as windows and doors that are
traditionally common sources of water leakage. With the overhangs
preventing the wall systems from getting wet, the risk of water intrusion
through these elements is greatly reduced.
Figure 8: Roof Drainage The vented cathedral ceilings and attics
are designed with the interior plane of air tightness is located at the
plane of the interior gypsum board. All the joints in the gypsum boards
must be taped and sealed. In addition, any penetration through the gypsum
must be air sealed, and all light fixtures should use air tight electrical
boxes. In order to maintain the continuity of the air seal between the
roof and the wall, the interior gypsum board is sealed to the wood framing
at the top of the wall assembly at band joist locations or to the wall
gypsum board. In order to maintain the interior air seal at the band joist
location, sealants or gaskets are used to seal between the framing
members.
Figure 9: Roof Air Barrier
The vapor control strategy of the
assembly is to promote drying primarily to the exterior and to reduce the
amount of vapor able to diffuse from the interior environment into the
roof structure. This roof assembly has continuous back-venting from eave
to ridge of the structural roof deck, providing higher drying potential of
the assembly to the exterior. With the low vapor permeability of the rigid
insulation on the interior of the catherdralized ceilings and the latex
paint finish on the ceiling of the vented attic portions, the assembly
will prevent interior moisture from penetrating into the roof assembly,
however, this will also reduce the drying potential to the interior during
the summer months.
Figure 10: Roof Vapor Management The thermal resistance of the
cathedral assembly is provided by the R-30 blown in cellulose insulation
and the 1 inch of rigid XPS insulation installed to the interior of the
rafters. With cathedral ceilings, the framing members are thermal bridges
through the insulating layer. These thermal bridges will reduce the rated
R-value of the insulation approximately 20%. This means that the 2x12
rafters with a span of 24 inches on center, and with rated R-30 blown
cellulose will in reality have an effective R-value of around R-24 for the
entire assembly. The rigid insulation however spans the rafters. This
means that close to the entire rated insulating value of the rigid
insulation will be effective in providing thermal resistance. 1 inch of
rigid XPS installed to the interior of the structure will have an
effective R-value of R-5. Added together, the R-value of the assembly is
approximately R-29.
Figure 11: Roof Thermal Resistance Wall Design
The wall water management system is designed with a shingle lapped
vinyl siding. While no intentional ventilation and drainage gap is
provided behind the vinyl siding, research has shown that the cladding is
still very effective in draining water back out to the exterior and open
enough to allow for air flow behind the cladding to help with drying of
the cladding and wall assembly. The drainage plane of the assembly is the
rigid insulation. For the rigid insulation to be effective as a drainage
plane, all the vertical joints in the insulation must be taped and sealed,
while at the horizontal joints a through wall flashing of polyethylene is
installed. Additional protection at the vertical joints could be provided
by using an insulating sheathing material with ship-lapped joints
(fiberglass faced rigid insulation boards are not acceptable in this
application due to problems of adhering membrane flashings and sheathing
tapes to the fiberglass facing. All other flashings such as head flashing
and step flashings should be regletted into the face of the rigid
insulation (ensure that the cut does not fully penetrate the foam
sheathing) and the top edge taped to seal against water penetration. In
some cases, such as areas of increased risk of water infiltration due to
increased rain and wind exposure, it may be warranted to install a layer
of housewrap over the exterior of the insulating sheathing. This housewrap
will become the drainage plane for the wall assembly.
Figure 12: Wall Drainage
Figure 13: Drainage plane and Window Flashing The interior plane
of air tightness for the wall assembly is located at the plane of the
interior gypsum board. In order to maintain the continuity of the air
seal, the interior gypsum board is taped and sealed at all the joints. At
the roof to wall connection the air seal is maintained by sealing the
gypsum to the top plate of the wall assembly and sealing the framing
members at the band joist with sealants or gaskets. For the foundation
connection similar strategies are used. In addition, any penetration
through the gypsum must be air sealed. All electrical penetrations should
use air tight electrical boxes that are sealed to the gypsum. An exterior
plane of air tightness is created through the taped and sealed rigid
insulated sheathing. This exterior air sealing element will help reduce
some of the negative effects of wind washing of the insulation.
Figure 14: Wall Air Barrier
The 1 inch of rigid insulating
sheathing is designed to elevate the condensing surface in the wall
assembly to reduce the risk of condensation occurring within the assembly.
During the winter months, the interior humidity levels should be kept
lower to limit the amount of moisture able to diffuse into the wall
assembly. During the summer months the vapor drive will primarily be from
the exterior to the interior. To accommodate this, the assembly is
designed to be able to dry to the interior through the use of
semi-permeable latex paint on the interior gypsum. Drying to the interior
is important, therefore, interior vapor barriers should not be installed.
Figure 15: Wall Vapor Management The thermal resistance of the
assembly is provided by the R-19 blown in cellulose cavity insulation and
the 1 inches of rigid insulation installed to the exterior of the
structure. Similar to the cathedral ceiling discussion in the roof design
section, the wall framing members reduce the rated R-value of the wall
assembly upwards of 35% to 40%. This means that a 2x6 stud wall with a
rated R-19 cavity insulation will in reality have an effective R-value of
around R-13 for the entire assembly. In order to limit the amount of
thermal bridging that occurs, the house is designed with advanced framing
techniques (advanced framing uses 2x6 studs at 24 inches on center, single
top plates, two stud corners, and headers over windows only on load
bearing walls). This can reduce the framing fraction of the wall from
approximately 23% down as low as approximately 16%.
Figure 16: Wall Thermal Resistance In order to reduce material
use and construction waste, the layout of the walls on the floor plan
follows a 24 inch grid. This 24 inch grid makes use of standard material
dimensions for sheathing and insulation products. This reduces cutting and
material waste on site. Following this, the walls are designed with the
use of advanced framing techniques and insulating sheathing as the primary
sheathing material to reduce the overall material used in the design.
Though the primary sheathing is the rigid insulating sheathing, OSB
sheathing is still required to be placed at the corners to provide for
lateral load resistance for the house. At these locations, the insulating
sheathing thickness is reduced to ½ inch to accommodate the thickness of
the OSB. Foundation Design
The foundation is designed with a condition crawlspace. The exterior
foundation walls are cast-in-place concrete with a gravel floor covered
with polyethylene. At grade, a layer of impermeable soil (such as
compacted clay) sloped away from the foundation, should be installed to
direct rain water away from the foundation and prevent water from
absorbing into the soil in the immediate area around the foundation. Below
grade, the exterior of the concrete is coated with a dampproofing to
prevent liquid water from penetrating through the concrete. In addition,
the back fill material around the foundation should be free draining to
allow ground or rain water to drain down to the perimeter drain installed
at the base of the footing. The perimeter drain is also connected to the
gravel bed below the polyethylene on the crawlspace floor through pipes
cast into the footing. This allows for any water in the gravel bed to be
drained away as well.
To prevent moisture migration between the concrete foundation and the
floor structure above, a capillary break (a closed cell foam sill sealer
or gasket) is installed between the top of the concrete and the sill
plate. This isolates the framing from any source of moisture that may be
either in or on the concrete foundation. Using sill sealer on all walls
maintains the same wall height. Similarly, to limit the amount of ground
water absorbed through the footing, a capillary break (polyethylene) is
installed between the footing and the concrete wall. The four-inch deep,
3/4-inch stone bed functions as a granular capillary break below the
polyethylene, a drainage pad, and a sub-polyethylene air pressure field
extender for the soil gas ventilation system. Without it, a soil gas
ventilation system is not practically possible.
Figure 17: Foundation Drainage
The interior plane of air
tightness is maintained through the taped and sealed foil faced
polyisocyanurate (polyiso) rigid insulation and the polyethylene ground
cover. The polyethylene ground cover should be returned up the walls and
sealed the wall foil faced polyiso. At the rim joist the concrete wall is
sealed to the sill plate through the use of a sill gasket and sealant.
Figure 18: Foundation Air Barrier The concrete foundation wall is
able to dry to the exterior through the exposed above grade portion of the
wall. On the interior, the 1 ½ inches of foil faced polyiso insulation
(foil faced polyiso is a Class 1 vapor retarder - at 0.03 perms)
considered to be vapor impermeable, would eliminate interior moisture from
being able to diffuse through the assembly and condense on the interior
surface of the concrete wall, however this also prevents the assembly from
being able to dry to the interior. Due to this, it is recommended to wait
to install the foil faced polyiso until near the end of construction to
allow for as much drying of the concrete as possible. After installation,
the concrete will still be able to dry through the exposed portion above
grade. For exterior soil moisture, the dampproofing on the foundation
walls is used to control the migration of moisture from the exterior soil
into the foundation walls. For the ground, the vapor control layer is
provided by the polyethylene ground cover over the gravel bed.
Figure 19: Foundation Vapor Management
The thermal resistance for
the crawlspace is provided through the 1 1/2 inches (R-9.75) of foil faced
polyiso insulation installed on the walls. With cavity insulation, the
framing members (studs, top and bottom plates, window headers, etc) are
thermal bridges through the insulating layer. These thermal bridges can
reduce the rated R-value of the insulation upwards of 35% to 40%. This
means that a 2x6 stud wall with a rated R-19 fiberglass batt will in
reality have an effective R-value of around R-13 for the entire assembly.
For this design, since the insulation is installed in a continuous layer,
concerns with thermal bridging of the insulation are essentially
eliminated. This means that close to the entire rated insulating value of
the insulation will be effective in providing thermal resistance for the
crawlspace walls.
Figure 20: Foundation Thermal Resistance Windows and Doors
The window and door installations are designed to be drained systems. A
pan flashing is installed below every window and door to direct any water
that may leak through or around the window back out to the exterior. The
nailing flanges of the window are sealed with a membrane flashing on the
jambs and head of the window. The sill is left open to allow the water to
drain out. At the head, a head flashing should be regletted into the XPS
sheathing and the top edge taped (Please refer to window installation
sequence details on drawing A-6).
Figure 21: Window Pan Flashing The continuity of the air barrier
is maintained by installing a bead of non-expanding urethane foam between
the window frame and the rough opening on all four sides of the window.
The foam is installed from the interior prior to the installation of the
interior trim. The foam should also be closer to the interior so as not to
block drainage of the pan flashing at the sill of the window.
Figure 22: Window Air Barrier Continuity The thermal resistance
of the window is provided by the overall U-value of the window assembly as
well as the Solar Heat Gain Coefficient. The values used for this home
were a U-value of 0.33 and an SHGC of 0.28 and are representative of what
is available on the market. For cold, it is recommended to minimize the
overall U-value of the windows for all orientations, however having a
higher SHGC on the South elevation can be of some benefit through
increased solar gain in the winter months offsetting the heating loads for
the house. While this is a good idea in theory, finding a window that has
a low U-value and a high SHGC can be difficult. In general windows with
lower U-values also have lower SHGC’s. Other Penetrations
There are many other penetrations that are often overlooked in the
design of houses. These are from dryer vents, bathroom exhaust fans,
exterior electrical outlets, exterior lights, gas lines, etc. These
penetrations must be designed into the water management system. Pipe
penetrations such as bathroom exhaust vents or dryer vents should be
stripped into the drainage plane with membrane flashing. Where the
electrical box are installed flush with or penetrates through the drainage
plane, the box should be stripped in with a membrane flashing to create a
flanged seal to the drainage plane. Alternately there are products
available on the market that have flanges as part of the electrical box or
mechanical vent. With these products the flanges can be then integrated
into the drainage plane.
All penetrations through the plane of air tightness should be sealed
with caulking or spray foam in order to maintain the continuity of the air
barrier.
These penetrations are thermal bridges. In order to minimize the effect
of the thermal bridging, the insulation should be installed as close as
possible to the penetration to minimize the impact of the disruption of
the insulating layer.
Energy Model Results
The results of the building enclosure upgrades represented a reduction
in energy consumption of 6.0% when compared to the energy consumption of
the Building America Benchmark house design. |
