Designs That Work
Very Cold 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 a rain screen approach. In a rain screen approach, the
exterior primary plane of water shedding (cladding, shingles, metal
roofing, etc) is not relied upon to be completely perfect. A secondary
drainage plane (usually a housewrap or taped insulating sheathing) is
installed behind the main exterior water shedding surface. This drainage
plane in combination with flashing details allow any water that may
penetrate through the exterior water shedding plane to drain back out to
the exterior.
Figure 5: 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 air seal must be created. This air seal must 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; however, special attention is often needed at transition details
between different assemblies and systems.
Figure 6: 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 hot humid climates such as this, the assemblies are designed to
prevent hot humid exterior air from diffusing into the assemblies, while
allowing the assemblies to dry to the interior.
To control thermal transfer, the intention is to maximizing 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 wind and seismic resistance must always be
incorporated into the design.
Roof Design
The roof is designed with asphalt shingle installed over a layer of
building paper on OSB sheathing. Below the OSB sheathing is a 4 inch
ventilation space created by installing 2x4 studs on edge on a 4 inch wide
strip of ½ inch plywood that is screwed through to the rafters. This
ventilation space will help remove any heat loss through the insulation to
prevent problems with ice damming on the roof eaves.
Figure 7: Roof Construction Section
In addition the ventilation space will help to dry any moisture that
may penetrate past the exterior shingles. Below the insulation is a
drainage plane created by the housewrap, this is the final layer of
protection against any water intrusion into the assembly and must be
continuous. The overhangs from the roof are designed to extend a minimum
of 2 feet from the exterior wall. This amount of overhang will provide
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 attic is designed as an unvented attic. With unvented attics such
as this, the plane of air tightness is located at the plane of roof and
not at the ceiling plane as is common with vented attic designs. While the
attic is not vented to the exterior, soffit and ridge vents are installed
to vent the gap between the insulation and the exterior roof sheathing.
The air tightness for this assembly is provided by the housewrap
sandwiched between the rigid insulation and the interior layer of roof
sheathing. In order to maintain the continuity of the air seal between the
roof and the wall the housewrap must be continuous from the roof down onto
the wall with all the joints taped and sealed.
Figure 9: Roof Air Barrier
With all of the insulation installed to the exterior of the structure
common problems of condensation within the structure are eliminated. The
location of the insulation moves the dew point of the assembly exterior of
the structure and in a location where the materials used in the
construction are resistant to moisture damage. If condensation were to
occur, it is exterior of the drainage plane of the assembly and the
moisture would be able to drain out to the exterior.
Figure 10: Roof Vapor Management
The thermal resistance of the assembly is provided by the 12 inches of
rigid EPS insulation installed to the exterior of the structure. 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 exterior of the structure, concerns with thermal bridging of the
framing members are essentially eliminated. This means that close to the
entire rated insulating value of the insulation will be effective in
providing thermal resistance. 12 inches of rigid EPS installed to the
exterior of the structure will have an effective R-value of R-42.
Figure 11: Roof Thermal Resistance
Wall Design
The wall water management system is designed with a ventilated and
drained cavity behind the wood siding. The wood is held off of the rigid
insulation with 1x4 furring strips. These furring strips provide for an
air gap that acts both as a drainage gap and ventilation gap. This allows
any water that penetrates past the siding to drain to the exterior and
allows for air flow behind the cladding to help with drying of the cavity.
In order to protect the wood from moisture related problems, the wood
siding should be back primed (primed on all 6 sides including end cuts)
with an oil based primer and painted with two coats of latex paint. The
actual drainage plane for the assembly is the housewrap behind the rigid
insulation. Likely, any water penetrating past the cladding will drain
down the exterior face of the rigid insulation, however, some water may
still get past at the joints in the rigid insulation boards. For this
reason it is still important that the continuity and integrity of the
housewrap drainage plane be maintained. All flashings should be tied back
to this plane and shingle lapped into the housewrap.
Figure 12: Wall Drainage
The air tightness for this assembly is provided by the housewrap
sandwiched between the rigid insulation and treated OSB sheathing. The
continuity is maintained at the top by ensuring continuity with the roof
housewrap. At the connection to the floor, the housewrap is continuous
past the rim joist and sealed to the OSB sheathing. The air seal is then
maintained by sealing the OSB sheathing to the rim joist of the floor
assembly.
Figure 13: Wall Air Barrier
With all of the insulation installed to the exterior of the structure
common problems of condensation within the structure are eliminated. The
location of the insulation moves the dew point of the assembly exterior of
the structure and in a location where the materials used in the
construction are resistant to moisture damage. If condensation were to
occur, it is exterior of the drainage plane of the assembly and the
moisture would be able to drain out at the bottom of the wall assembly to
the exterior.
Figure 14: Wall Vapor Management
The thermal resistance of the assembly is provided by the 8 inches of
rigid EPS installed to the exterior of the structure. As mentioned in the
roof design section, with cavity insulation, the framing members 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 8 inches of rigid EPS installed to the exterior of the
structure will have an effective R-value of R-28.
Figure 15: Wall Thermal Resistance
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 (advanced framing uses 2x4 studs at 24 inches on center, single
top plates, two stud corners, and headers over windows only on load
bearing walls).
The lateral load resistance is provided by completely sheathing the
wall area with OSB sheathing.
Foundation Design
The foundation is designed as a pier foundation with the floor elevated
off the ground. This foundation allows for more construction options in
areas where the ground is frozen for long period of the year and uneven
rocky conditions make creating level footprints more difficult. In
addition, the open nature of the foundation will allow for snow to blow
through, preventing severe drifting of snow up against the house.
Figure 16: Foundation Drainage
The air tightness for this assembly is provided by the housewrap
sandwiched between the rigid insulation and the OSB subfloor. At the
connection to the wall, the housewrap is draped over the exterior of the
rim joist and sealed to the back of the wall OSB sheathing.
Figure 17: Foundation Air Barrier
The insulation is installed above the framing structure of
the floor. The assembly is designed to dry to both the interior and the
exterior. The EPS insulation is semi permeable and will limit the amount
of moisture that is able to diffuse into the assembly. Any moisture that
does will be able to dry to the exterior.
Figure 18: Foundation Vapor Management
Similar to the wall assembly, the thermal resistance of
the assembly is provided by the 9 3/8 inches of rigid EPS insulation
installed in the floor structure. For this design 9 3/8 inches of rigid
EPS installed to the underside of the structure will have an effective
R-value of R-33.
Figure 19: Foundation Thermal Resistance
For this design, the strength of the floor structure is
provided by the EPS insulation sandwiched between two layers of OSB
sheathing. Load from the exterior walls is transferred through the floor
structure by the double 2x10 perimeter rim joist. At interior partition
walls additional 2x10 floor joist are installed to transfer the load to
the pier foundation. This reduces the amount of framing that is
traditionally required in a standard floor assembly while also providing
for superior 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 window is located in the wall so that the flanges of the
window are at the same plane as the housewrap drainage plane behind the
rigid insulation. 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, the housewrap should be
lapped over the membrane flashing to prevent a reverse flashing from being
created (Please refer to window installation sequence details on drawing
A-7).
Figure 20: 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 21: 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 0.3 and
are representative of what is available on the market. For very cold
climates, 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 23.7% when compared to the energy consumption of
the Building America Benchmark house design. |
