Sealing the Building Envelope
Jan Brandjes | 07 Apr 2021
Sealing the building envelope of a home during construction is not only extremely cost-effective for the builder, but also will ensure true energy savings and high levels of comfort for the home owner.
Above: Click the image to watch the playlist on wrapping and sealing the building
Air Sealing Details
The air sealing details in this article attempt to minimize the effort required to reduce air leakage in residential buildings. The idea is simple -- reduce the number of sealing steps and delay them until as late as possible in the construction process.
Air sealing consists of two main parts:
Subfloor sheets and plasterboard wall and ceiling and ceiling finishes (excellent air barrier materials that cover 90 percent of the building) and the joints between the sheets.
A standard set of other components and penetrations in the building envelope that require sealing.
The air sealing techniques focus on details typical in site-built, timber-frame construction in which most of the walls and ceilings are covered with plasterboard.
•Floors over Crawlspaces
•Rim Joists
•Attic Access
•Fully Sealed Exterior House-wrap
•Connections Between Floor and Exterior Wall
•Intersections of Interior Partition and Exterior Wall
•Intersections of Interior Partition and Exterior Ceiling
•Windows
•Doors
•Bath Drain
•Plumbing Vents
•Recessed Lights
•Electrical Boxes
•Stairs on Outside Walls
•Areas Closed During Framing
•The Last Resort
•Fireplaces
•Chimneys
•Combustion Appliances
•Forced-Air Heating and Cooling Systems
•Unnecessary Sites
Floors over Crawlspaces
In spite of the earlier statement about sealing as late as possible in the construction sequence, floors over crawlspaces are easiest to seal while the subfloor is being laid. (However, it can be done later.) Most builders use adhesives in addition to nails or staples to attach the sheathing to the floor structure. This provides an excellent opportunity to seal the floor, making an airtight subfloor that runs continuously under all exterior and interior walls. For this to work, the bead of adhesive must be continuous -- without any gaps.
Rim Board
The rim board of a floor between heated spaces is a major leakage site and one that can be difficult to seal well. Because circumstances vary from job to job, you'll want to have several methods for sealing boards.
Fully Sealed Exterior House Wrap
Many builders already use house wrap, which is a vapor permeable sheet applied to the outside surface of walls. House wrap has several advantages: it's durable, easy to inspect and unaffected by timber shrinkage. Properly installed, it seals the entire wall, including the rim board. It prevents heat loss due to air leakage and blocks wind that can penetrate walls and reduce insulation effectiveness. It pays to discuss the importance of a sealed house wrap with the bricklayer!
To accomplish all this, the house wrap must be installed in a way that seals the whole wall.
Connector strips must be installed at the top and bottom of the wall. They connect to the air barriers on the floor and ceiling. These connector strips are probably the most difficult part of the job.
All joints must be sealed. Tape is the most common material, but some locations may require caulk.
All penetrations must be sealed to the house wrap, including: pipes, wires, windows, doors, vents, etc. As a precaution, it would be a good idea to seal these penetrations on the inside of the wall, too.
Remember that even fully sealed house wrap only seals the exterior walls. Ceilings and floors, where much air leakage occurs, must be sealed too.
Connections Between Floor and Exterior Wall
Some builders use caulking or gaskets under the bottom plate of an exterior wall. To expand and contract, caulking requires a joint that’s at least 3 mm x 3mm. Setting a wall on top of a bead of caulking simply mashes the caulking, leaving nothing to fill a gap that may open in the future. Another gasket is needed between the bottom plate and the floor.
Here’s another way:
Seal the joint between the subfloor and the plaster or other interior wall covering. Sealing this leakage site after the plaster saves the framing crew an extra step while they’re trying to put up the walls. It also makes it possible to do the sealing work when the home is weather-tight. This step can be postponed until just before the finish floors go down. This method requires some surface preparation. Brush away dust and loose debris before caulking.
Cavity sliding doors
Cavity sliding doors are installed during the framing stage. As they are mostly located within an internal wall, little consideration is given to potential air leakage. However, in many cases they create an open air path to the ceiling space, or external walls. It takes little effort to seal these air leakage paths during the framing stage.
Windows & Doors
There are several options to consider when dealing with windows and doors. Option 1 consists of sealing between window/door jamb and house frame. This can be done by using a backer rod with caulking over top, or a low pressure expandable foam with caulking over top. Option 2 is to create the seal after the plaster is installed. This seal is created by caulking the window/door jamb to the frame, or caulking the architrave to the plaster. Air sealing always occurs on the inside, and is easiest when you wait until the latest possible stage of construction. In windows with plaster returns, run a bead of sealant between the plaster and window frame. Run another bead under the sill, sealing it to the plaster.
Windows
Windows need to be sealed between the jamb and house frame, or if later, between the jamb/architrave and the plaster. Smaller gaps (30 -50 mm.) can be sealed with high quality elastomeric caulking. Larger gaps require foam sealant, although some builders use foam for all gaps more than 30mm. wide.
Doors
Doors also should be sealed between jamb and frame or after plaster. Because the gap between the door jamb and the plaster is often over 30mm. wide, you’ll probably need to use urethane foam sealant. Use your preferred caulking on the threshold.
Rough-in plumbing & electrical: pipes and wiring through framing
With the framing completed, plumbers and electricians will start on the rough-in of water and drainage lines and wiring of all sorts. It is not necessary to seal every hole drilled through the framing, however the following areas need to be addressed:
- 1: All holes drilled through top plates and bottom plates
- 2: All holes drilled through studs at junctions of internal wall & external walls
Rough-in of ducting for mechanical systems
Whole-house cooling or heating systems require the use of ducting. Most common is the use of insulated flexible ducting, installed in the ceiling space or under the floor. As the ducting system is part of the inside of the building, proper sealing of joints ( Y &T’s) is crucial to prevent energy loss. The fact that these systems pressurize the ducts when operating makes any kind of leakage even more prominent. Ensure that all outlets through plaster are tightly fitted.
Rough-in of ducting for mechanical systems
Most whole-house heating systems will have a return air plenum installed in a internal wall void. This void is connected to the ducting in the attic. Ideally a return plenum is made from sheet metal and will have a collar on top to allow for a connection with the ductwork. Check for sealed connection after the system is roughed-in, but before plaster.
Bath Drain
Generally, the bath drain and overflow pipes require a large hole in the subfloor. Below are two sealing solutions to be considered with raised timber floors and concrete slab.
Timber floor application: The Box
The existing floor joists form two sides of the box. Add blocking between the joists to close the other two sides. Make a cover for the bottom with exterior grade plywood or its equivalent. The cover should have two pieces that meet at the drain pipe. First, cut the plywood to the correct size. Locate and drill a hole slight larger in diameter than the drain pipe. Saw the plywood into two pieces by cutting through the hole. The box can also be installed before the plumbing is in place. Cut an extra large hole in the subfloor where the bath drain will go. Build the box as described above, except cover it with a single piece of plywood. Once the cover is fastened, caulk all the cracks where framing meets from the inside.
(The extra large hole in the subfloor allows you to get the caulking gun below floor level.) The plumber can drill a hole for the drain and seal the gap around the pipe.
Concrete slab application
Holes made in the slab to facilitate any plumbing should be filled with either a concrete mix, or a sprayfoam insulation.
Try to avoid breaking through the slab and exposing natural ground.
Plumbing Vents
Vent stacks extend from the inside to the outside, exposing them to significant temperature differences, especially when extremely hot or cold water flows through the pipe. Under these conditions, vents tend to expand and contract. Because of this movement, a flexible gasket is the best way to seal a vent.
Gaskets for plumbing vents can be purchased at any plumbing supply store, or you can make your own.
To makes a gasket, cut small sheets of EPDM(Ethylene Propylene Diene Monomer (M-class) rubber) , (inner tubes from old tires work well, too). Make a circular hole that's smaller than the vent.
Because the gasket needs to slipped over the vent as it is installed, you’ll have to provide them for the plumber. Bring them to the site in advance and staple them to the top plate near each vent location.
The plumber slips the gasket over the pipe so that it's above the top plate. After the plumber is finished, run a generous bead of sealant around the hole on top of the plate. Gently, press the gasket into the sealant. Staple around the edge to hold the gasket in contact with the sealant.
Filling the void between pipe and top plate with a low pressure expandable foam product that are also flexible, is a quick and easy way to solve this problem.
Plumbing Drains
Plumbing drains through floors or walls are usually found in all the wet areas as well as the kitchen. The plumber will always cut a bigger hole than the actual drain pipe and this can become an air leakage path to the outside. Most drains through the floor can be sealed at rough-in stage. Drains through the wall should be sealed after the plaster is hung, but before laundry/bath/kitchen cabinets are installed. A simple bead of caulking will do the job. For excessive holes, use a gasket or expandable foam. For a better finish, also apply a bead of caulking (No More Gap) at the back/floor of the cabinet.
Recessed Lights
Leakage through recessed fixtures is only a problem when they penetrate outside ceilings. Leakage doesn't occur through lights installed in a ceiling with conditioned space above, as in the first floor ceiling of a two story house. Here are several lighting options that reduce air leakage.
Option 1 - Surface-mounted Fixtures
To avoid the leakage problem, use surface mounted fixtures when possible. Then you simply seal the electrical wiring through the plaster.
Option 2 - Sealed Bulkheads
One way to stop leakage is to build a sealed bulkhead below the insulated ceiling, so the recessed light is effectively inside the building. For this to work, the ceiling above the bulkhead must be covered with plaster and the joints taped. That means the bulkhead will have to be framed after the plaster is installed, which could be a scheduling headache.
Option 3 - downlight cover
These covers should be installed before insulating the ceiling. Ideally covers are seated on the plaster with a bead of caulking.
Electrical Boxes
Meter boxes in outside walls and ceilings should be sealed. Although the leakage through each box might be small, there quite a few of them on the outside walls of the house. There are several options that work for meter boxes in walls and ceilings:
• Install a gasket behind the face plate. In order to work the gaskets must press firmly against the plaster on all sides.
• Apply sealant between the box and plaster. Also seal the hole where the wire enters the box and any other holes in the box. Your job will be easier if you choose plastic boxes without holes.
Stairs on Outside Walls
Like the rim board of a floor between conditioned spaces, a stair built against an outside wall could pose a potential air sealing challenge. The simplest solution is to schedule the work in such a way that the plaster wall -an effective air barrier- is in place before installing the staircase.
There might be instances where a staircase must be installed prior to the plasterboard. In those cases the best option is to insulate the wall behind the staircase and cover the whole area with a sheet of clear plastic (polyethylene). For best results, apply a thin bead of caulking on the studs and plates where the plastic sheet terminates. You now have created a perfect seal behind the staircase.
Interrupting the construction process
Sometimes there's no avoiding it. To insulate and seal properly, you need to interfere with framing work. A bath enclosure is a good example. Because it's too big to fit through the door, a one-piece bath or shower unit has to be carried into the bathroom before the walls are framed. Before the rough plumbing locks the unit in place, you should insulate and seal the wall.
Your goal is to install a section of the air barrier on the wall so that it can be connected to the final air barrier later. For the air barrier material, you have several choices: drywall, exterior grade plywood (or equivalent) or 6-mil polyethylene sheet. Poly and plywood are probably easier to have on the site during framing. For poly, be sure to run the air barrier one stud past the tub on each side. That provides solid backing for a good seal.
As mentioned, this method also works for stairs, dropped ceiling, eaves and other tricky sealing problems.
The Last Resort
When all else fails and no amount of head scratching provides the answer, you can try this. Assuming the building has board sheathing, such as insulated foil board , you can use the outside sheathing as part of the air barrier. To do this you simply caulk the sheathing to the studs and plates. A gasket or bead of sealant on the face of the end studs and the top plate seals the plaster to the studs. The joint between the subfloor
and plaster is sealed normally.
This option comes with a risk! The outside is sealed and covered with a material that is relatively impermeable to water vapor. Any vapor that gets into the wall cavity would have trouble escaping. For this reason, use this option with caution. It's not recommended for high moisture areas, such as bathrooms or laundries. However, it might be a good idea for small wall sections, such as behind stairs or recessed wall heaters.Consider the use of a breathable membrane as a better option!
Combustion Appliances
All combustion appliances that draw air from inside the building present potential hazards. These include water heaters, furnaces and stoves. The safest and simplest solution is to install “sealed” combustion appliances that draw air from outside the house. A good example is the Rinnai Energy saving space heater with fresh air intake. These units get all the air used for combustion and draft from the outside. There should be no opportunity for flue gases to enter the building.
Unfortunately, sealed combustion gas stoves are not available. A properly operating gas stove with an exhausting range hood (200 cfm minimum) should not cause a problem. However, over time the burners may become dirty and produce more harmful gases. Plus, the occupants may not operate the exhaust fan every time they turn on the stove or oven. These factors make the use of a gas stove risky.
For a complete list of ways to ensure a building is air-tight, and detailed instructional videos, complete the Building Science and Energy Efficiency Course with the Green Building Institute.