Building a Tight Thermal Envelope
Monday, April 28th, 2008The building envelope is the part of the house which separates the indoors from the outdoors, and consists of the floor on the bottom level, the ceiling on the top level, the exterior walls, the exterior floor bands, and of course the doors and windows. The standard house envelope leaks air like a sieve and accounts for something like 30 percent of climate control costs due to air infiltration alone. Fortunately during construction it is cheap and easy to make the thermal envelope much tighter and to make a considerable improvement in the energy efficiency of the finished home. According to the Federal Energy Efficiency and Renewable Energy website “Such airtight homes often consume one-third less energy when compared to similar unsealed homes.”
During the Framing Process
Seal under the exterior walls - I recommend that you use construction adhesive to glue your exterior walls down and seal them to the floor all at the same time. You will usually have it on the job anyway for gluing down the floors. Alternatively you can use caulk, foam sealant, or special wall gasket. I’ve used them all and they all work, but I like construction adhesive the best. Use caulk to seal building paper or housewrap (tyveck or equivalent) to the exterior framing at the floor band just below the walls, and to the very top outside of the exterior walls. Tape all building paper or housewrap seams, and repair any rips or tears. Tape building paper or housewrap at windows. Caulk under exterior doors before installation. Seal around windows and exterior doors using caulk or poly foam sealer (great stuff) that is specified for doors and windows. Don’t use the regular expanding foam sealant around windows and doors or it will cause them to bind later on. I highly reccomend the Pella Window and Doors website for its outstanding guides to installing any brand of window or door correctly.Framing and Insulation
The way most houses in my area are framed there is an area at the junction of exterior walls at the corners and where interior walls join the exterior walls that framers call “Tees, and Corners.” Tees, and corners also refer to a framing detail at these locations that are usually each made out of two studs and three or more blocks of wood, these devices serve as a convenient place to attach the walls together, and to attach the drywall. The problem with tees and corners is that they are almost impossible to insulate so you end up with a three and one half inch area at every one of these locations that has no insulation. In the typical home this adds up to 4-6 feet of un-insulated wall. That is not good. To avoid this situation, don’t use the conventional device; instead just use a single 2×6 stud installed sideways in place of the tee or corner. This gives you a fine place to attach your walls, and drywall. It also saves a lot of labor compared to building the old style devices, and costs less for material, while leaving room to install insulation. Another area of framing that often gets no insulation is the headers over windows and exterior doors. Whenever possible use only one piece of lumber (usually either a 2×10 or 2×12) for the header and put it to the outside of the wall so that there is space left on the inside for some insulation. Usually you will be fine with openings up to four feet wide to use a single header, but this is highly variable because of local codes considerations. Check with your codes inspector first. If your attic will have blown in insulation in it, build a box around the attic access hole so that the blown in insulation can come all the way to the access hole without spilling into the house.During the Rough in process
Insulate the area of exterior walls where tub/shower units will be installed before the fixtures go in. Try to avoid using recessed fixtures in the thermal envelope whenever possible. They are usually full of holes that leak air, and they cause a void in the insulation. If you must use recessed fixtures always use the kinds that are approved for direct contact with insulation. Often plumbers cut a big rectangular hole under each bath tub for the pipes to go through. This is a challenge to seal up neatly, but I have found that I can do a good (although ugly) job by attaching tyveck from under the floor, and then using a lot of foam to cover and seal it from the top. You could also use pieces of plywood laboriously fitted and then sealed I suppose. If anyone finds a better way, I would like to hear it. After wiring, plumbing and HVAC have been roughed in and before installation of insulation or drywall squirt foam sealant in ALL of the holes that have been drilled through the top or bottom plates of ALL walls - not only the exterior walls. The interior walls are important too, because air can be exchanged from the interior to the crawl space and the attic through interior walls. Seal around all penetrations of the exterior wall sheathing. Electricians are notorious for cutting nasty ragged holes to install exterior wall boxes. You will need to arm yourself with tape, caulk, and foam sealant for this job. Use caulk and foam to seal all of the many holes in all electrical rough in boxes including where the wires go through. This sounds like a bigger job than it is. Since you don’t have to worry about being neat, it really doesn’t take very long.While Drywall is being Installed
Apply a continuous bead of glue to the top and bottom wall plates behind the drywall. The glue at the bottom prevents air from getting into the wall cavity from inside the house, and the glue at the top prevents air from being pulled from the wall cavity into the attic. Also glue the drywall to the frame around doors and windows for the same reasons. If you are contracting out the drywall installation I highly recommend that you hang out while it is being done just to make sure that they both understand what you want and then actually do it. Subs are notorious for ignoring things like this if they aren’t supervised, because once it is done there is no way to inspect it. Tape and mud every single drywall joint even if it will be hidden behind trim, cabinets or other finishes. It doesn’t have to be pretty in these areas but you want to seal them up.After Drywall is Installed
Caulk or foam around electrical outlet boxes, recessed fixtures, and where plumbing pipes penetrate the drywall. Before the attic is insulated, get up there and foam or caulk everywhere that you see light coming through around light boxes, bathroom exhaust fans, other recessed fixtures, or anywhere else. If you will have an attic access door, make sure that it is insulated, tight fitting and sealed in place with weather stripping. I usually just use a nice piece of plywood screwed to the ceiling and sealed with stick on weather strip. Use an interior paint or primer that is rated as a vapor barrier.This is based upon the Air Tight Drywall Approach developed by the Canadian Government and is intended to result in a tight thermal envelope which is able to resist inappropriate condensation. What you are after is to prevent drafts from moving through the wall cavities, and ceiling. In conventional (not all that tight) construction a couple of things happen which exchange air between outside and in: 1) wind simply pushes air in through the various cracks making the house “drafty” 2) A thermo siphon or chimney effect causes air that heats up in the attic to draw through the wall cavities pulling your conditioned inside air into the wall cavities (by way of receptacles, fixtures, and cracks) and out via the attic.
