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Air sealing

House Leakage, and "How Tight is Too Tight?"

A frequently asked question is "how tight is too tight? Can't you make a house too tight?" No, you can't. If your house were so tight it could float, all the better. Tight construction makes it all the easier to control ventilation. Uncontrolled ventilation, or accidental air leakage, is random, so you have no guarantees in the average house-leaky or tight-that you're getting the fresh air you need when you need it. The grammatically incorrect phrase "Build tight and ventilate right!" is absolutely the way to go.

The following quote from Jim White, senior researcher with the Canadian Mortgage Housing Corporation, is a straightforward summary of this point: "Both people and houses need continuous ventilation to stay healthy, but natural ventilation does not provide continuous ventilation. Therefore, we need good mechanical ventilation. Good ventilation means balanced ventilation, and balanced ventilation works best in tight houses. Therefore, we need tight houses with good balanced ventilation systems in them. Anything less is just not good enough.By the way, it was the leaky houses in our Wallaceburg study that had the most mold and the sickest kids. Going back to leaky houses is not the way."

To build a tight home, even when using a relatively tight building system like SIPs, you need to consistently find and seal every home's largest holes. When you have finished sealing, the only way you can be certain that you did a good job is to inspect it, because when it comes to tightness, you get what you inspect, not what you expect. The only reliable tightness inspection includes a blower door-a mechanical device that mechanically exhausts air out of the home, pulling it in where leaks are still unsealed. Talk is cheap; testing is proof.

After two decades of experience with house tightness testing, we've learned that the leakiest parts of a stick-framed house are, in approximate order of largest problems: attics, vented crawl spaces, mud sills, cantilevers, rim joists, exterior walls, windows and doors. The most health-threatening air leaks in a house are actually ductwork leaks (see HVAC section). The second-most-dangerous leaks in the average house are leaks between the house and garage; they let in carbon monoxide. Your most structurally damaging leaks are those under pressure: e.g., ductwork leaks that can pressurize floors, forcing warm moist air out to and through rim joists leaks, causing paint failure, mold, mildew, even rot.

SIP panels make house tightening easier, but only where they are used in the shell. Using SIP panels just in your walls does not address the largest holes. I've tested two SIP walls-only houses that were only marginally tighter than the average home. Ways to make further progress in tightening are covered in the next two sections.

Once you've tested a house and know that it's tight, remember that you have to provide mechanical ventilation. There are a number of approaches (see Indoor air quality section).

Air Sealing Attics and Foundations

Attics. The largest leaks from homes into conventional attics come from partition walls, chimneys, other chases, and dropped soffits . Your most surefire approach is using SIP panels for your roof system. If your budget won't allow this, then do the following: before insulating the attic,

This home's attic will require careful attention to air sealing, since the builder used roof trusses instead of panels.
  • seal over all dropped soffits with either rigid material or staple on some bubble-pack foil-faced insulation;
  • caulk or spray-foam over the top of each partition wall, along either side where the drywall is supposed to meet the top plates (there are often nearly continuous 1/8"-1/4" gaps here, over time);
  • seal off all chases into the attic;
  • use sheet metal and high-temperature caulk to seal off flues where they penetrate the attic;
  • either use a scrap piece of SIP for your attic hatch, and weather-strip where it bears on the 5/4-by-2 trim casing of your hatch access hole, or relocate the attic hatch to the garage;
  • use EPDM gasket around plumbing penetrations up into the attic and attach them to plates; and
  • use a dense blown-insulation such as cellulose insulation, which slows down (but does not eliminate) air exfiltrating from the house up into the attic.

Rims. In a SIP house, the best way to reduce rim joist leakage is to eliminate the traditional rim-joist system or at least "move it inside." For example, in houses on basements or crawl spaces, use 10' tall panels. Attach a base plate to a mud sill wide enough to bear both the panel plus 1 �" of floor joists plus a "mini-rim" out of thinner engineered wood You build the floor first, then slip the panels down over the base plate on the mud sill. You now have a wall that rises 9-feet above the floor deck and that sits just adjacent to the floor system. On the second floor, you can hang the upper from joist hangers; this eliminates the rim and strengthens the wall. In both of these cases, you'll still need to seal the resulting joints; however, the job is massively simplified and the problem of having to establish an air barrier inside the traditional rim is eliminated.

If you stand SIP walls up on conventionally placed floors with rims, you'll need to pay extra attention to those rims. The best way to insulate a conventional rim is to cut-to-fit foam blocks for each joist space; however, whether you're using dimensional 2xs or I-joists for your floor framing, "cut to fit" won't create an air barrier that stops water vapor from reaching the rim, condensing and causing structural and/or indoor health problems. At this juncture, you should spray the entire rim system with two-part expansive foam in large packs (not little cans). At the foundation rim, this does double duty as you will be sealing the foundation to sill, the sill to rim and the rim to floor decking. (The latter shouldn't be very important, since you should glue your decking to the rim and floor joist with construction adhesive.) The best way to cap this off would be to spray an elastomeric paint sealant over the foam, to make it vapor impermeable.

Mud sill. Over the life of a building, the foundation-to-mud-sill joint is most susceptible to movement; therefore, to seal this joint, you need a sill sealer that moves. Don't use a fiberglass sill sealer; once flattened by a frame, it won't "bounce back." Furthermore, another name for fiberglass is "filterglass," which is why you use it in your furnace to filter gross dust particles without reducing air flow that much. Don't use roll-out foam sill sealers that have air bubbles in them; once compressed, the air bubbles pop, which drastically reduces the ability of the sealer to re-expand. The best product for the job is EPDM gasketing, available in rolls from nationwide specialty supply houses; after compression, EPDM gasketing will bounce back as wood shrinks or foundations settle a bit. Foam-rope caulks can also work, though they tend to slide around while you're placing your mud sills. Spraying foam sealant over this joint will help, but foam gets rigid, eventually breaking one edge of the foam joint with slight movement.

Crawl spaces. In my area, houses over vented crawl spaces are roughly 25% leakier than houses built over basements and about 10% leakier than houses on slabs. The problem is that crawl-space vents are "low-side holes" in the building; all winter long, they intentionally let cold air enter, which gets drawn up through holes in the floor or ductwork and into the building, eventually exiting up through holes in the attic. The crawl space air brings with it unhealthy soil gases, pesticides and unwanted moisture.

In most cases, the best solution is to build an unvented crawl space. Eliminating venting means that, with the combined mud-sill and rim-joist treatment described above, you can have a tighter, safer, more comfortable and more durable house. But you need to do this "unventing" correctly; for example, you'll need to completely cover the soil with a durable vapor retarder that is mechanically fastened all around the edges to the foundation wall and middle support piers.

If your codes or problems with your seasonal water table prevent use of an unvented crawl space, then take a serious look at using SIP floors. They massively reduce the problems with sealing and insulating a floor above a ventilated crawl space. However, this increases the importance of insulating your water lines and any ductwork in the crawl space. (Try to keep the ducts inside the conditioned space; see the HVAC section.)

Air-Sealing SIPS and Other Elements

Sealing SIPs. Panels are relatively easy to seal. The larger the SIP panels you use, the less sealing you have and the tighter the shell should be. Most sealing is achieved as part of applying adhesives at wood-to-wood joints: bottom plates, top plates, end plates, and spline joints.

Trust me on this: combined blower-door tests and infrared camera scans show that while most panels fit snuggly at spline joints, that won't always be the case; that's life in the big city and construction in the real world. So it's important to seal the spline joints with sufficient adhesive or foam caulk; otherwise, any leakage path would allow water vapor to migrate to the exterior skins and slowly cause damage over the life of the panel.

At wall and ceiling intersections, use expansive foam sealants after the structure is in place. You can also apply adhesive on the tops and bottoms of cant strips-ripped wedge-like pieces of wood that spread the load at wall-roof intersections. Along the ridge line, be generous with your use of expansive foam. Be sure to have an attachment for your foam canisters that allows you to shoot foam into very small gaps.

Penetrations. Sealing wall and ceiling penetrations through SIPs is much the same as with standard practice: use highly expansive foam sealants wherever fire-rated caulks aren't required. The one other exception is plumbing stacks; since they move more than nearly any other item in your building, use EPDM gasketing wide enough allow the stack to slip tightly through a cutout, then staple the edges of the gasket to the panel's bottom facing.

Cantilevers. Gas fireplaces and entertainment centers, built on floors cantilevered out two feet, are commonplace in our region of the country. So are associated comfort problems during the winter. Cold air tends to flow in between the insulation (along the outside wall) and the drywall (attached to the inner framing). The problem is lack of an air barrier. In most cases, air can bypass the insulation, rendering it virtually useless. The solution is to install a durable air barrier in all sides of the cantilever, covering the insulation and most importantly covering beneath any air passageways up beside chimneys or into leaky attics (no matter the size). You may have to do this prior to installing insulation elsewhere in the building shell, so keep some batt insulation on hand at the jobsite.

At the floor level, every cantilever should be stuffed full of insulation; the more dense the insulation, the better. Then each cavity between the house and cantilever must be sealed with an air barrier material; oversized cut-outs of foil-faced, bubble-back insulation work well when stapled into place and foam-sprayed along the edges. Note that a garage ceiling floor is effectively a large "cantilevered floor;" insulate and seal the floor cavities between the house and garage in the same manner as described above.

Windows and doors. When installed per manufacturers' specs, reasonable quality windows and doors don't leak much air. Typically, the largest potential leakage sources are between the rough openings in your SIP walls and the window or door frames. The best way to seal these is with foam-rope caulk; have various sizes on hand, from 1/4" to 1". However, using low-expansive and non-expansive foam sealant also works very well. Stay away from high-expansive foam; they can create enough pressure to bow frames enough to break glass.

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