Schematic of a solar water heater hookup (C) - Lennox Industries How to Minimize Air Leaks on a Building

  • AIR LEAK MINIMIZATION - CONTENTS: Keeping it Tight: how production builders keep air infiltration to a minimum with careful planning and plenty of caulk. Guide to preventing building air leaks & air infiltration. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
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How to minimize building air leakage:

This article discusses how builders minimize air leaks during construction, planning, layout, and use of caulk sealants.

We discuss details for building an air-tight building shell, framing details at each floor level and at building corners to make sealing and insulating effective and easy, how we seal air leaks around windows, doors, other building penetrations, and using interior finishes to help seal the buiding.

We discuss using blower door tests for air tightness in new construction of buildings.

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Solar Age Article on minimizing air leaks in buildings by proper design and layout as well as judicious use of sealants

Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Cutting the air exchange rate in a building from 0.50 to 0.25 air changes per hour will typically produce the same range of savings as doubling wall R-values from R-20 to R-40 and can be done for much less money. Dollars put towards caulking and sealing is money well spent. Builders, snooping around with smoke pencils and infrared cameras using thermography (thermal images showing areas of unwanted heat loss or gain) have identified a number of trouble spots in new construction - some obvious and some not.

This article focuses on proper air sealing in new building construction. For a detailed article on weatherization retrofit procedures to find and stop un-wanted heat loss or gain in existing buildings

Making an Air-Tight Building Shell

Schematic of a solar water heater hookup (C) - Lennox IndustriesEnergy conscious builders establish their first line of defense while roughing the building shell.

First floor framing sealing against air leaks

Begin at the beginning with an inexpensive fiberglass or foam sill sealer, compressed between the sill plate and the foundation wall (illustration at left). This is a tricky area to caulk later, so get it right the first time, doubling the sill sealer where necessary to fill larger gaps between the sill plate and the top of the foundation wall.

As the band joists (rim joists) are laid up, add a bead of caulk between the band joist and the sill plate.

Follow with a bead of caulk between the subfloor and the rim joist and sill.

Continue with caulk between the subfloor and the band joist, and between the subfloor and the bottom plates of the outside walls.

This glued-up assembly will help prevent air from flowing under the exterior walls or between basement and outdoors. If insulating sheathing is used, butt and seal rigid wall insulation to exterior foundation insulation where they meet at the sill. If possible, keep wall and foundation insulation flush on the building exterior so that the siding can be carried right over the joint with no flashing. This makes for tight, neat, and easy exterior detailing.

Second floor sealing against air leaks

In a two-story structure, caulk around the second floor band joist the same as at the first floor. Caulk the upper top plate to the band joist, band joist to subfloor, and subfloor to the bottom plate of the second story wall.

Sealing at building corners

Building corners tend to be leaky and un-insulated. They deserve special attention. As in other exterior joints that coincide with breaks in the interior finish, caulking is helpful here. Seal either sheathing to sheathing, or sheathing to framing. If insulating sheathing is used, run a length of tape up the corner.

See FRAMING DETAILS for BETTER INSULATION for details of building corner framing techniques to improve insulation and sealing.

If a permeable sheathing is used, the entire building, from sill to top plate, may be wrapped and taped with an air and water barrier that "breathes" such as Dupont's Tyvek® or Parsec's Airtight White™. If these materials are fully taped at seams and door and window openings, they could eliminate the need for much of the exterior caulking.

See HOUSEWRAP AIR & VAPOR BARRIERS for details on selection and use of these products.

Sealing Air Leaks at Interior Building Framing

Builders have been surprised to discover how much air leakage occurs through cracks and holes in interior building partitions. Convective heat loops also occur in uninsulated building interior partition walls, even if the walls are sealed at top and bottom.

Stop these leaks at the source by isolating interior stud cavities from basement, attic, and outside walls. Holes and notches for plumbing and wiring are the main culprits. But interruptions in the interior vapor barrier, where partitions meet outside walls and ceilings are also potential leaks. To prevent breaks in the interior air/vapor barrier, secure a piece of polyethylene on the ceiling or outside wall before erecting the interior partition. This piece can then be overlapped to join the full poly vapor barrier when later installed. Make these and all laps in the poly barrier over a framing member (many prefer two) so that the sheets will be compressed together by the drywall.

Holes drilled later through the top or bottom plates into unconditioned spaces should be sealed with a shot of foam or, if smaller, a glob of caulk.

Sealing Air Leaks at Doors & Windows

The standard approach to caulking doors and windows - between exterior casing and siding rarely does an adequate job. These notorious heat thieves merit a double dose of protection. So seal them from both outside and inside.

Aluminum and vinyl-clad windows generally install by nailing through an exterior flange. A bead of caulk gunned under the flange seals these units with minimal effort. Similarly, a bead of caulk under the exterior casing on all-wood units may do the trick. but in these a little more care is needed at joints and miters in the exterior trim.

On the inside, aim for an airtight seal between the jamb and rough opening. Some builders choose to foam this gap. This is best done with a small bead shot deep into the gap where the window unit can resist the forces of the expanding foam. (Windows have actually cracked!). As extra protection, a tight fitting length of wood can be tapped into the window jamb to hold it true while the foam sets up. The remainder of the space can then be stuffed with fiberglass.

If the poly barrier is sealed intact to the window jamb, then fiberglass alone should suffice in the rough opening. To achieve a good seal here, run the vapor barrier right across the window opening and make diagonal cuts from window corner to window corner. Insert these flaps between the window jamb and the interior casing. Trim the excess with a sharp knife. A low infiltration window, sealed thus, should leave you and your smoke stick draft free.

Interior Finish Sealing Details Against Air Leaks

Airflows seem to ignore baseboards, drywall joints, and miscellaneous interior trim, so seal your vapor barrier well to subfloors and lap it at corners between walls and between wall and ceiling.

At this point, things are looking pretty good except those messy cuts around pipes and electrical boxes. these necessary evils to a well-sealed home can be dealt with in several ways. Strapping out the drywall 3/4-inch off the studs and vapor barrier adds both an insulating air space and a convenient conduit for plumbing feeds and wiring. Shallow boxes can be installed here without puncturing the vapor barrier.

WARNING: wiring and copper piping in locations exposed to damage from future nails and screws that may be driven through drywall to hang a picture, say, must be protected by steel plates, or routed deeper into the wall cavity. One of our -DJF- friends called to say that on moving into his new home he drove a picture nail through drywall, only to hear a hissing sound. His mistake was to pull the nail, converting a small water supply leak from a punctured copper pipe into a large one before he turned off the water.

Some builders have had good luck with slitting a tight X in the vapor barrier and taping the flaps to the inside of the box with polyethylene tape. Use a plastic box (without all the holes) and caulk where the wire enters the box. WARNING-DJF: Be careful: adding any combustible material inside of an electrical box or panel may violate electrical codes and could add to a future fire risk.

The Vapor Box™, a shallow plastic pan, designed to fit around and seal off electrical boxes, is manufactured in Canada.

Electrical boxes should be sealed on inside walls as well, but without a vapor barrier, different strategies apply. Caulking or foaming plastic boxes to the drywall should do the trick. WARNING-DJF: if you're using foam to seal around electrical boxes, take not not to squirt foam inside of the box itself. The foam gaskets work ok if electrical box and drywall are lined-up just right. On ceilings, try to keep wiring and shallow electrical boxes on the warm side of the vapor barrier by strapping over the poly. WARNING: see our note above about protecting wiring from damage - a lower risk at ceilings.

Seal off plumbing where it crosses from unconditioned to conditioned spaces. Plumbing in outside walls will prove harder to seal and exposes pipes to possible freezing. One or two strategically placed plumbing chases make it easier to recognize and seal these leaks - but don't forget to take a look at CONVECTIVE LOOPS & Thermal Bypass Leaks when using this approach.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Blower Door Test Results on New Construction

Trying to take some of the guesswork out of the battle against air leaks, solar designer Terry Brennan of Red Wing in upstate New York, puts his homes, half completed, to a blower door test and routinely finds the equivalent of a 1- to 2-square foot gaping hole in leftover cracks - even after a conscientious sealing job has been done on the building shell. Brennan prefers to run the test with exterior walls insulated and drywalled, and interior partitions open. Subfloors are still exposed and the attic is uninsulated to allow access to ceiling and partition leaks.

In new construction after building sealing, these air leaks appear at junctions of floors, walls, ceilings, and around chimneys and electrical fixtures. He has found interior partitions as leaky as exterior walls - due primarily to unsealed holes in top plates. On more than one occasion, he has found wide open recessed light fixtures (required to be left open by code) installed contrary to specifications. So much for specifications!

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Original article

"Keeping it Tight: production builders are keeping air infiltration to a minimum with careful planning and plenty of caulk" - links to the original article in PDF form immediately below is preceded by an expanded/updated online version of this article.

Building Air Leakage Articles


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