Weatherization: Air sealing a building | Pro Construction Guide

Weatherization: Air sealing a building

Done correctly, weatherization by air sealing is the most cost-effective energy-saving measure you can take in a building. It will save energy dollars and reduce drafts and humidity fluctuations inside the building, and HVAC systems will operate more effectively and efficiently.

Before weatherization and any other energy-saving measures are taken in a building (old or new), air leaks should be sealed as much as possible. Replacement windows and additional insulation can be effective, but only after the building’s envelope is sealed. The benefits of replacement windows will be greatly reduced if air is still leaking through gaps and cracks in the building. Adding insulation before sealing leaks can be counterproductive, as it will make it harder to find the leaks in the future.


Proper air sealing requires using the appropriate materials at certain locations in the building envelope.

The first step in weatherization

Whole-building weatherization projects begin with identifying the weak points in the building’s envelope. An energy audit usually involves a blower door test to locate and identify lair leaks; duct blasters do the same with HVAC ductwork and an infrared camera can reveal weak spots in the insulation system (missing insulation or large gaps and voids). After the first test, remediation work can begin. Then when it’s finished, conduct a second test to show you how well the improvements are working.

The energy audit will help you locate cracks and leaks that aren’t obvious and help you select the best materials for the job. Some leaks can be easily fixed with some caulk or weatherstripping, while others will require specific materials, such as duct mastic, airtight electrical boxes or high-tech sealing and caulking materials. The illustration below shows the most common air leaks in a building’s thermal envelope.

Weatherize a building

Two options for sealing the rim joist on a masonry foundation. Professional spray foam is quicker but more expensive than using a mix of canned foam and insulation.


Attacking the problem

With an energy audit report, weatherization contractors usually begin by making a checklist. This will tell them which areas of the building need sealing, what materials are needed, and what replacement components, if any, need to be ordered (new windows, an outdoor air supply to connect to combustion appliances, etc.).

The checklist will also help you order and prepare the materials needed to weatherize the structure. By deciding in advance what material will be used for each task, you can select the fewest possible types and order them ahead of time. This reduces the amount of unused/spoiled sealant and helps avoid overstocking materials.

Weatherize a building
Use a combination of insulations and elastomeric sealant to seal a rim joist above a stick-framed wall.

Sealants for all seasons

To match the sealant to the weatherization for best results, you’ll need:

  • Caulk – One-part urethane caulk sticks to most materials, resists moisture, and maintains its elasticity over time.
  • Gasket – Polyethylene backer rod available in ¼- to 2-inch diameters makes good filler for larger cracks and can be applied in any weather. Sill sealer is designed for use between mudsills and foundation walls, but is not as effective under sole plates.
  • Foam – Spray foam insulation is very popular for weatherization for its wide array of applications. Expanding foam is great at sealing plumbing and electrical penetrations between floors, and can be used with rigid foam board and construction adhesive to seal cavities in the rim joist. Where overfilling of gaps would appear unsightly or lead to more cleanup, look for low or no expanding foam. To completely fill rim joist cavities with foam, which is the easiest solution but also the most expensive option, many experts recommend icynene foam for its low CFC content.
  • Adhesive – A continuous bead of construction adhesive where the bottom wall plate meets the subfloor is a very effective air sealer. It can also be used on the tongue-and-groove edges of subfloor panels. Use it to adhere rigid foam board squared in rim joist cavities and then spray some expanding foam around the edges for an inexpensive and effective air seal.
  • Sheet goods – Patches or boots made from EPDM or rubber can be very effective for sealing the hole where plastic pipes penetrate top and bottom plates. Cut holes in the sheet a bit smaller than the pipe diameter and slip the pipe through for new construction, or slice the patch and then seal the seam with silicone for a retrofit. Housewrap can also be used for this purpose.
  • Weatherstripping – Made from a variety of materials and profiles, weatherstripping is very effective at stopping air leaks around doors and windows. Replace worn or missing weatherstripping in these units, and insert scraps and leftover pieces into gaps and cracks throughout the home.

­–By Rob Fanjoy

Featured Products

Sponsored Messages