Weatherproofing

Air sealing and weatherproofing are the foundation of shelter energy efficiency and moisture control. A drafty, unweathered structure can lose 30–40% of its heating or cooling energy through air infiltration alone — before any insulation is considered. Weatherproofing costs $200–$800 for a typical home as a DIY project and pays back in reduced fuel costs within 1–3 years under normal conditions.

In a grid-down winter, a well-weatherproofed shelter can stay livable on a single small wood stove or space heater. An unweathered shelter may require 3–5× the fuel to maintain the same temperature. Do this work before it becomes an emergency.

Related: Insulation covers R-value specifications that build on a weatherproofed shell. Wood Heat covers heat source sizing for a weatherproofed shelter.

Weatherproofing Sequence

Always weatherproof in this order — each step depends on the previous one:

Attic air sealing (highest impact, done before insulation)
Basement and crawl space sealing
Exterior wall penetrations (pipes, wires, vents)
Window sealing (interior and exterior)
Door sealing (sweeps, compression seals, threshold)
Exterior envelope (caulk, housewrap, flashing)

Priority 1: Attic Air Sealing

The attic floor is the most important air sealing location in a house. Hot air rises and escapes through every penetration in the ceiling — and cold outdoor air is drawn in to replace it (the stack effect). A house with 10 attic penetrations can have the equivalent of a 14 in (36 cm) hole in the ceiling.

Common Attic Air Leakage Points

Location	Size of Typical Gap	Sealant
Top plates at wall intersections	1/8–1/2 in (3–12 mm) continuous gap	Acoustical sealant or spray foam
Recessed light fixtures	Large — entire fixture is a hole	Fire-rated spray foam or IC-rated cap
Plumbing chase (around pipes)	1–3 in (25–76 mm) per pipe	Spray foam + metal collar
Duct boots/registers	1/4–1 in (6–25 mm) at edges	Foil tape + mastic sealant
Electrical boxes	1–2 in (25–50 mm) at edges	Acoustical sealant
Chimney chase	1–6 in (25–150 mm) around perimeter	Sheet metal flashing + high-temp caulk
Attic hatch / pull-down stairs	Entire frame — large	Weatherstripping + rigid foam cover

Procedure: 1. Move any existing insulation aside using a rake or broom. 2. Apply acoustical sealant (e.g., OSI QUAD or Tremco Acoustical Sealant, inexpensive per tube) to all small continuous gaps with a caulk gun. 3. Apply single-component spray foam (Great Stuff or equivalent, inexpensive per can) to gaps up to 3 in (76 mm). Do not use expanding foam around chimneys or flues — use high-temperature caulk and metal flashing. 4. Cap existing recessed light fixtures with airtight covers made from rigid foam (inexpensive each) or replace with IC/AT-rated fixtures sealed at the ceiling. 5. Replace insulation after sealing. A well-sealed attic typically reduces a home's air leakage by 25–35%.

Cost estimate: $150–$400 in materials for a typical 1,200–1,800 sq ft (111–167 m²) attic. Time: 4–8 hours.

Field Note

The attic hatch is often the single largest air leak in a house. A standard pull-down stair has R-1 insulation and gaps equivalent to leaving a window slightly open. Install a rigid foam insulation cover box over it ($60–$120 prefab; or DIY with 2 in / 50 mm XPS foam for $15–$20 in materials). This one fix often reduces attic air leakage by 10–15% on its own.

Priority 2: Caulk Selection Guide

Choosing the wrong caulk for a location causes premature failure — cracking, shrinking, or bond failure within 1–3 years instead of 10–20.

Caulk Type	Best Applications	Not Suitable For	Cost (10 oz tube)	Lifespan
Silicone (100% silicone)	Tub/shower, exterior glass, metal-to-metal	Paintable surfaces, porous masonry	$6–$12	20+ years
Latex/acrylic	Interior trim, baseboards, drywall gaps	Wet areas, exterior joints	$3–$6	5–10 years
Siliconized latex	Interior/exterior trim, paintable exterior	High-movement joints	$5–$8	10–15 years
Polyurethane	Exterior wood joints, masonry, driveways	Glass, painted surfaces (stains paint)	$8–$15	15–20 years
Acoustical sealant	Attic air sealing (stays flexible, never hardens)	Exposed exterior surfaces	$6–$10	30+ years (interior)
Butyl rubber	Window glazing, roof flashing	Interior use	$5–$8	15–20 years
High-temp (RTV silicone)	Around flues, chimney, exhaust pipes	Standard trim/siding	$8–$15	15+ years at high heat

General rules: - Use silicone everywhere moisture is present (bathrooms, below grade, window sills) - Use acoustical sealant (never hardens) everywhere you need a permanent air seal that may flex - Use polyurethane for high-movement exterior joints on wood and masonry - Never use latex caulk in continuously wet locations — it fails within 2–3 years

Application Procedure

Clean the surface. Remove all old caulk, dirt, and loose paint. Surfaces must be dry and free of oil.
Mask adjacent surfaces with painter's tape for a clean bead.
Cut the nozzle at 30–45 degrees, opening sized to the gap: 1/8 in (3 mm) for hairline cracks, 1/4 in (6 mm) for standard joints.
Apply a continuous bead with consistent pressure. Work steadily — don't stop and restart in the middle of a joint.
Tool (smooth) the bead immediately with a wet finger or plastic tool. This forces caulk into the gap and creates a concave surface that sheds water.
Allow to cure per manufacturer instructions before exposure to water (silicone: 24 hours; latex: 2–4 hours).

Priority 3: Door Sealing

Doors are the highest-traffic air leakage point in a shelter. A standard exterior door with worn seals may leak 5–10 cubic ft per minute (cfm) of air — equivalent to a 2 in (5 cm) diameter hole in the wall.

Door Sweep Types

Type	Application	Effectiveness	Cost
Automatic drop sweep	Best for drafty exterior doors — lifts when door opens, drops on close	Very high	$35–$75
Door shoe (integrated threshold)	Replacement of entire threshold assembly	High	$25–$60
Adhesive vinyl sweep	Budget option, easy install, short lifespan	Moderate	$8–$15
Heavy aluminum door bottom with pile seal	Interior doors, quieter	Moderate	$15–$30
Door draft guard (fabric sausage)	Temporary measure only	Low	$10–$20

Installation — automatic drop sweep: 1. Remove the door from hinges and lay flat. 2. Mark the attachment holes — the sweep should touch the threshold with approximately 1/16 in (1.6 mm) compression when door is closed. 3. Pre-drill and attach with screws provided. Check for binding — the door must open freely. 4. Test by sliding a piece of paper under the closed door. The paper should have noticeable resistance.

Door Compression Seals (Perimeter)

The door jamb stop (the strip the door presses against) requires compression weatherstripping around all four sides:

Material	Best Use	Cost (per 17-ft / 5.2 m door set)	Lifespan
EPDM rubber foam tape	Aluminum or steel doors	$12–$20	3–5 years
Bulb seal with flexible vinyl	Wood doors with slight irregularities	$15–$25	5–10 years
Magnetic seal (like refrigerator)	Steel doors, highest performance	$45–$90	15+ years
V-strip (tension seal)	Side of door in jamb groove	$8–$15	5–10 years

Priority 4: Window Sealing

Permanent Sealing Options

Exterior caulking: Apply paintable siliconized caulk around the exterior window frame where it meets siding. This joint fails regularly due to thermal movement — inspect annually and reapply as needed (inexpensive per window).

Interior rope caulk: Removable putty-like rope (DAP Rope Caulk, inexpensive per 90-ft / 27 m roll) pressed into window gaps for seasonal sealing. Strips out cleanly in spring. Excellent for older double-hung windows where operating sashes have developed gaps.

Window film insulation kits: Interior plastic film stretched over window frame and heat-shrunk tight. Adds approximately R-1 by trapping a dead-air space. Particularly effective on single-pane windows. - Cost is affordable for a kit covering 5 windows - Effect: Reduces drafts, adds ~1 in (25 mm) dead air space = roughly R-0.9–1.2

Exterior insulating shutter: Solid foam-core panel mounted over window exterior. Adds R-5 to R-10 depending on thickness. Used in severe cold or emergency scenarios where maximum retention is needed and windows aren't needed for light.

Secondary Glazing

For single-pane windows: magnetic secondary glazing panels (Innerglass, Indow) can be installed over existing windows from the inside without permanent modification. These add R-2 to R-3 per panel. - Cost is a moderate investment per window professionally measured and installed

Priority 5: Basement and Crawl Space

Rim joist / sill plate: The joint where the wood framing sits on the foundation is one of the highest-priority air sealing locations. It is typically accessible from inside the basement. Apply 2 in (50 mm) strips of cut-and-cobble rigid foam (XPS or polyiso) into each joist bay, sealed with spray foam at all edges. Alternatively, spray the entire rim joist with 2 in (50 mm) of closed-cell spray foam. - DIY cost: $150–$350 for a typical foundation perimeter - Contractor cost: $500–$1,200

Basement windows: Old basement windows are common air and moisture entry points. Replace with glass block (mortared in, excellent thermal resistance) or install compression-fit foam inserts for the winter.

Crawl space vents: In mixed and cold climates, sealing crawl space vents and conditioning the space improves moisture control and reduces floor drafts significantly. See Insulation for full crawl space conditioning procedure.

Vapor Barrier Integration

Weatherproofing and vapor control work together. Key specifications:

Class I vapor retarder (<0.1 perm): 6-mil polyethylene sheeting ($0.10–$0.15 per sq ft), foil-faced board. Use on warm-in-winter side of insulation in cold climates.
Class II vapor retarder (0.1–1.0 perm): Kraft-faced batts, vapor-retarder paints (like Zinsser BIN). Acceptable for most cold-climate walls.
Vapor-open assembly (>10 perm): Uses multiple drying paths. Required in mixed-humid and hot-humid climates.

The condensation plane (where warm moist indoor air meets cold and moisture condenses) must be in a manageable location. In a cold climate with exterior rigid foam, the condensation plane moves to within the foam layer — outside the framing — which prevents interstitial condensation and rot.