How to Seal Your Windows from Outside Air: DIY to Professional

Diagnosing Your Window Air Leaks

Before you seal anything, you need to know where air is actually leaking. Guessing wastes time and materials. Here are four reliable methods to find your leaks, ranging from free and simple to professional and comprehensive.

The Incense Test

Light a stick of incense and hold it about two inches from the window edges -- along the sides, top, bottom, and meeting rail (where upper and lower sashes meet on a double-hung window). Move slowly. Where the smoke stream wavers, bends sideways, or gets pulled toward the window, you have found an air leak. Mark each spot with a small piece of painter's tape.

Do this test on a windy day for the most obvious results. Test with the window closed and locked.

The Wet Hand Test

On a cold day (under 40 degrees outside), hold a slightly wet hand near window edges. Evaporation from moving air creates a cooling sensation that your skin can detect even when the air movement is too gentle to see. This method is less precise than the incense test but works when you do not have incense on hand.

The Light Test

On a sunny day, turn off all interior lights and look for daylight around window frames. In a dark room, even small gaps show as thin lines of light. This method identifies the most severe gaps but may miss smaller ones.

Professional Blower Door Test

For a definitive diagnosis, hire a home energy auditor to perform a blower door test. This involves sealing your home's exterior doors and using a calibrated fan to create negative pressure inside the house. The auditor then uses smoke pencils or infrared cameras to identify every air leak. Many Utah utility companies subsidize these audits for $50-$150 -- a worthwhile investment that identifies all leaks, not just the ones you can find manually.

A blower door test also gives you an ACH50 number (air changes per hour at 50 Pascals), which lets you compare your home's leakage to benchmarks. Homes built in the 1990s with original windows typically test at 8-12 ACH50. Modern building codes target 3-5 ACH50. After sealing or replacing windows, you can retest to verify improvement.

Level 1: Quick Fixes (Under $20 Per Window)

These solutions work best as emergency or temporary measures -- to stop a draft right now while you plan a longer-term solution. They are also effective as seasonal treatments for homes where replacement is not yet in the budget.

Rope Caulk

Cost: $4-$6 per package (covers 3-5 windows) Time: 5-10 minutes per window Effectiveness: 20-40% draft reduction

Rope caulk is a pliable, putty-like cord that you press into gaps around window frames. It sticks to clean surfaces without adhesive and can be removed in spring without damaging paint or finishes. It works best for gaps between the window frame and the wall trim where you feel drafts.

How to apply: Peel a length of rope caulk from the package. Press it firmly into the gap between the window frame and the interior trim, or along the sash where it meets the frame. Overlap ends to prevent gaps. For best adhesion, apply when the surface temperature is above 50 degrees.

Shrink Film Window Insulation Kits

Cost: $15-$20 for 5 windows Time: 15-20 minutes per window Effectiveness: 30-50% draft reduction, minor energy savings

Shrink film creates an additional air barrier over the entire window opening. It consists of clear plastic film applied over the interior window frame with double-sided tape, then shrunk tight with a hair dryer.

How to apply: Clean the window frame surface. Apply double-sided tape around the frame perimeter. Press the film onto the tape, leaving some slack. Use a hair dryer on medium heat to shrink the film until it is taut and wrinkle-free. Trim excess film with a razor blade.

The film is visually noticeable up close but nearly invisible from across the room. It prevents air from reaching the window assembly at all, making it effective even on very leaky windows. The downsides: you cannot open the window without removing the film, and it needs to be reapplied each season.

Draft Stoppers for Window Sills

Cost: $5-$15 per window Time: 2 minutes per window Effectiveness: 10-20% draft reduction (bottom edge only)

Foam or fabric draft stoppers placed along the window sill block air leaking at the bottom edge. These are the simplest option but address only one of several potential leak points.

Level 2: Weatherstripping Replacement ($20-$60 Per Window)

Replacing worn weatherstripping is the most impactful DIY improvement for windows that are structurally sound but sealing poorly. It restores much of the original air-sealing performance.

Identifying Your Weatherstripping Type

Before purchasing materials, open your window and identify the existing weatherstripping:

• Fin seal (brush/pile): Fine fibers on a backing strip, typically found in a channel along the sash sides. Common on 1990s-era double-hung and sliding windows.
• Compression foam: Foam strips that compress when the window closes. Found on better-quality double-hung windows and some casement windows.
• Compression bulb: Rubber or silicone tube-shaped seal. Found on casement and awning windows. The most durable type.
• Spring bronze or V-strip: Thin metal strips that flex to fill gaps. Found on older wood windows. Extremely durable but less common in modern windows.

Replacing Fin Seal Weatherstripping

Most 1990s-era vinyl and aluminum windows use fin seal in a track (kerf). Replacement fin seal is available at home centers for $5-$10 per 17-foot roll.

1. Open the window and locate the weatherstripping channel along the sash sides.
2. Pull out the old weatherstripping. It should slide out of the channel. Use needle-nose pliers if needed.
3. Clean the channel with a damp cloth to remove dust and debris.
4. Cut the new fin seal to length. Insert the backing flange into the channel, pushing firmly. The fin fibers should face toward the sash surface.
5. Close and lock the window. Check for remaining drafts with the incense test.

Upgrading to Compression Foam

For better performance, consider replacing fin seal with self-adhesive compression foam ($8-$15 per window). This is especially effective along the meeting rail of double-hung windows.

1. Clean the surfaces where the foam will adhere with rubbing alcohol.
2. Measure and cut foam strips to length.
3. Peel the adhesive backing and press firmly into place.
4. Close the window to compress the foam. It should compress about 25-50% when the window is closed -- if it prevents the window from latching, use a thinner foam.

Level 3: Caulking and Frame Sealing ($30-$80 Per Window)

Caulking addresses the gaps around the window frame that weatherstripping does not cover. For complete sealing, you need to address both the interior and exterior.

Interior Caulking

Use paintable silicone or latex caulk ($4-$8 per tube) to seal the joint between interior window trim and the wall. Even if this joint was originally caulked, settling and paint cycles create gaps over time.

1. Remove old, cracked caulk with a putty knife or caulk removal tool.
2. Clean the joint with a damp cloth and let dry.
3. Apply a thin, continuous bead of caulk along the trim-to-wall joint. Tool the bead smooth with a wet finger or caulk tool.
4. Repeat at the trim-to-window-frame joint if there is a visible gap.

Exterior Caulking

Use exterior-grade polyurethane or silicone caulk ($6-$12 per tube) on the joint between the window frame (or exterior trim) and the siding.

1. Remove old, failed caulk. Power-wash or brush-clean the joint.
2. If the gap is wider than 1/4 inch, insert a foam backer rod first to provide a base for the caulk.
3. Apply caulk in a continuous bead. Tool smooth.
4. Allow to cure per manufacturer instructions before painting (typically 24-48 hours).

Behind-the-Trim Insulation

For the most thorough sealing, remove interior window trim and insulate the gap between the window frame and the rough opening. In many older homes, this gap was filled with loose fiberglass batt or nothing at all.

1. Carefully pry off interior trim with a flat pry bar, protecting the wall with a putty knife behind the bar.
2. Inspect the gap. If you see daylight, loose fiberglass, or open gaps, there is room for improvement.
3. Apply low-expansion spray foam insulation into the gap. Use the "window and door" formulation, which expands gently and will not bow the frame. Do NOT use standard expanding foam, which can warp window frames.
4. Allow foam to cure (usually 8-12 hours), then trim flush with a utility knife.
5. Reinstall trim and caulk the trim-to-wall joint.

This is the most effective DIY sealing method but requires more time and skill. If you are uncomfortable removing trim, a handyman or window installer can do this for $40-$80 per window.

Level 4: Interior Storm Windows ($100-$250 Per Window)

Interior storm windows are removable panels that mount inside your existing window frame, creating an additional air barrier without replacing the primary window. They are a strong middle ground between DIY sealing and full replacement.

How They Work

An interior storm window is a clear acrylic or glass panel in a thin frame that compresses against your existing window trim or sash using magnets, clips, or compression gaskets. The sealed air space between the storm window and the primary window provides both air sealing and added insulation.

Benefits for Air Quality

Interior storm windows reduce air infiltration by 50-70% on average. They block air from reaching the primary window assembly at all, which means even severely degraded weatherstripping on the old window becomes less of an issue. The storm window's own seal is what matters.

Several companies offer custom-made interior storm windows sized to your exact openings. Indow, Innerglass, and others ship directly to Utah homeowners. You measure, order, and install yourself -- no contractor needed.

Limitations

Interior storm windows add bulk to the window opening and make the primary window harder to open (you must remove the panel first). They work best on windows you rarely open. For windows you open frequently -- bedrooms that need ventilation in mild weather, for example -- they add inconvenience.

They also do not address exterior sealing issues or failed insulated glass in the primary window.

Level 5: Full Window Replacement ($300-$1,200 Per Window)

When windows have structural failures -- warped frames, broken hardware, failed insulated glass seals, or rotted wood -- no amount of sealing addresses the root problem. Full replacement is the only path to reliable, long-term air sealing.

What Replacement Delivers

Modern replacement windows achieve air leakage rates of 0.1-0.3 cfm/ft² -- a 70-90% improvement over typical older windows. Combined with proper installation (spray foam in the frame gap, sealed trim, exterior caulking), a new window creates a nearly airtight barrier.

Beyond air sealing, replacement windows provide updated energy performance (U-factors of 0.20-0.30 vs 0.50-0.70 for many older windows), noise reduction, improved operation, and curb appeal.

Cost Ranges for Utah

Window Type	Per-Window Cost (Installed)	Air Leakage Rating
Vinyl double-hung (standard)	$300-$500	0.20-0.30 cfm/ft²
Vinyl double-hung (premium)	$450-$650	0.10-0.20 cfm/ft²
Vinyl casement	$350-$600	0.05-0.15 cfm/ft²
Fiberglass double-hung	$500-$900	0.10-0.20 cfm/ft²
Fiberglass casement	$600-$1,200	0.05-0.10 cfm/ft²

For comprehensive pricing data including labor, permits, and potential tax credits, see our complete window replacement cost guide.

Choosing the Right Approach for Your Situation

Your ideal approach depends on your window condition, budget, and timeline.

Go With Quick Fixes (Level 1) If:

• You are renting and cannot modify the windows
• Budget is extremely limited right now
• You need immediate relief during an active inversion or smoke event
• You are planning replacement but need a stopgap for one more season

Go With Weatherstripping and Caulking (Levels 2-3) If:

• Windows are under 15 years old with intact glass seals and straight frames
• Hardware works properly and sashes stay in position
• Specific windows have localized drafts that can be traced to worn sealing
• You want to extend the life of functional windows by 5-10 years

Go With Interior Storm Windows (Level 4) If:

• Windows are older but in reasonable structural condition
• You want significant improvement without the cost of full replacement
• Historic or architecturally significant windows need to be preserved
• Budget allows $100-$250 per window but not $300-$1,200

Go With Full Replacement (Level 5) If:

• Windows are over 20 years old with multiple failure signs (fogging, drafts, broken hardware, warped frames)
• You have single-pane windows
• Energy bills are notably high and you want the full package of benefits (air sealing + insulation + noise + appearance)
• You plan to stay in the home for 5+ years (sufficient time to recoup the investment)

For guidance on whether your specific situation calls for replacement, see our detailed framework in the Utah air quality and windows guide.

Professional vs DIY: When to Call In Help

Most Level 1-3 work is manageable for a homeowner with basic tools and a weekend afternoon. Weatherstripping replacement, caulking, and shrink film are straightforward projects with widely available materials.

Call a Professional When:

• Spray foam behind trim (Level 3): If you are not comfortable removing and reinstalling trim, or if the gap is larger than expected, a professional ensures the job is done without damaging the frame or wall finish. Expect $40-$80 per window.
• Interior storm windows (Level 4): Measurement is critical. If you are unsure about measuring or your openings are not square (common in older homes), having the manufacturer's installer do the work ensures proper fit.
• Full replacement (Level 5): Window replacement involves removing the old window, preparing the rough opening, setting the new window plumb and level, insulating, flashing, and finishing both interior and exterior. Improper installation can cause water leaks, air leaks, and frame failure. Professional installation is strongly recommended.

Finding a Utah Window Professional

For window replacement, get quotes from at least three local installers. Ask each one specifically about their air-sealing practices -- do they use spray foam in the frame gap? Do they seal interior trim? Do they caulk the exterior frame-to-siding joint? A quality installer addresses all three. If an installer only plans to "pop the new window in and trim it out," they are leaving air-sealing performance on the table.

For Utah-specific cost expectations and what to look for in an installer, see our window replacement cost guide. And for the broader context on why window sealing matters so much for Utah homes facing inversions, wildfire smoke, and allergies, our comprehensive air quality and windows guide covers everything from PM2.5 science to seasonal preparation strategies.