How Do Mosquitoes Get in the House?

Mosquitoes can enter in 2–5 seconds at dusk or dawn when Culex is most active. They actively follow people through doorways. (2) Torn, bent, or ill-fitting window and door screens — corner gaps where screen meets frame are as dangerous as visible holes. Standard 18x16 fiberglass screen (openings 1.33x1.52mm) reliably excludes adults. (3) Attached garage doors with degraded bottom seals — NH's most underestimated pathway. Mosquitoes shelter in garages, then enter through the unweatherstripped interior door. (4) Gaps around exterior doors — failed weatherstripping and missing door sweeps. Any gap that lets a dollar bill slide free is a mosquito entry. (5) Utility penetrations — plumbing, gas, electrical, AC line-sets, cable drops, and dryer vent flaps stuck open by lint buildup. (6) Soffit, gable, ridge, and attic vents — especially relevant for Culex, which prefers higher entry points and attic overwintering. (7) Basement bulkhead doors and fieldstone foundation mortar gaps — specific to NH and rarely addressed. (8) Chimneys without caps, uncapped plumbing vent stacks. (9) Hitchhiking on clothing, pets, and bags — genuine but low frequency. (10) Pet doors — continuously unscreened openings with lower flow than exterior doors.

Standard 18x16 fiberglass window screen has openings of approximately 1.33x1.52mm, which reliably excludes adult mosquitoes but not no-see-ums or the smallest midges. For Seacoast homes or properties adjacent to freshwater swamps, 20x20 no-see-um mesh drops the opening to 0.8–1.0mm and is what you want for total exclusion. The WHO mosquito-netting specification is 1.2mm maximum mesh opening. The commonly cited '1/16 inch' figure (1.59mm) is at the upper threshold — small mosquitoes can still squeeze through it. Treat any structural gap 1.5mm or larger as an active mosquito entry and seal it.

They are weak fliers at approximately 1–1.5 mph, which is why fans work and why positioning a box fan between you and the door is effective defense. Peak entry activity is dusk (roughly one hour before and after sunset) and again at dawn. Light matters less than homeowners assume. Peer-reviewed work (Wilson et al. 2023) shows Cx. pipiens is most attracted to blue-rich cool-white LEDs and least attracted to warm-white (<3000K), amber, and red wavelengths. The best lighting fix is not changing the bulb — it is moving porch lights away from doors so mosquitoes congregate somewhere other than your entrance, or switching to motion-activated fixtures.

First, fieldstone basement foundations common in pre-1940 homes have mortar gaps that admit mosquitoes directly into the coolest, most humid part of the house — exactly where Culex pipiens wants to overwinter. These gaps require hydraulic cement (not standard caulk) and spray foam for rim-joist cavities. Second, basement bulkhead doors are ubiquitous in NH (Bilco-style hatchways) and the gasket-to-masonry joint is rarely sealed properly — check the top-door gasket, caulk the masonry-to-frame joint, and weatherstrip the interior bulkhead door. Third, attached garages — nearly universal in post-1970 NH construction — act as mosquito staging areas. The garage door bottom seal degrades, mosquitoes shelter inside the warm garage, and the interior door to the kitchen or mudroom is almost never weatherstripped to exterior standards. This three-step garage pathway is the most overlooked and most fixable entry route in NH homes.

Install a pest-rated door sweep on every exterior door (Xcluder Low-Profile Rodent-Proof Door Sweep, approximately $25–30, is the professional standard). Replace garage-door bottom seal if cracked, compressed, or curled (Frost King RV18, approximately $25); add side and top seals (Frost King GaraSeal kit, approximately $30). Windows and screens: Walk every screen with a flashlight held obliquely — backlight reveals pinholes and corner gaps. Replace with Phifer 18x16 fiberglass for standard protection, or Phifer No-See-Um 20x20 charcoal fiberglass for Seacoast or swamp-adjacent homes. Use self-adhesive screen patches for small tears. Utility penetrations: Clean dryer vent and confirm the backdraft flap closes fully; replace with a magnetic or spring-loaded damper if worn. Seal plumbing, gas, electrical, and AC line-set penetrations with foam plus exterior-grade caulk. Basement (NH-specific): Inspect and caulk the bulkhead door gasket and masonry-to-frame joint. Fill fieldstone foundation mortar gaps with hydraulic cement. Seal rim-joist cavities with two-part spray foam. Cover sump pit with a sealed lid.