How do I create positive air pressure in my house?

You can create positive air pressure in your house by supplying more fresh outside air into the house than is being exhausted out, so that air pressure inside is slightly higher than outside and air leaks outward rather than inward. The most common way is to use your HVAC system to bring in outside air, many modern systems have a fresh air intake duct connected to the return plenum that brings in filtered outside air when the system runs. You can also install an energy recovery ventilator or heat recovery ventilator that supplies balanced fresh air while conserving energy, or a dedicated makeup air unit that blows filtered outside air in. Another method is to adjust your HVAC to have more supply air than return air by partially closing return vents or opening supply vents, though this should be done by a professional to avoid straining the system. Running a supply-only ventilation fan that pulls outside air in and filters it will create positive pressure. You should first reduce excessive exhaust by not running large range hoods, dryers, and bathroom fans simultaneously without makeup air, and seal leaky return ducts in attics. Positive pressure helps reduce infiltration of dust, pollen, and humid air through cracks, which is beneficial in humid climates and for allergy sufferers, and prevents backdrafting of combustion appliances. Aim for slight positive pressure, about 2 to 5 pascals, too much wastes energy and can push moist indoor air into walls causing condensation in cold climates. An energy auditor can measure pressure with a manometer.