an important basis for such rules, buildings with high interior humidities during cold weather (e.g., museums, swimming pools, humidified or poorly ventilated airtight homes) may require different rules, as can buildings with drier interior environments (e.g., highly ventilated buildings and warehouses). The 2009 International Residential Code embodies more sophisticated rules to guide the choice of insulation on the exterior of new homes, which can be applied when retrofitting older homes. A vapor permeable building wrap on the outside of the original wall helps keep the wind out allows the wall assembly to dry to the exterior. Asphalt felt and other products such as permeable polymer based products are available for this purpose, and usually double as the water resistant barrier / drainage plane as well. Interior retrofits are possible where the owner wants to preserve the old exterior siding, or where setback requirements limit space for an exterior retrofit. Sealing the air barrier is more difficult and the thermal insulation continuity compromised (because of the many partition, floor, and service penetrations), the original wall assembly is rendered colder in cold weather (and hence more prone to condensation and slower to dry), occupants are exposed to major disruptions, and the house is left with less interior space. Another approach is to use the 1/3 to 2/3 method mentioned above—that is, to install a vapor retarder on the inside of the existing wall (if there is not one there already) and add insulation and support structure to the inside. This way, utilities (power, telephone, cable, and plumbing) can be added in the new wall space without penetrating the air barrier. Polyethylene vapor barriers are risky except in very cold climates, because they limit the wall's ability to dry to the interior. This approach also limits the amount of interior insulation that can be added to a rather small amount (e.g., only R-6 insulation can be added to a 2×4 R-12 wall). Costs and benefits In new construction, the cost of the extra insulation and wall framing may be offset by not requiring a dedicated central heating system. In homes with numerous rooms, more than one floor, air conditioning or large sized, a central furnace is often justified or required to ensure sufficiently uniform temperatures. Small furnaces are not very expensive, and some ductwork to every room is generally required to provide ventilation air in any case. When peak demand and annual energy