Attic insulation is very high on the priority list of energy efficiency measures, because warm air rises to escape through ceilings with low pressure boundaries, and warm air always flows towards colder air. In other words, the energy you pay for during winter months to stay warm rises and can escape through numerous small areas of an unsealed ceilings that can add up to the equivalent of always having a door or window open. With the wrong type, or having insulation that was improperly installed, a drafting effect can occur causing extreme energy loss.
During summer months the opposite happens with hot from the attic drawing towards the cooler living areas. An unvented attic can allow hot expanding hot air to radiates through the insulation and ceiling causing air-conditioning systems to run for long periods to keep a home cooled way into an evening until this pressure dissipates and the attic cools down.
Spray foam insulation is currently the most effective insulation on the market, and can be installed on the underside of of the roof decking, or between ceiling joist of an attic.
Blown in fiberglass or Rockwool insulation have a specific usefulness in certain applications, but like fiberglass batt insulation, these are not highly effective materials due to their inability to prevent air movement. These products are designed to hold air for optimal performance, and need to be installed within a closed-sealed cavity where no air infiltration is allowed.
Fiberglass batts are commonly found in older homes, and unfortunately, this type of insulation does not provide the level of thermal protection needed to stop energy loss. The more fibrous an insulation is, the more air is allowed to penetrate in and around it, and this is known as a wind washing, and this can actually lower the effective R-value of the insulation.
Blown in cellulose is an environmentally friendly, cost effective insulation. Cellulose insulation is made from recycled newspaper and treated with borate (salt mineral mined from the earth) for an effective pest control, and this mineral provides the insulation with a Class 1 fire rating. The even distribution of blown in cellulose insulation makes for a high performance insulation that has come into increasing favor among green builders and building energy efficiency experts.