How to Insulate a Finished Attic

By: Bill Kibbel , Contributing Writer
In: Obsolete Design Elements, Home Improvement Tips

In response to an e-mail, you were kind enough to explain how to best insulate our old house attic. We are now seriously considering finishing the attic. I’m guessing this would change how to insulate. Would we just put the insulation between the roof rafters, or am I missing something? If you have time, I would really appreciate some guidance.

Finishing an attic seems straight forward and simple, but there are some things to consider. Planning ahead, research, and correctly installing materials can help make an attic into comfortable extra living space.

Attic spaces are typically very hot in the summer and very cold through the winter. Without properly insulating, your finished attic room will also be cold in the winter. Without properly ventilating the roof, your finished space can be just as unpleasant as an unfinished attic. Since parts of a finished attic are usually very close to the roof, insulation often blocks proper ventilation that is needed under a roof structure.

Ventilation
The best method for ventilating any roof is to have some type of venting up high on the roof that allows for rising hot air or warm/damp air to escape. For this to be fully effective, low vents should be installed to replace the air that is escaping. Continuous ridge vents and soffit vents are often the most effective, but it’s best to have an experienced professional evaluate to determine the best method.

Insulation

I suggest designing the walls and ceilings to minimize the area where insulation behind these surfaces is near the roof as this can restrict air movement.

  1. The area near where the roof rafters meet the floor doesn’t have much clearance so “knee walls” are often constructed. The area behind these walls is often used for storage of seasonal items and luggage.
  2. Instead of the ceiling following the slope of the roof all the way to the peak, a flat ceiling can be easily installed by attaching joists horizontally across pairs of rafters.

Both of the above creates some spaces where a good amount of insulation can be installed and areas are created for ventilating under the roof. These spaces aren’t really useable in a room anyway.

Where insulation will have to be installed between the roof rafters, “baffles” should be installed first. Baffles hold the insulation away from the underside of the roof decking, creating a channel for air movement.

Finished Attic

Finished Attic

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