How Much Attic Insulation Does a Boise Home Need?
Most Boise area homes need about R-49 of attic insulation, which works out to roughly 16 to 20 inches of blown-in fiberglass or 14 to 17 inches of cellulose. Dodd Roofing and Exteriors regularly finds Treasure Valley attics with half that or less, and the shortfall shows up on the roof as uneven snow melt, ice at the eaves, and shingles that work harder than they should. If you are adding insulation anyway and the framing has room, R-60 is a better target.
What R-value should a Treasure Valley attic have?
Ada and Canyon counties fall in climate zone 5 under the model energy code, and attics in newer homes here are generally built to about R-49. Homes from the 1970s and 1980s, including much of the Boise Bench, the North End, and older parts of Nampa and Caldwell, often have a thin layer that was fine when it was installed and falls well short today. Your city building department can confirm what applies to your specific project, since Boise, Meridian, Eagle, and Star each handle permits on their own.
How can I tell how much insulation I have?
Open the attic hatch with a flashlight and a ruler. Step only on the joists, never on the drywall between them. Measure the depth in a few spots and note the type: fluffy white or pink fiberglass, gray paper-like cellulose, or rolled batts. If you can still see the tops of the joists, you almost certainly have less than 6 inches, which is far under target. Matted, wet, or dirty patches are worth noting too, because compressed insulation delivers less than its label says.
Why does attic insulation matter for my roof?
Heat that leaks into the attic warms the roof deck. On a cold night after a sunny day, that warmth melts snow on the upper roof, and the water refreezes at the cold eaves. We see this most in the foothills and in Eagle and Star, where nights drop well below freezing. The Boise ice dam question is less severe than in North Dakota, but the same physics applies.
Moisture is the other piece. Warm indoor air that leaks upward condenses on cold wood during inversions and cold snaps, which can stain decking and dampen insulation. Our post on attic frost and moisture in winter explains how to tell condensation from a real leak.
Can I add more on top of what is already there?
Usually yes, but air seal first. Gaps around recessed lights, bath fan housings, plumbing stacks, and the attic hatch let warm air bypass the insulation, and more fluff on top will not stop that. Just as important, never bury the soffit vents. Rigid baffles at each rafter bay keep a clear channel for air to travel from the soffit to the ridge. Our guide to attic ventilation requirements in Idaho covers how much vent area you need.
Should I insulate before or after a roof replacement?
Order matters. If the roof is leaking, fix it first, because wet insulation compacts and has to be replaced. If the roof is sound, the best moment is often around a replacement. A roof replacement is when blocked soffits, missing baffles, and bath fans venting into the attic are easiest to correct, and the insulation crew can then work from the inside in any weather. Dodd's free inspection covers the roof, decking, and ventilation side. For the insulation itself, have a qualified insulation contractor quote the work.
Are there rebates, and where do I start?
Idaho Power and Intermountain Gas have offered weatherization incentives at various times, but terms change, so confirm current details before you book. If a roof project is part of the plan, our financing options can spread the cost, and the homeowner guide library has more attic and roof basics. For a roof and attic check anywhere in the Treasure Valley, call (208) 295-9421 or use the link below. We will tell you what we see, and we will not push a replacement the roof does not need.
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