Common Insulation Mistakes

Installing insulation is like painting. Looks easy. Almost everyone thinks they can do it. And almost everyone makes mistakes. Knowing what mistakes to avoid makes your DIY insulation efforts more successful.

Insulation Mistakes

Insulation Mistakes and How to Avoid Them

Improperly installed insulation usually causes colder or hotter houses and a less comfortable home. It costs more to heat and cool. Moisture can be a problem that causes mold and mildew growth and attracts pests like termites. Insulation mistakes can even cause fires.

Not Sealing Out the Air

Insulation is a thermal barrier that slows heat transfer. Most of it is not an air barrier. Air will pass right through products like unfaced fiberglass. Before insulating, seal all holes and gaps with spray foam in a can or caulking. (Acoustic caulking never dries and must be covered.)

  • Electrical boxes and plumbing and mechanical penetrations to the exterior.
  • Plumbing and electrical penetrations into attics. Also bath fans and lightboxes.
  • Wood-to-wood seams such as wall sill plates to the floor and cripple-to-stud construction–which is not done in the picture above. This is a standard building code requirement in many jurisdictions.

Blocking Attic Air Flow

The attic air above the insulation blanket on the attic floor should be close to the temperature outside. Air movement is essential to achieve this. Attic ventilation also helps control moisture. Never block the vents.

When installing or adding attic insulation, install vent baffles to allow unimpeded air movement between soffit vents and roof vents or gable vents. All vents should be designed to keep out pests and water.

Removing Existing Insulation Can be Unnecessary

In many cases, removing existing insulation is a waste of time and money. Adding more loose-fill insulation to an attic that contains fiberglass or cellulose is perfectly acceptable. Installing faced fiberglass over unfaced fiberglass in wall cavities adds R-value as long as it is not compressed too much.

Removing Existing Insulation Can be Essential

Some situations require removing existing insulation–for safety or to make a good job of installing the new product. Adding more insulation without fixing the problem causes more headaches and costs.

  • Wet Insulation. Most wet insulation has reduced R-value. Remove it and fix the leak.
  • Mold and Mildew. Mold and mildew growth usually means moisture is getting in. Mold can cause health problems and stinks. Solve the problem before adding insulation.
  • Pest infestations. Rodents and insects nesting in your insulation certainly reduce insulation value. The insulation and pests need to be removed before new insulation is installed.
  • Asbestos. Removing asbestos or leaving it alone are the best ways of dealing with the problem. Blowing loose fill insulation over it seems like a good idea but even that process will send asbestos fibrils into the air. Some states mandate professional removal.

Forcing In Too Much Insulation

The insulation value in products like fiberglass batts comes from the air trapped in them. More is not better if it is compressed. The batts lose R-value. Compressing a 5 ½” batt into a 3 ½” cavity will not give you R-20 walls. It will give you R-12 with more expensive insulation. Use the appropriate size for optimal results.

Attention to Detail

Batt insulation is meant to fill wall cavities stud-to-stud and sheathing-to-drywall. Close enough isn’t good enough. Compressing batts behind electrical boxes, wires, and pipes will leave cold spots. A batt that does not fill a corner leaves another cold spot. Enough cold spots and the whole wall is compromised.

Cut out the appropriate amount of insulation to get a good fit around obstructions. Split bats to fit around wiring and pipes. Pull the batt into corners. Cut batts to fit properly into non-standard stud cavities. Cutting off an inch is better than compressing the batt.

Using the Wrong Insulation

With all of the types of insulation available, it can be easy to make the wrong choices. Here are a few things to keep in mind.

  • Bubble Wrap insulation and reflective insulation are effective in some situations. Totally worthless in others.
  • Fiberglass is not the best choice for basements. Rigid foam board insulation or spray foam insulation are moisture-resistant and better choices.
  • Mineral wool Insulation is best for soundproofing.
  • Spray foam insulation is the wrong choice for attic floors.
  • Safety. Do not use flammable insulation near heat sources like furnaces and light fixtures.

Applying Vapor Barrier Incorrectly

Do not install another layer of faced insulation over existing faced insulation. Moisture can be trapped between the layers. The vapor barrier part of faced insulation should always be installed against the warm side of the wall.

Some locations require a 6 mil poly vapor barrier on the inside of the studs. No need to use faced batts. Poly must be sealed and overlapped properly.

Ignoring Local Building Codes

Ninety percent of US houses are under-insulated. House construction has become highly regulated. Local codes change and often require permits and inspections. Make sure you know the requirements before starting your project. Not adhering to the regulations costs time, money, and aggravation.

Fire Safety

Many types of insulation are flammable. Leave sufficient space around light boxes and never insulate over pot lights. Do not cover or plug vents. Keep insulation away from furnaces, water heaters, or any other source of open flame.

Ignoring Window and Door Insulation

Windows and doors can be responsible for a large amount of heat loss. Make sure you install a new weatherstrip where required. Insulate the cavity between the frames and studs. Remove the existing insulation. Use a combination of low-expansion spray foam and fiberglass. Do not pack the fiberglass tight.

Hire Experienced Contractors

If you do not feel comfortable insulating your house or renovation, consider hiring a professional. It will be more expensive but will likely be done correctly with the proper product in the appropriate locations.

There are about 10,000 insulation installers working in the US. Your brother-in-law who insulated his basement may not be one of them. Insulation installer is a non-compulsory trade. It is learned on the job without an apprenticeship program.

Make sure to do your homework on insulation contractors in your area. Hire a company with a track record and a stable workforce. You may spend a little more money but the result should be more satisfying.