Can You Over Insulate An Attic? [And what happens then?]

If you’re building your own house or your current residence doesn’t trap heat as well as you’d like, you may wonder how much insulation you should place in your attic and if it’s possible to install too much. We researched this topic to find out everything that you need to know!

It’s possible to over-insulate an attic as too much will cause moisture buildup and eventually result in mold. Eventually, adding more insulation leads to diminishing returns in trapping heat as well. The right amount depends on the region and climate you live in, your attic’s size, and how energy efficient you want your home to be.

We know that’s a lot to hear, which is why we’ll discuss all of it in detail in this post! Read on and we’ll tell you how to figure out the right amount of insulation for your attic, how often you should insulate it, whether or not you should keep old attic insulation, and touch on roof insulation as well. Let’s get started!

Professional worker putting mineral rockwool insulation in attic wall, Can You Over Insulate An Attic [And what happens then]

How Much Insulation Does Your Attic Need?

Let’s get this out of the way first: insulating your attic to your local government code is usually insufficient. If you insulate your attic to code, you’ve only achieved the bare minimum allowed by law, which is rarely the ideal amount of insulation. In fact, far more homes suffer from under-insulation than over-insulation.

As we said before, how much insulation you need depends on your region’s typical climate. Its R-value measures insulation’s effectiveness at trapping heat. The higher the R-value, the more effective it is. The Department of Energy recommends your attic have an R-value of R-30 for hot climates, R-38 for temperate climates, and R-49 for cold climates.

Obviously, you’ll need more insulation if you live in a colder region. This article has a map and data table that show recommended R-values for different regions in the US. You should have enough insulation for it to extend higher than your floor joists, and you have trouble seeing the joists when you look across your attic.

Most homes are under-insulated, but there is such a thing as too much insulation. Excessive insulation leads to moisture buildup, mold, and worse air quality in your home. Additionally, you’ll reach the point where adding insulation will cost you more in installation fees than it will save you in heating bills. Finally, you should also check to ensure that the insulation isn’t blocking any vents, or your attic’s ventilation will be compromised.

How Often Should You Insulate Your Attic?

Your insulation’s longevity depends on what type you have. Spray foam or polyurethane foam insulation can potentially last a lifetime. Fiberglass insulation can last 80 to 100 years unless it’s damaged. Cellulose insulation usually works for 20 to 30 years, but it can lose its effectiveness after 15 years.

Generally speaking, your insulation should last at least 15 years before needing replacement, and you should have an insulation company check it once it’s that old to make sure it’s still working. However, external factors can compromise its integrity and require you to get new insulation sooner. These factors include leaks, mold, pest infestation, wet or punctured insulation, or severe weather. If your insulation gets moldy, replace it immediately.

If you constantly feel drafts indoors or you’re always fiddling with your thermostat to keep the temperature bearable, then your insulation might need replacement. Additionally, you should get new insulation if your home was built in the 1970s or earlier as manufacturers have significantly improved their products since that time. This recommendation especially applies if your house has never had its insulation replaced.

You should have an expert conduct an annual inspection of your attic to determine whether or not your insulation needs replacing or reinforcement. Insulation gradually loses its abilities over time, so you should get in the habit of regularly checking the material in your attic.

Should I Remove Old Attic Insulation Before Adding New Insulation?

We’ve already discussed this topic in detail in a previous post, so you can read that if you want the whole story. However, we’ll give you a quick run-down here.

Most of the time, you don’t need to remove old attic insulation before adding new material. There are only a few exceptions to this rule, and they should sound familiar since we discussed some of them in our last section.

If the existing insulation is wet or looks like it has been wet or exposed to moisture, you should remove it because it can cause mold and rot. As we mentioned before, you must immediately replace any moldy insulation. Any insulation that has suffered fire damage should also be replaced.

If you find evidence of a current or previous rodent/pest infestation, you must ensure all rodent urine and/or feces is removed as well as any nearby insulation. You should also have your attic disinfected before you add any new insulation.

A Warning About Vermiculite Insulation

Those who own a house built before 1990 also have a crucial safety consideration to be aware of: vermiculite insulation. From 1919 to 1990, a Montana mine was the source of 70 percent of all vermiculite sold in the US. This mine had a deposit of asbestos, so all the vermiculite was contaminated.

If your house was built before 1990 and has vermiculite insulation, do not disturb the material. If you do, you might release and inhale asbestos fibers, leading to serious, potentially fatal, illness. Don’t store anything near any vermiculite insulation or allow children to approach it. Hire a professional asbestos contractor to remove the vermiculite insulation. Do not try to remove it yourself.

Should You Insulate Between Roof Rafters?

The interior view of a roof's gable. The beams of the home are exposed and there is aluminum insulation showing

If you’re storing anything in your attic that needs to stay warm or if you want to use it as a living space, then you should insulate between the roof rafters. This insulation will keep the attic roughly as warm as the rest of the house and reduce heat loss from your roof. If you plan to keep your attic unfinished and don’t need to heat it, then insulating only the attic floor is sufficient.

Insulating between your roof rafters is generally harder and more expensive than insulating the attic floor joists. Ensure you allow for ventilation below your roof’s shingles or tiles as it’s necessary to prevent moisture buildup. Leave space for air to flow up one side of the roof parallel to the rafters and down the other side.

You have several options regarding which type of insulation to install here. You can use batts of fiberglass, cellulose, or mineral wool. You could also cut a polystyrene slab to size and use it. Expanded polystyrene insulation panels are another great choice as they’re meant to squeeze between rafters. Finally, you could try polyurethane spray foams as long as there is nothing under the shingles/tiles (including roofing felt) and there are no missing or slipped tiles or shingles.

If you want to read more about which types of insulation work best for attics, feel free to read our post on the topic.

What Thickness Should Roof Insulation Be?

You’ll generally want to install 250-270mm of roof insulation, though we recommend you go for the 270mm maximum.

Final Thoughts

Insulating your attic is a worthwhile endeavor considering the reduced HVAC bills and lower carbon footprint that can result from it. Just make sure you don’t add too much insulation and check once a year to make sure it’s still in good condition. Once you’ve determined the right amount of insulation for your home, you’re well on your way to an energy-efficient future. Good luck!

For more information on attics and other home heating and cooling subjects, take a look at our other informative guides here at!

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