A dark attic with a skylight and fiberglass insulation in the floorboards

Attic insulation is a significant concern for homeowners in northern areas with frigid winters. You may have noticed that some attics have vapor barriers installed while others don’t. 

So, are vapor barriers necessary for attic insulation, and should you install one in your house? 

This article will discuss what a vapor barrier does and when you need to install one in attic the attic. We’ll also discuss how to install a vapor barrier in attic insulation properly. 

And if you’re looking for a calculator to see what installing a vapor barrier might cost you, try out our Vapor Barrier Cost Calculator here.

Does Attic Insulation Need a Vapor Barrier?

Attic insulation doesn’t always need a vapor barrier, but if you live in a climate with colder winters, you may need to install one. A vapor barrier can prevent moisture buildup in the attic insulation, preventing the insulation from getting damaged. 

What Does a Vapor Barrier Do for Attic Insulation?

As the name suggests, a vapor barrier, also known as a vapor retarder, prevents water vapor from building up in your attic insulation. This problem is more common in the winter when the attic is colder than the rest of the house. 

As the water vapor generated inside rises towards the ceiling, it enters the attic through porous areas. If you don’t have a vapor barrier, the attic will become more humid in the winter, and the insulation may get damp. 

While most types of fiberglass insulation are waterproof, too much moisture can reduce the lifetime of your ceiling and may damage the drywall in your house. 

A worker wearing a face mask installing insulation in a roof space without attic insulation batts.

A vapor retarder prevents the water vapor from getting deposited in attic insulation and will circulate it through the ceiling. This application can make your house warmer in the winter since the indoor humidity levels will increase. 

However, manufacturers didn’t introduce vapor barriers until the arrival of non-porous attic insulation. Previously, water vapor would circulate out of the house through chimneys and air vents since water vapor tends to move toward cold air. 

However, in modern homes without air vents and fireplaces, the vapor has no other way to exit the house and will get deposited in the attic. 

This doesn’t mean you shouldn’t have airproof insulation or open a blocked chimney—studies have shown that an open fireplace will lose 70% of its heat and can actually make your house colder in the winter. 

In such climates, a vapor barrier can prevent moisture buildup in your attic without compromising your house’s insulation system. 

When You Should Install a Vapor Barrier With Attic Insulation 

You should only install a vapor barrier in attic insulation if you live in a climate with more than 8,000 heating degrees days. For example, if you live in the US, you’ll need a vapor barrier in climate zones 6 and 7. This map of US climate zones will show you which zone you’re located in. 

In rare cases, you may need to install a vapor barrier with attic insulation if you live in climate zone 4 or 5, although it’s usually unnecessary. You’ll only need a vapor barrier with attic insulation if you live in an area where it’s freezing in the winter, and your house is completely insulated. 

This necessity is because houses in colder climates will produce more indoor water vapor in the winter since doors and windows are rarely opened. While a sound air heating system may circulate the water vapor, it may also make your house more humid. This moisture may get stuck if your attic doesn’t have a vapor barrier. 

You’ll need to install a vapor barrier before the attic insulation if your ceiling is porous in different spots due to lighting installations or other fixtures. This sequence will prevent the water vapor from going through the ceiling to the attic, and you won’t have an excess moisture problem. 

An unfinished attic with a partially-installed vapor barrier

When You Don’t Need a Vapor Barrier 

You don’t need a vapor barrier if you live in a moderate or hot and humid climate, as it will cause more harm than good. In the US, this will be anywhere in climate zones 1-3, although houses in climate zone 4 often don’t need vapor barriers either.

Installing a vapor barrier in a hot climate will only cause your house to heat up significantly during the summer, and you’ll have to deal with more humidity indoors. 

Homes in warmer climates rely more on proper air circulation to lower indoor temperatures in the summer. So while these houses still benefit from attic insulation, they don’t need vapor barriers. 

Another downside of installing a vapor barrier in a hot and humid climate is that it may damage your walls. The excess humidity won’t be able to circulate through the ceiling properly, and it will damage your drywall over time. This consideration is one of the reasons why houses in humid areas need more circulation and vents. 

Faced insulation also doesn’t usually require a retarder since it traps moisture. However, you’ll have to consult your local laws to check whether houses with faced insulation are exempt from having vapor barriers. 

If you have to install something to prevent moisture buildup in a humid climate, you can install an air filter instead of a barrier. 

Why You Shouldn’t Install a Vapor Barrier Over Existing Insulation 

Never install a vapor barrier over existing insulation, as it will only damage the ceiling. The correct way to install one is first to remove the existing insulation and then install a vapor retarder. Then, you can install a new layer of insulation over the barrier to completely seal your attic. 

If your old insulation is damp because of the absence of a vapor retarder, it’s best to replace the insulation when installing a barrier. While this may be expensive, it’s an excellent long-term investment, and you shouldn’t have to replace your insulation for at least 10-20 years. 

Unfortunately, you can’t really seal the attic completely, so you’ll have to install a vapor barrier over the ceiling drywall. This process is only effective if you paint the drywall with permeable paint (like latex paint) to allow the water vapor to circulate through the ceiling. 

Always install a vapor retarder in the interior of the insulation if you live in a colder climate since the weather will remain cold or mild for most of the year. 

Other Factors To Consider When Installing a Vapor Barrier

Vapor barriers aren’t perfect, and you’ll have to look at other factors before installing one in your attic. These include the type of cladding on your roof and walls and the barrier’s permeability. 

Houses with absorptive cladding are more likely to retain moisture, so you’ll have to install the retarder on the exterior of the insulation. This application should protect the cladding from getting damp and will prevent structural damage to your house. 

The roof of a home under construction with a vapor barrier partially installed

Another factor to consider is the permeability of the barrier. Vapor barriers with low permeability keep out more water vapor and are more effective at protecting attic insulation in colder climates. 

However, it would be best if you always compared the indoor and outdoor humidity levels when looking at the permeability of the retarder. For example, if the indoor humidity is much higher and the air is drier outdoors, you should use a vapor barrier with lower permeability. 

Final Thoughts 

A vapor barrier can protect your attic insulation from getting damp and moist, but you don’t always need it. However, if you live in a cold climate with moderate summers, attic insulation will benefit from a retarder. 

Always ensure that the barrier is installed correctly and has some permeability to prevent moisture buildup in the ceiling. 

Best of luck in your endeavors!


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