When do you need a vapor barrier for insulation?

Undergoing a renovation? Maybe, you’re building a new wall, basement, roof, swimming pool, a spa room, or simply an additional floor.

Insulation and vapor-barrier is something you cannot skip thinking about! Confused whether you need a vapor-barrier for insulation? Well, it depends on your home location, climate conditions, cladding type, wall location, and similar.

Buying a right vapor barrier and installing it properly is must otherwise you undergo damages in the long-term. Also, it prevents mold, mildew, moisture, rot, etc.

So, stay with us till the end to discover everything about vapor barrier. We’ll talk about factors like weather conditions, cladding type, wall location, permeability, etc to consider before installing vapor-barrier.

When do you need a vapor barrier for insulation

When do you need a vapor barrier for insulation?

Here are factors that say whether you need a vapor barrier for insulation or not:

Climate type

Type of Home area/location

Vapor-barrier placement
















Hot & humid



Cladding type

Climate Type

Vapor-barrier placement

Absorptive cladding







Cold-&-Dry climate

No need of vapor barrier

Non-absorptive cladding




Cold-&-Dry climate






Permeability Rating

Material Type


less than 1 U.S. perm rating

Glass, Rubber membrane, Polyethylene sheet, Sheet metal, Exterior-grade plywood, Foiled rigid insulation board, Vapor-retardant paint, etc


1 to 10 U.S. perm rating

Interior grade plywood, Tar paper/30-pound Asphalt-coated paper, Unfoiled extruded or expanded polystyrene, Paper or foil faced batt insulation, Bitumen-coated kraft paper, Oil-based or moisture-retardant latex painted gypsum board, etc


10 or above U.S. perm rating

Brick, House wrap, Board lumber, Concrete block & slabs, Unfaced fiberglass insulation, Cellulose insulation, Tar paper/15-pound Asphalt-coated paper, Drywall/Unpainted gympsum board, etc

Things to consider

Vapor-barriers or vapor retarders are needed for almost every home & building! Just the place where you put it – interior or exterior differs! It depends on weather, cladding type, wall location, and vapor-barrier’s permeability factor.


Climatic conditions change the position of vapor barrier insulation inside wall structures.

If you live in colder regions or extreme weather conditions, you should install a polyethylene plastic vapor barrier on interior sides. Put it likewise, even though in summer, the temperature can go hotter! As it is a cooling-dominated region, heat is likely to come from the home’s interiors. That’s why you put vapor-barrier on the interior against cladding or insulation. Doing so, you will get more heat during long winters. You can keep the interiors warmer!

If you live in mixed-humid climates, you’ve always got a lot of humidity outside! So, there is vapor! So, a vapor barrier is a must-have by building codes too. It’s trickier to decide the place of vapor barrier in walls though! It depends more on specific area types. If the area of your home is cooling-dominated for longer durations, install a wall vapor barrier on the interior. Other way round, if the area of your home is heating-dominated for longer durations, install a wall vapor barrier on the exterior.

If you live in mixed-dry climates, you might not require a vapor barrier at all. This is because the outside air is dry and doesn’t include humidity. As there is no-to-little rainfall and thereby vapor, there is no moisture and thereby no mildew damage. The chances the heat will produce are from inside a house. So, install a vapor barrier on the interior. Also, if building code mandates, consider adding a vapor barrier. 

If you live in a hot & humid climate, you should install a vapor barrier on the exterior sides before the cladding. The reason being, heat is excessive on the outside so a vapor barrier stops heat from entering into the home’s cooler interiors. 

Cladding Type

Cladding is application of materials over one another that acts as a skin of the building. It is the base layer of a building. The cladding is made up of absorptive materials like brick, wood siding, stucco, stone, lumber, fiber cement siding, etc. As per U.S. Census Bureau 2009 statistics, almost all new constructions are clad with absorptive materials.

Such absorptive materials hold moisture & water inside which gradually makes the interior damp. The retained moisture inside absorptive material can also release vapor, worsening the water damage. So, in a nutshell, cladding type can highly impact the vapor barrier position. You must see its position before putting a vapor barrier.

In a mixed humid weather like climate zones 4C (marine), 5, 6, 7 and 8, you can place a vapor barrier on the exterior of the cladding type. In a cold & dry climatic condition, you can skip the vapor barrier if it’s an absorptive cladding. On the other hand, if it’s a non-absorptive cladding, you may need to consider adding a vapor barrier.

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Wall Location

Where do you put a vapor barrier with insulation? On the wall’s interior or exterior? Well, it’s worth-asking! Decide it in relation to climatic conditions and home’s humidity levels. It is a ratio of outside to inside humidity levels. While deciding, consider the entire home’s humidity and not of separate rooms.  

Generally, it is installed on the exterior walls of houses as it’s where the most of outside humidity gets in contact with the inside. Doing so, it elevates comfort inside a house! Also, it reduces moisture & mildew issues arising due to water-trapping inside walls.

Also, it’s not needed to install a vapor barrier on every wall insulation! Put a vapor-barrier where it’s most-necessary! Say, in mixed-dry or cold climates, you do not always need to put vapor-barrier on every wall. 

Attention ! : Water vapor condenses as it meets temperature below dew point. Such condensation is a threat to a building’s shelf life. Always use a vapor barrier, where necessary!

In case of crawl spaces, you can put a vapor barrier insulation on exposed walls so it prevents moisture.


You can use many materials as a vapor barrier. Each different material has different permeability. It is determined as a perm rating as per the U.S. permeability system.

The lower the perm rating, the lower the ability to let vapor diffuse.

Materials having less than 1 U.S. perm rating, known as Impermeable:

  • Glass
  • Rubber membrane
  • Polyethylene sheet
  • Sheet metal
  • Vapor-retardant paint
  • Exterior-grade plywood
  • Foiled rigid insulation board

Materials having 1 to 10 U.S. perm rating, known as Semi-permeable:

  • Interior grade plywood
  • Unfoiled extruded or expanded polystyrene
  • Paper or foil faced batt insulation
  • Tar paper/30-pound Asphalt-coated paper
  • Bitumen-coated kraft paper
  • Oil-based or moisture-retardant latex painted gypsum board

Materials having 10 or above U.S. perm rating, known as Permeable:

  • Brick
  • Concrete block & slabs
  • House wrap
  • Board lumber
  • Unfaced fiberglass insulation
  • Drywall/Unpainted gympsum board
  • Cellulose insulation
  • Tar paper/15-pound Asphalt-coated paper

Attention ! : Never add a vapor barrier on both sides of a wall as it traps moisture if entered which then increase water damage

All these materials prevent vapor from reaching roofs, crawlspaces, ceilings, attics, building walls, etc. That otherwise leads to water dampness yielding growth of mould and rot. 

Attention ! : Seal all air gaps and ceiling cavities to get better protection from vapor effects.

How do I know if I need a vapor barrier?

How do I know if I need a vapor barrier

Whether you need a vapor barrier insulation or not depends on the above-mentioned factors – climate, cladding, wall location. Which kind of vapor barrier material you need is dependant on its permeability.

Always consider your climate, home location & cladding. Also, know the vapor-barrier material’s permeability. Once you anticipate the use of a vapor barrier against an insulated wall, you can fix its position accordingly. It then gradually yields greater benefits.

Say, interior walls of houses may not require a vapor barrier, however wall exteriors may require. The reason being humidity isn’t concentrated heavily at home’s interior walls but on exterior walls (only in case of hot-&-humid regions).

What happens if I don’t use vapor barrier?

What happens if I don't use vapor barrier

A vapor barrier stops outside cold & heat from seeping into insides. Without it, you welcome all unwanted heat, moisture, & cold inside your home!

You use a vapor barrier for ease of ventilation inside the house. Outside temperatures & weather are always severe. Also, building construction is made using bricks, stucco, wood sidings, and many other things. These things can retain water & moisture causing rot and mold to spread out. Such water damage can also affect rigidity of building constructions, affecting its longevity. To avoid this, you need a vapor barrier. 

Also, it eases comfort & energy-efficiency inside homespace. You are protected from severe sun’s rays and colder winters. In summers, you feel cooler because of the combined effect of vapor barrier & insulation. In winters, you can retain heat inside because of the combined effect of both. Moreover, it reduces your costs associated with wear-and-tear.

Consider these to ensure maximum energy-efficiency and durability.

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When should you not use a vapor barrier?

When should you not use a vapor barrier

Do you live in dry or cold regions? If yes, then probably you do not need a vapor barrier.

In this type of weather-conditions, there is light or no rainfall. So, there is no humidity in the air. The air-quality is rough and dry. This is why there is no vapor and no problem of moisture, mildew, mold, discoloration, and rot. As there is no vapor, there is no question of stopping vapor movement using a barrier!

So, avoid use of vapor barrier in mixed-dry or cold weather conditions.

What side do you put vapor barrier on?

What side do you put vapor barrier on

You put a vapor barrier either on the interior or exterior before cladding or insulation. It depends on weather-conditions, cladding type, and wall location.

In very-wet or very-cold climates, you can put it before cladding on interiors. This way, moisture does not pass through the cladding to the house’s interiors. There is a vapor-impermeable layer always protecting! This in turn also reduces condensation.

In hot & humid climates, you can put it before cladding on exteriors of outside walls. This way, outside heat & humidity does not make its way to home’s interiors! A vapor-barrier exists there to prevent humidity.

Should the vapor barrier be on the warm side or cold side of the wall?

The vapor barrier must be on the warmer side of the wall. The vapor diffusion always moves from warmer to colder point.

In conclusion, no matter from where the heat generates! Be it from home’s interiors due to laundry, cooking, etc or from home’s exteriors due to heavy humidity, always put a vapor barrier on warmer sides!

Can you put vapor barrier on both sides of the insulation?

No, you mustn’t put a vapor barrier on both sides of the insulation in any case!

Poor air sealings or window leaks can always cater to humidity leaks inside. If at any time moisture leaks into such insulation, it will trap between both barriers. This in turn will spread water damage. Also, making it hard to get it off easily. So, it’s always a bad idea!

Can you screw through Vapor barrier?

No, you cannot screw through the vapor barrier as it punctures it. Doing so, will decrease the vapor barrier’s efficiency. So, screwing holes into a vapor barrier is a bad idea of all means!

Still, if you’re gonna screw, make sure that you do not pierce the vapor barrier. Just penetrate the drywall, use a ‘not too long’ screw, and press the plastic anchor in.  

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So, yeah, you need a vapor barrier in almost all cases! Just consider your weather-conditions, cladding type, and wall location.

It’s not about installing any vapor barrier in what-so-ever way! You need one with optimal permeability and one that matches your weather-conditions, cladding type, wall location, etc. Read this short post to discover more!

After selecting a vapor-barrier, it must be installed properly. You need to skip air infiltrations to ensure efficiency!

After all, vapor barriers are a one-time investment in lifetime! It puts you at comfort for a lifetime and you achieve energy-efficiency too! So, prefer proper research before choosing a vapor barrier!

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