Village Science

Insulation & Vapor Barriers


A 2, 9
B 1
D 1, 3, 4



Forms of energy

A vapor barrier is usually a sheet of plastic, commonly called “Visquene,” through which air and water cannot pass.

A home that has a vapor barrier installed properly will be warm, dry, and will last a long time.

A modern Alaskan house that doesn’t have a vapor barrier, or has one installed improperly, will have major problems.


Wood will not rot if it is kept dry. The bacteria that destroy wood fibers need four conditions to grow:

  • Wood (their food)
  • Oxygen from the air
  • Water (moisture)
  • Heat

If any of these four is removed, the bacteria cannot grow. In the frame of a house, there is both wood and oxygen. As soon as there is moisture and enough heat, bacteria can grow, destroying the wood fibers. We must keep water out of our ceilings, walls, and floors whether it is water leaking from the roof or water from vapor.



Warm air can hold more water vapor than cool air.

When air is warmed, it removes water from other surfaces until it is saturated. This is why we hang our clothes outside in the summer or over the stove in the winter. Warm air removes the water from the clothes by evaporation.

Water that is suspended in air is called vapor. When warm air is cooled, it must release some of the water vapor it carries. The water condenses out of the air. Heat is released when vapor condenses.


Need for Vapor Barrier in the House

If the air in a house had no vapor, there would be constant static electricity, much of the woodwork would crack, people would have a hard time breathing, and would often get bloody noses. We see a degree of this when it is -40° or -50°F and there is very little vapor in the air.

People breathing, coffee pots brewing, and cooking pots all add moisture to the air. When we go outside in the winter we “see our breath.” In reality, we see the vapor in our breath condensing. We never see the carbon dioxide and other gasses. When we are inside a warm house, the same amount of vapor is in our breath but we don’t see it.

In winter, the presence of vapor in our home is proven by frost on the inside of windows and around cracks in the door.

Vapor in walls or ceiling

What happens when the vapor in the house enters the walls or ceiling?

As the air passes through the insulation, it cools. Some of the vapor condenses in the insulation. Wet insulation does not insulate as well as dry insulation. The rest of the vapor reaches the outside wall. Frost and water droplets form. Bacteria present immediately start working, breaking down the wood fibers. Rotting begins.

We see the same effect when we breathe through a scarf in very cold weather. Water condenses and freezes on the outside of the scarf.


Vapor Barrier

Warm air carrying large amounts of vapor must be kept from the insulation in our walls and ceilings. This is done by a vapor barrier, a large sheet of plastic that will not allow air to escape from the room into the walls. As the room is being constructed

  1. insulation is placed in the walls,
  2. vapor barrier is stapled over the insulation and studs, and
  3. paneling or drywall is nailed or screwed to the inside surface of the wall.

The vapor barrier keeps the warm air from escaping through cracks into the walls or ceiling.

Some builders don’t seal the electrical outlets. Warm air escapes through them into the walls and ceilings causing damage.

I framed a house. The owner later had major vapor problems in his attic. He blamed me for not cutting enough ventilation holes. However, the problem was caused by tremendous amounts of warm air escaping around the stovepipe into the attic. The solution was to seal around the stovepipe so the warm moist air couldn’t enter the attic space.


Other Locations in the United States

Interestingly enough, vapor barriers are not used in warmer areas of the United States. Warmer places don’t have the severe differences in temperature and vapor content in the air. In Alaska, the thin sheet of plastic vapor barrier, properly installed, can mean the difference between a house lasting a short fifteen to twenty years, or a healthy sixty to a hundred years.


Vapor Barrier and Insulation Work Together

  • A room with insulation but no vapor barrier will soon have vapor and frost in the walls and ceiling.
  • A room with a vapor barrier and little or no insulation will frost on all cold spots. A window is, in effect, a vapor barrier without insulation. The frost during cold temperatures is obvious.
  • A room that has a good vapor barrier and adequate insulation will keep the moisture necessary for good health in the room, will keep the moisture out of the walls, and will keep the room warm in all parts.


There are several types of insulation adequate for house construction.

  • Fiberglass
  • Foam
  • Sod

Fiberglass insulation actually works two ways:

  • It insulates from heat transfer.
    Glass does not conduct heat well at all. Put the end of a glass tube in a hot flame, and hold the other end of the tube with bare hands for a long time. If you do the same thing to a copper or steel rod, the heat will quickly be conducted up the rod to your hand.
  • Fiberglass traps air.
    The glass fibers trap air, preventing circulation. In an open space, air circulates (convection). Warm and cold air constantly mix. When air is trapped in small pockets and prevented from circulating, it is an excellent insulator. The thousands upon thousands of glass fibers woven together keep air from circulating, making a very cozy nest of dead air pockets that insulate the house.

The man-made fibers and down feathers in winter clothing operate on the same principle of creating dead air pockets. That is why wind is so chilling. It blows through the dead air spaces in the fibers, removing the insulating dead air.

Disadvantages of fiberglass

  • wetIt is uncomfortable to install.
  • It is destroyed in floods. The insulation on the bottom of a wall gets wet and very heavy. It doesn’t dry well at all. With the bottom of the insulation wet and heavy, it sags under the weight, pulling the insulation from the top of the wall.
  • Exposed to the outside, small animals constantly carry fiberglass away, making nests with it.


There are many kinds of foam insulation. Some are waterproof, others are not. Some are damaged by sunlight, others are not. Foam has some wonderful insulating qualities for the same reasons fiberglass does. It also adds structural strength to a building that fiberglass does not. Most kinds of foam are unaffected by flooding, frost, and vapor. Foam insulation is excellent for insulating pipes below ground level. It stays completely dry for decades.


Foam insulation in a house is expensive. It is also deadly when it burns, giving off poisonous gasses.


Thermal Conductivity of Different Materials



sodOld time Alaskans insulated their roofs in a different way than we do today. The ridge pole was covered by poles, or split spruce trees. The poles were covered with birchbark and/or grass which shed water.

Sod was placed over the grass, and over the sod, dirt. This type of roof had no vapor barrier, but it “breathed”. The vapor passed through the grass, sod and dirt, and into the air. Frost problems were unheard of. Grass grew on these roofs. They were very warm and felt quite cozy.

Unfortunately, sod roofs could not have a steep pitch as the dirt would wash away. Sod roofs were so heavy they needed a very large ridgepole in the middle. The weight of the roof frequently buckled the gable ends, and repairs were often necessary. The wood in sod roofs did rot after a period of time and required replacing.

“R” Rating

The R rating of insulation tells how well it does its job. Two inches of foam is usually R10. Six inches of fiberglass is R19. R32 is very good insulation.



Today’s steel roofs are relatively light. They easily shed snow and are completely waterproof. Ceilings are insulated with fiberglass.

Some houses have icicles hanging from the eves and others do not. There is a reason for this.

If the roofing material is warmed by the building heat, the snow in contact with that roofing melts. The overlying layer of snow insulates it. The water runs down the roof toward the edge. However, when it gets to the cold uninsulated overhang, it freezes, causing the water to back up under the snow layer.


This goes on for some time until there are large icicles hanging from the edge of the roof and water is seeping into the ceiling of the house. I built a roof like this once, and walked in overflow on my roof at -30°.

Although this is very damaging, people continue to build roofs in Alaska in this manner, particularly in Anchorage.

The solution is to make sure there is cold air circulating between the roofing material and the insulation. That way no heat can escape to melt the snow.



  1. While in a warm house, close the cover on a jar. Bring the jar outside or put it in a freezer. Is there condensation inside the jar when it is cooled? Bring the jar into the warm house again. What happens?
  2. Breathe on a plate or piece of metal that has been cooled outside in sub zero temperatures. What happens. Bring it inside and watch what happens. Where does the frost go?
  3. The next time it gets –40° or –50°F, scuff your feet on a rug and touch a doorknob. Is there a spark? Why do you think this doesn’t happen when it is warm?
  4. The next time it gets very cold, put a blanket against the bottom of a cold window and leave it overnight. What happens? Why?
  5. During cold weather, observe windows that are single, double, and triple pane. What difference do you see?
  6. How are winter shoe packs with felt liners like a wall without a vapor barrier? What happens in very cold weather when you try to take the liners out of the boots after wearing them all day? Why does this happen? Can you think of a way of preventing this?
  7. Compare shoe packs with felt liners to the white “bunny” boots or VB (vapor barrier) boots as they are called. What are the similarities and differences?
  8. Check the houses in the village. Ask what kind of insulation is in the walls and ceiling. Is there a vapor barrier?
  9. Check the roof of an old abandoned cabin in your area. What kind of insulation was in the walls and ceiling?
  10. Try to find an old abandoned log cabin with a sod roof. Study the roofing materials.
  11. Test wet and dry insulation (wet & dry socks?) for their conductivity of heat.
  12. Ask the oldtimers how they could detect a bear hole during winter months. Does one of these signs relate to condensation?
  13. Put a glass tube or other piece of glass in a hot flame. Does it conduct heat well? Compare this with a metal coat hanger or other piece of long metal. Compare these with wood.
  14. Visit a house under construction or talk to local carpenters. Do you see the vapor barrier? What do the carpenters say about vapor barriers?
  15. Ask oldtimers about sod roofs. Were they warm? Did they leak?
  16. Submerge a piece of closed cell foam (usually blue or pink) after weighing it. Leave it under water for a few days. Weigh it again. Did it absorb any water? What is the R factor of two inches of foam?
Student Response

Student Response

  1. What four things do the bacteria require that cause wood to rot?
  2. What is vapor?
  3. What would happen if there were no vapor in the air of our homes?
  4. Which can hold more water vapor: warm or cold air?
  5. What happens to the vapor in warm air when the air is cooled?
  6. What happens when vapor gets into the walls of our homes?
  7. Draw a cross section of a wall that has insulation and a vapor barrier.
  8. What things in our homes naturally put water vapor into the air?
  9. If you tried to explain the use of a vapor barrier simply to someone who didn’t know, what simple rules would you give them?
  10. What two things make fiberglass good insulation?
  11. What are the three disadvantages of fiberglass insulation?
  12. What are two disadvantages of foam insulation?
  13. Draw a cross section of a sod roof. Did the oldtimers use a vapor barrier?


  1. A roll of Visquene is 8 x 100 feet. Assuming there are no overlaps (in reality there are). How many rolls of Visquene are necessary to put a vapor barrier in a house 24’ x 36’, with walls 8’ high. The outside walls and ceiling need a vapor barrier. How many square feet will be left over for overlap and other purposes?
  2. The above house needs _______________square feet of Visquene. It comes in rolls of 12’ x 100 for $47.21 or rolls of 8’ x 100’ for $29.52. What is the best combination of rolls that can be purchased and what is the total cost?
  3. Two inches of foam has an R factor of 10. Six inches of fiberglass has an R factor of 19 (round off to 20.) A piece of foam is 2’ x 8’ and costs $14. Fiberglass costs $37 for a bundle that contains 78 square feet. Which is the better insulation buy for a square foot?
  4. A building has 1276 square feet to insulate with six inches of fiberglass. The price of fiberglass landed on the jobsite is $.47 per square. What is the cost of insulating the house?
  5. Hank was building a 40’ x 56’ shop. He wanted to pour the concrete floor over 4” of foam. Each piece of foam is 2” x 2’ x 8’. How much would this cost if he could get the foam for $10.99 each? (Round off if you like.)

Questions or comments?
© Alaska Native Knowledge Network