Building Science ‐ Something to Sleep On

The control of moisture in a building is critical for mold control and ensuring the longevity of a structure.  Applying the principle that makes for a good wall design could help you sleep more comfortably.  The same physics that govern moisture flow in a wall also determine the flow of heat and moisture in a mattress.  In spite of the obvious similarities, I have not seen common engineering principles correctly applied to control moisture, mites and mold in the bedroom.

Relative and Absolute     Humidity

Both a wall and a mattress contain a thick layer of porous insulation. During the winter a wall is exposed to warm moist air on the inside and dryer cooler air outside. Similar conditions exist across a mattress.  There is a warm moist side under the covers and a cooler dry side underneath the mattress.

Under cold wet conditions a cubic foot of outside air may contain less moisture than a cubic foot of inside air. The reason for what may seem counter-intuitive is that warm air has the capability of holding much more water vapor than cold air. Water vapor is water in gaseous form as opposed to a liquid. The relative humidity (RH) is the percent of water vapor in the air compared to the maximum amount of water vapor that the air can hold at that temperature. If the temperature inside is 70ºF and the relative humidity is 50%, under cold wet outside conditions (40ºF, 90% RH), the air inside will actually contain three times more water vapor than the air outside.

The actual amount of water vapor in a cubic foot of air is called the absolute humidity. During the winter the warm air inside can hold more water vapor and have a higher absolute humidity than the cold air outside.  The air inside has moisture added to it as a result of cooking, plants, showers, and human respiration increasing the absolute humidity.  If there is a difference in absolute humidity across a porous piece of insulation, water vapor will migrate away from the side of higher absolute humidity.  Moisture then migrates into a wall during the winter and into a mattress all year.

If a volume of air is cooled, its ability to hold moisture is reduced. When the relative humidity reaches 100% moisture in the air will condense. The temperature at which this happens is called the dew point.

The movement of water vapor into a wall can be retarded, by putting a vapor barrier on the warm side of the wall.  These generally consist of an impermeable sheet of plastic. Sheetrock painted with an appropriate paint can also retard the movement of water vapor into the wall.

The outside surface of the wall should be breathable so that moisture leaves the wall faster than it enters.  If the porosity of the outside wall is too low and it is unable to breathe, the relative humidity could reach the dew point and water vapor will begin to condense on the outer wall.

Mattress Moisture Theory

A person can lose about 8 oz. of moisture each night.  Combined with body heat, this forms a layer of warm humid air under your blankets.  Cool, drier air is under the mattress.  These are the same conditions found across an insulated wall during the winter.

Moldy Matreess On A Cold Floor – Drawn by Anna Ludd

The same principle used to control moisture in a wall can be used to control moisture in a mattress.  The strategy is to reduce the flow of moisture into the mattress and then provide for ventilation under the mattress so that any moisture traveling through the mattress can exit through its lower surface.  This will decrease the moisture content of the mattress.

If the mattress is sitting on a cool impervious surface, the relative humidity could increase to 100% (the dew point).  Moisture will then begin to condense on the surface below the mattress.  The accompanying illustration shows the result of putting a mattress on a cool floor that is not insulated:  a damp molding floor.

The moisture entering the mattress can be almost completely stopped by putting a vapor barrier on the top of the mattress.  For comfort, the vapor barrier could be a flexible piece of plastic or coated nylon placed under the mattress cover.  An effective vapor barrier can eliminate the need for air circulation under a mattress.

A vapor barrier can also aid in the performance of clothing and sleeping bags used in extremely cold weather.  Sleeping bags used by arctic explorers have turned into solid lumps of ice as moisture condensed and froze the bags outer skin.  The layer of ice would become thicker each day.  Using a vapor barrier inside the bag would prevent this problem.

Reducing the Biological Activity in Your Mattress

Dust mites and mold need food and moisture for their survival.  The skin particles we shed are a major nutriment source for dust mites and mold.  Most mattress covers available have a very tight weave that keep dust mites below and skin particles above.  The breathability of these mattress covers is promoted as a positive feature.  However, because the covers breathe, moisture in the form of water vapor is still transferred into the mattress.  Both dust mites and mold can extract moisture from the air when the relative humidity is relatively high, above 80%.  More moisture becomes available to these critters as the relative humidity approaches 100%.  A vapor barrier would eliminate both the flow of moisture and nutrients migrating into the mattress.

If the mattress is placed on a solid non-breathable surface, the use of a vapor barrier also eliminates the possibility of condensation on the mattress’s lower surface.  At night the relative humidity near the lower surface of a mattress, even if well ventilated, can reach 80% or more providing the moisture needed for a healthy population of dust mites.  My mattress is sitting on a sheet of plywood over a bottom-mounted set of drawers.  The room stays fairly cool all year.  Without the use of a vapor barrier in the mattress cover, the plywood would be wet and probably moldy.  With the vapor barrier it’s perfectly dry.  The measured moisture content of the plywood is the same as the moisture level of the other wood structures in the room.

The situation is similar for a pillow.  Warm moist air is breathed out and then migrates into the pillow where dust mites and mold can grow.  A vapor barrier on the top of the pillow would alleviate this situation.  The bottom of the pillow should remain breathable.

The vapor barrier I used was a very thin sheet of polyethylene plastic.  Polyethylene is a fairly inert plastic.  We are also investigating the possibility of using other materials as a vapor barrier.  The vapor barrier was placed under a quilted mattress cover so its presence is not noticeable.  The mattress cover and sheets are both washed for dander control.

A co-worker has a similar sleeping arrangement, but they have no vapor barrier.  Their setup consists of a foam mattress sitting on a plywood deck.  Because of the moisture collecting on the plywood, every few months both the mattress and plywood must be dried out.

It is somewhat surprising that these easily applied engineering principles are not more readily applied to the household environment.  With or without the use of a vapor barrier it is a good strategy to let your bedding dry out during the day.  It may be good news that making your bed may not be the best strategy.