The Science Behind Our Energy Saving Spa Covers

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Heat Energy Transfer

Heat Energy Transfer


is direct heat flow through matter (molecular motion). It results from contact of a warm area of an object with a cooler area or with another object. The greatest flow of heat possible between materials is where there is direct conduction between solids. Heat is always conducted from warm to cold and always moves via the shortest and easiest route. In general the denser a substance the better conductor it is. Because air has low density the percentage of heat transferred by conduction through air is comparatively small. A less dense mass has less flow of heat by conduction thereby resulting in a better insulating material.


is the transport of heat within a gas or liquid, caused by flow of the material itself (mass motion). Natural convection heat flow is largely upward and somewhat sideways. This is called “free convection”. A warm stove loses heat by conduction to the cooler air in contact with it. This added heat activates (warms) the molecules of the air which expand, becoming less dense, and rise. Cooler, heavier air rushes in from the side and below to replace it. Convection when mechanically induced by a fan is called, “forced convection”.


is the transmission of electromagnetic rays (infrared) through space. Infrared rays occur between light and radar waves. Radiation refers only to infrared rays. All objects radiate infrared rays from their surfaces in all directions in a straight line until they are reflected or absorbed by another object. Traveling at the speed of light these rays are invisible and they have no temperature only energy. Heating an object excites the surface molecules causing them to give off infrared radiation. When these infrared rays strike the surface of another object the rays are absorbed and heat is produced in the object. This heat spreads throughout the mass by conduction. The heated object then transmits infrared rays from exposed surfaces by radiation as they are exposed directly to an air space. Radiation, which accounts for 50% to 80% of all heat transfer, will pass through air with ease just as radiation travels the many million miles that separate the earth from the sun.

Water vapor

is the gas phase of water and as a gas it will expand or contract to fill any space it may be in. In a given space with air at a given temperature, there is a limited amount of vapor that can be suspended and any excess will turn into water. The point just before condensation commences is called 100% saturation. Vapor laws: (1) The warmer the temperature the more vapor the air can hold.(2) A larger space holds more vapor. (3) The more vapor in a given space, the greater its density. (4) Greater vapor density flows to areas of lower vapor density. (5) Permeability of insulation is a prerequisite for vapor transmission, the less permeable means less vapor transfer.

R factor

or resistance to heat flow is a measure of insulation’s ability to slow conductive heat flow however doesn’t consider radiation or convection loss. EPS foam becomes a conductor of heat and loses its insulating efficiency when the relative humidity increases because of moisture absorption. Insulations with mass normally contain at least the average amount of moisture in the air and must be completely dried out before achieving lab rated R values. The R value of EPS foam spa covers is reduced by over 35% with typical ambient moisture content (from R12 rated to R8 effectual). As EPS spa covers continue to gain water weight from radiant energy they increase energy costs exponentially.


In both winter and summer, 65% to 80% of heat is lost through radiation. Aluminum has low thermal emissivity (0.05%) and when coupled with the low thermal conductivity of air it is possible to practically eliminate heat transfer by radiation and convection. Aluminum insulation can reflect 95% of all radiant energy and is impervious to water vapor and convection currents. The performance of most aluminum insulation is unsurpassed for winter heat and adds efficiency for summer by minimizing convection currents. Aluminum inhibits infrared rays from penetrating its surface while reflecting energy back. There is no such thing as, “dead air space”, as far as heat transfer is concerned even with an air-tight compartment such as a Thermos bottle. Convection currents are inevitable when differences in surface temperatures exist and since air has some density, there will be some heat transfer by conduction when space is heated.

Expanded Polystyrene Foam (EPS) and the Spa Cover

Expanded Polystyrene Foam (EPS)

Expanded Polystyrene Foam (EPS) and the Vinyl Clad Portable Spa Cover

Moldy EPS Foam CoversDiscarded expanded polystyrene does not biodegrade for many hundreds of years and is resistant to photolysis. Because of this stability, very little of the EPS discarded in today’s modern, highly engineered landfill biodegrades. Because degradation of EPS creates potentially harmful liquid and gaseous by-products, American landfills are directed to minimize contact with the air and water required for degradation. By impeding the natural degradation process, EPS waste can stay buried in our landfills for up to 2000 years.

Polystyrene foam is a major component of debris in the ocean, where it becomes toxic to marine life. Foamed polystyrene blows in the wind and floats on water, and is abundant in the outdoor environment. Polystyrene foams are produced using blowing agents that form bubbles that expand the foam.

Although polystyrene can be recycled at recycling facilities most polystyrene is not. The EPA (United States Environmental Protection Agency) estimates that 25 billion polystyrene cups are tossed every year. Since polystyrene degrades very slowly – more than 500 years for a single cup – the EPA considers this a serious environmental problem. Several green leaders, from the Dutch Ministry of the Environment to Starbucks’ Green Team, advise individuals reduce their environmental impact with reusable coffee cups.

Vinyl clad, expanded polystyrene spa covers have been around for about 30 years with few questions regarding claims made of their R value. A single sheet 3” thick of 1.5 lb foam could have an R value of 11.5 in a dry test chamber. In ambient weather with moisture a common factor, that R value could be reduced by 35% to 7.4 ”. If that sheet was tapered it would be as energy efficient as the thinnest portion as energy follows the path of least resistance. A 4” to 2” tapered spa cover would have an R value of considerably less than most claim due to its conductive vinyl, gaping hinge and the 2” thick tapered edges. Further reducing insulating benefits is the conductive nature of condensation or water accumulating in the foam core from radiant energy. This natural occurance eventually leads to an unwieldy weight and an early demise of the components that comprise the cover. Mold and mildew from water permeated spa covers can promote foul odors and health problems. The typical EPS foam spa cover (7’x 7’) represents 3520 styrofoam cups. When multiplied by the number of replacement spa covers in the U.S. annually that amounts to over 6 billion cups headed to landfills with 7 billion additional being created for existing and new spas, all destined for land fill sites.



NOTE: If hot tub cover is waterlogged or smelly it should be replaced. A waterlogged cover will likely be infested with mold, mildew and bacteria and continuously inoculate the spa water with more micro-organisms.


Moldy EPS Foam CoversBiofilm is a complex association of micro-organisms which adhere to hot tub surfaces, particularly inside the plumbing, where they are difficult to remove. Biofilm may contain bacteria, viruses, mold and parasites as well as dirt and polysaccharides to improve adhesion to surfaces. It grows within the plumbing systems and jets, forming a slippery film. Micro-organisms inside the biofilm layer are somewhat protected from sanitizing agents. They present a danger to the bather if exposed before sanitizers can kill them. It is important to remove biofilm and prevent it from reforming as it can, under certain circumstances, cause disease.

There are several types of micro-organisms, which can grow in spa water: bacteria, fungi, protozoan, and viruses; some of these are pathogens. These organisms can come from a variety of sources including other humans and animals, from the ground, water or even airborne sources. A poorly maintained spa can form biofilm in the plumbing system and on surfaces to allow organisms to multiply.

Culprits include various forms of Pseudomonas, which can affect ears, eyes, skin (rashes) and the respiratory tract. Another is Mycobacterium avium, which can cause flu-like symptoms.

Interestingly, researchers have found that many people who complain of “allergic reactions” to sanitizing chemicals are actually suffering from skin rashes caused by bacteria due to inadequate sanitizer levels!

Hot tub folliculitis is a condition caused by Pseudomonas bacteria, and is often seen where spa sanitation is absent. The most common symptom is an itchy rash or small reddish bumps, sometimes confused with bug bites. It can develop into more serious problems. This condition usually clears without scarring. It may recur if the infected hot tub is not properly cleaned and disinfected.

Portable Spa Confirmed:

Portable Spa Confirmed

“Most energy consuming appliance in America”
Spa may be getting bad rap!

(Major heat loss detected at foam spa cover, “dual hinge”)

A typical bi-fold EPS foam spa cover has a dual vinyl hinge connecting the two halves to provide a lift bar support feature for the cover during spa use. This 1.25″ to 2″ wide hinge allows heat to escape continually as can be witnessed by observing where the neighbors’ cat sleeps on chilly nights!

A steam blocking vinyl covered pad is sewn at each end of the hinge to obviate concern. These, “Wedgies”, keep the two foam cores at a measured distance as they decompose from water sanitizing efforts. Cover lifting bars strain the cover stitching and hinge continually degrading and breaking it down. Black metal lift bars can exceed 175 degrees in the sun to help speed this along.

Outdoor testing at sunset on the dual hinged foam cover provides evidence of accelerated heat loss due to evaporation at that hinge (the, “Chimney Effect”).

A sixty degree evening temperature in October does not reveal the thirty two degree cover surface due to evaporation!

Spas are required to have covers and few covers are made without the dual hinge. EPS foam spa cover quality also declines as manufacturers meet lower competitive pricing. Dual hinged, foam spa covers have helped to degrade the portable spa to the least efficient appliance in America while increasing energy costs and promoting premature land fill.

2 Year Old Vinyl Spa Cover

2 Year Old Vinyl Spa Cover

Energy Efficiency Testing

Energy Efficiency Testing

As dual hinged foam spa covers get water heavy their insulating ability drops considerably. Tests reveal spas maintaining 100 degree standby temp with a dual hinged foam cover older than 18 months can use as much as 14.5 kWh of power daily. (1 -kWh @ $0.14 x 14.5 = $2.03 per day)

The APSP energy efficiency testing at Cal Poly in 2008 concluded that a Coverplay spa convincingly outperformed all other North American manufacturers of acrylic spas with a 3.57 kWh rating. No other acrylic spa entered was even close to that rating, in fact 2 out of 3 spas tested were so inefficient as to be non-compliant for sale in California. Spas that met compliance were minimally so and cover test results now suggest they would be out of compliance shortly after their sale because of that dual hinged EPS foam cover.

North America uses dual hinged covers and these number in the millions. Using the Cal Poly test results, the Airframe spa cover design could save America millions of energy dollars every month while helping to remove the portable spa from the dubious distinction of, “Most energy consuming appliance in America”.

The California energy company (PG&E) determined from a 10 year study the portable spa to be the most energy consuming appliance in America. Through Title 20 legislation the California Energy Commission now restricts sales of portable spas in California. U.S. Congress bill HR 2454 supports that Title 20 energy bill for enforcement on a national level.

As EPS foam covers become water permeated their insulating capability is compromised. Water, a conductor, makes covers heavy and increasingly more costly to heat the spa. This costs consumers up to 75% more in stand by power.

Until now consumers replaced their old water saturated cover with another EPS foam cover. Water saturated moldy foam covers end up in America’s land fills at an alarming rate of 145,000 per month. Many US cities and countries like China have banned EPS /Styrofoam from being sold there. California and Oregon have pending legislation to be among the first in the nation to ban EPS state wide. An average spa cover represents about 3520 Styrofoam cups or 510.4 million cups a month to our land fill sites in America alone. It can take up to 20 centuries for EPS to break down in today’s land fills as foam board must be protected from the water supply.

TheAirframe spa cover is a rigid aluminum frame with a banded core that insulates with layered chambers of air amid low emissive radiant barriers.

TheAirframe does not retain water, lasts for more than 10 years and can be repaired, recovered and recycled.

The multi air chambered frame is covered in fitted, water repelling, gas permeable fabric for durability and longevity not petroleum based vinyl. Eliminating EPS foam from use in spa covers is a, “Green”, idea that could avoid replacing the equivalent of 7 billion Styrofoam cups in the USA annually and costing spa owners unnecessary millions to heat spas with water saturated covers on them.


The Airframe Spa Cover

TheAirframe Spa Cover

TheAirframe spa cover doesn’t gain water weight and is the first spa cover to offer a 10 year warranty. This sustainable design was awarded a rebate from the Oregon Energy Trust, uses no EPS foam and relies on the science of chambered air technology. TheAirframe has the potential to be one of the most innovative energy saving sustainable concepts ever for portable spas.

(U.S. and other patents pending)

Thank you for your interest in TheAirframe spa cover as this guide is intended to help you understand and care for this sustainable, patents pending cover. Better choices will help improve our planet by reducing EPS foam* from being buried in our land fills…future generations depend on our principled behavior now.

TheAirframe spa cover is comprised of poly banded aluminum frames with layered chambers of air that insulate amid radiant barriers and covered with attractive outdoor water repellent fabric. This sustainable design is recyclable and repairable with no EPS foam and built to withstand the elements without becoming water saturated.

The pliable surfaces meet ASTM regulations for safety and will not break like EPS foam board covers. Cover tie downs are durable cable restraints.

Airframe fabric is gas permeable, (breathable) adjusts to climate changes and selected for that purpose. As it, “breathes” it is less inclined to allow mold and mildew as vinyl spa covers are noted for.

Radiant energy barriers surround the air chambers to limit emissivity and condensation. As layered air chambers have minimal conductivity (no mass) they insulate better than new EPS foam.

The patented Angle Integrated Removal System (AIRS cover assist) allows the patented angle of compression hinge a better energy efficient seal while allowing easy access to the spa and safer lower storage of the cover. An optional CBS (counter balance system) is available to further assist cover removal if desired.

The Airframe is an energy winning design using scientific principles to insulate and patented elements to create a superior light weight spa cover made in the USA. Airframe has no threads or zippers to fail and no EPS foam to gain water weight. The Energy Trust of Oregon offers a $100 cash rebate for Airframe’s energy efficiency.

…Welcome to the 21st century.

The Airframe Stabilizer Strap

The Airframe Stabilizer Strap

TheAirframe Spa Cover Rear Stabilizer Strap for portable spas. US Patent No. 7496975

Our elongated stabilizer strap is a patented design developed to improve convenience, transition and insulating value of a portable spa cover when implementing a cover removal system. During spa cover removal the stabilizer strap limits cover side-slip optimizing effort to minimize torque. When closing, the strap helps return the cover to its original efficient position and replaces the need for cheap plastic clips on a cover. When closed the elongated strap provides protection against unauthorized access.

TheAirframe tie down assembly consists of a post anchored to the frame with a chrome steel link through it. When attaching the tie down cable pass the looped end through the chrome link and then pass the swivel post bracket assembly through that loop and pull tautly until the ferrule sheath on the cable passes slightly through that loop. (See photo)

With the cover centered on the spa extend the tie down cable vertically to the cabinet without slack where the swivel post bracket assembly can be installed with 4 pan head screws provided. (A too taught cable will impede removal)

A little silicone spray on rear edge of spa acrylic will ease cover removal.

Titan tie-down

US Patent No. 7496975