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Doors Passive Home Windows

Aluminum vs. Fiberglass vs. RAU-FIPRO

January 26, 2021


An aluminum window profile comes with many benefits:

  • is a tremendously versatile structural material
  • it is very rigid, with a modulus of elasticity (E) of approximately 60 GPa, although not as rigid as steel (E~200 GPa)
  • it is also strong
  • it only has approximately one third the density of steel – with the proper design it can yield a lighter, stronger structure than steel can.
  • most alloys have very good corrosion resistance.

All of these properties make it an ideal building material for windows and doors – for warm to moderate climate. Aluminum conducts heat extremely well so here’s the disadvantage:

  • they suffer in cold climates
  • they offer poor energy efficiency
  • they have low resistance to condensation

Nonetheless, it is still commonly used in cold climates. We all know what it is like sitting next to an aluminum window on a cold day in a restaurant or commercial building. You can feel the heat being sucked from your body. However, structure comes first, and until recently, there have been few alternatives.


In the 20th Century there was a revolution in composite materials, typically some sort of fibers in a plastic matrix, the most familiar being fiberglass, glass fibers in a plastic matrix. Composite materials commonly began to complement or even replace aluminum in the aerospace industry. The most common matrix is a thermoset plastic. A thermoset plastic, once cured, cannot be melted for reshaping or joining.

Companies have been using fiberglass for window profiles for decades. Here’s some of the benefits of Fiberglass:

  • it has excellent structural properties
  • it has low thermal conductivity

Conventional fiberglass profiles have become a popular alternative to aluminum in very cold climates though time has shown a few drawbacks to these systems: 

  • First, because they use glass fibers in a thermoset matrix, the only way to fasten the corners is mechanically or through the use of adhesives. These corners can fail over time, allowing water to infiltrate the building structure. 
  • Another problem is that conventional fiberglass does not hold screws well, as the screws tend to cut the glass fibers. In aerospace applications any mechanically fastened joints are specially engineered in a way not practical for window operators or hinges.


In the mid-2000’s REHAU revolutionized the window profile market with a new glass fiber composite material, RAU-FIPRO. This is a fiberglass, but a fiberglass in a thermoplastic matrix. As the matrix is thermoplastic, shapes can be extruded rather than pultruded, allowing more complicated shapes to be made. This means more chambers for better insulation and corners may be thermally welded, all but eliminating the possibility of failures in the corners and resultant water infiltration. It also means that any areas where hardware is fastened can be specially engineered for the profile itself to carry hardware loads instead of screws. Moreover, special screws were developed by our screw supplier in Germany for this material that will not loosen screw holes over time.

Access Windows and Doors made with RAU-FIPRO offer the advantages of fiberglass without the disadvantages. With our RAU-FIPRO glass fiber composite extrusions along with our reinforcement couplings, we can span window and door openings that used to be reserved for aluminum. We engineer all mullions so that the reinforcement is carrying the design wind loads – not the glass. This prevents premature glass failure. We offer vastly superior thermal performance than aluminum. With a thermal conductivity of one tenth of one percent of aluminum, the RAU-FIPRO in our 6 chamber, 86mm profiles outperforms even thermally broken aluminum.