Roof Insulation Systems in St. Louis

To extend the life of your low slope roof, it is extremely important to have adequate flat or tapered insulation installed.

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Low Slope Roof Insulation Systems

Roof insulation plays an essential role in your commercial building’s ability to maintain internal temperatures and prevent air from escaping. In addition to reducing extreme temperature fluctuations and improving energy efficiency, insulation can prevent sheathing and roof deck materials from rotting.

Flat Roof Versus Tapered Roof Insulation

A roof that is nearly flat or has a low slope is called a flat roof system; however, no roof should be completely flat — it must have a slight slope to allow water to drain. Roof membrane manufacturers require positive drainage for their system warranties to be honored. If the nearly-flat roof has inadequate drainage, tapered insulation can be installed to create a slope for the water to drain more easily. Flat roof insulation systems and tapered insulation systems use similar materials but have different approaches to installation.

Flat Roof Insulation

A flat roof is composed of several layers including a thermal layer, designed to trap air, which acts as the insulation layer of the roof. Flat roofs are common for commercial buildings because they are less expensive to install and easy to maintain.

There are two main types of flat roofing that require different approaches to installation:

  • Warm Flat Roofing – Insulation material is placed on top of the roof, rather than in-between the ceiling and the decking. The positioning of the insulation on top of the vapor control layer prevents condensation from becoming an issue. The waterproofing system is then applied over the top of the insulation.
  • Cold Flat Roofing – Insulation is placed in-between the joists (horizontal load-bearing planks) underneath the roof surface. The insulation material is placed between the supporting joists, leaving a gap between the roof and insulation to enable the airflow to circulate freely.

Insulating a flat roof requires ample ventilation to allow moisture accumulated inside to escape and to let hot air escape in the summer. If moisture has nowhere to go, it will gather and eventually undermine the insulation. Flat roofs do not have soffits (holes on the underside to allow air flow), so ventilation will have to come from side vents or from roof vents. Roof vents on flat roofs need to be sufficiently sealed and positioned off the roof so water cannot seep in through them.

Tapered Roof Insulation

A tapered roof system is where the slope of the roof is formed by the insulation itself. It comes in several different thicknesses but with a slope that allows precipitation to flow into the drainage system rather than pooling on what would be an otherwise flat roof surface and potentially leaking through the upper membrane. As more designers and roofing professionals understand the importance of positive drainage in good roofing practice for both new construction and re-roofing, the popularity of tapered insulation has increased dramatically.

Four different slope patterns are commonly used for commercial tapered roofing systems:

  1. The shed roof design
  2. The two-way slope
  3. The three-way slope
  4. The four-way slope

The pattern chosen for any commercial roof depends on a number of considerations, including:

  • Total square footage
  • Drainage systems already in place
  • Present fixtures such as skylights, HVAC systems, etc.

Roof Insulation Materials

There are several kinds of materials that work well when insulating a roof. All insulation has an R-value assigned to it, which is the amount of thermal resistance that the insulation has, and it is determined by the thickness of the insulation. The higher the R-value, the more effective the insulation. The material that works best depends largely upon preference in a given situation.

Polyisocyanurate (Polyiso)

This comes in a range of thicknesses, for example, a 1.1 inch of Polyiso has an R-value of 6.2 and a 3.5 inch of Polyiso has an R-value of 20.5. It is used under roofing membranes and placed between non-organic and organic felt. Pentane is then used to expand the foam. Flat panels are typically installed on low slope roofs that slant towards a guttering system. Tapered panels are angled to direct water to a drain.

Cellular Glass

This type of insulation has an R-value of 4.69 per inch, making it a good choice. It consists of glass that has been crushed and then heated while inside a mold. The glass composition makes it moisture, high heat, and fire-resistant.


This insulation will either be in expanded or extruded types. Expanded polystyrene flat roof insulation is combined with a foaming agent and typically has an R-value of 3.6 to 4.0 per inch of thickness. This insulation expands when it is exposed to heat. Extruded polystyrene flat roof insulation is created in several steps and has an R-value of 5.0. The polystyrene is heated and then exposed to the atmosphere, and then expanded.


The R-value of perlite insulation is measured at around 2.78 per inch. It is a kind of glass made from volcanic ashes. The perlite is combined with other organic fibers and binders, and pressed to form a board. Air bubbles are trapped inside the perlite and create the insulation for the flat roof. The perlite board is painted with an asphalt coating which limits moisture absorption.

Wood fiber

With the same R-value as perlite, it is just as effective and inexpensive. This is made out of purely organic wood fibers and it is formed into wood boards by mixing the fibers with various fillers and binding materials. The water resistance can be increased by using asphalt inside the fibers or painting a layer on the boards themselves. Wood fiber may not be the best choice for most situations, but it is an inexpensive, organic alternative to flat roof insulation.

Water on Low Slope Roofs

Low slope roofs, in particular, need insulation to prevent water from ponding and snow from sitting on the roof for long periods of time. Ponding water on roofs is a large threat and can result in more destructive damage than wind or other natural elements.

Roof damage from pooling water may include:

  • Contributing to the premature deterioration of the roof membrane, flashings, and coatings
  • Structural deflection (bending) of roof deck
  • Leaking and ruining the insulation
  • Collecting dirt that can cause the growth of bacteria or unwanted vegetation
  • Increasing the damaging ultraviolet exposure due to the water acting as a magnifying glass under the pond
  • Challenging the adhesives used, thus compromising the integrity of the roof system

Contact the St. Louis Roofing Insulation Experts

Our professional roofers will determine the best option for your low slope roof and provide reliable service that won’t break the bank. Contact South Side Roofing for a consultation today!