Roof R-value

[Rachelle Kun]

Bychoosing the right insulation and roofing for your building, you could save hundreds of dollars by cutting your energy bills. You can also create a proper roof system to last decades and protect your building inside and out.

At West Roofing Systems, we specialize in commercial roofing that insulates your building and saves you money through energy costs and building protection. We work with building owners and managers to calculate the R-value they want to achieve for their buildings and helping them reach their goals.

R-value is calculated by more than just the insulation. To calculate the R-value of a roof system, your contractor will take into account the underlayment, coverboards, adhesives, insulation and even the membrane.

The bright white color reflects UV rays, saving buildings up to 35% in energy costs from not needing as much air conditioning to keep the facilities cool. Utilizing a bright white coating application reduces energy costs as part of the energy star cool roof solution.

West Roofing Systems, Inc. installs sustainable and efficient roofing systems for commercial, industrial, manufacturing and municipal buildings. In over 40 years, we have installed over 60,000,000 square feet of commercial roofing, and our expert team is ready to help you.

One of the most important factors in selecting the optimum flat or low-slope commercial roof insulation is designing to the desired thermal efficiency. R-value is a measurement to quantify the insulating properties of a building material. The highest R-value per inch is the best, and this helps determine flat roof insulation thickness for building regulations. Understanding relative R-values will help rank the comparison of various materials choices.

Phenolic foam had high reported initial R-values of up to 7.5 per inch, but this product has not been widely produced in the U.S. since the early 90s because of litigation concerning the corrosiveness and leaching acidic impact on metal decks or metal fasteners under certain conditions. Some roof assemblies, especially lightweight concrete decks, may have this insulation still installed.

Currently, a European manufacturer is marketing a reformulation for restricted building materials usage in the USA and Canada. Specifiers can decide if the benefits of a high R-value insulation will outweigh other potential concerns.

Polyurethane in sprayed-on foam is a high R-value choice for flat roof insulations. The American Chemistry Council Center for the Polyurethanes Industry reports R-values of up to 7.2 per inch. For commercial flat roofing, either 1-component or 2-component spray polyurethane foam is commonly used as well as insulated metal panels for the building envelope. Cold storage buildings with freezers or coolers often utilize polyurethane insulated metal panels.

There are several advantages to polyurethane: structural performance, dimensional stability and moisture resistance. The major benefit of spray polyurethane foam (SPF) is that it can conform to irregular surfaces on a wide variety of substrates. There is typically long-term stability of the R-value as well as high fire resistance. Polyurethane does have a narrower window of application temperature and humidity limitations in some climate conditions.

Polyisocyanurate (polyiso) is closed-cell foam bonded to various facers, such as glass or foil. The blowing agents used to manufacture the product have evolved over time to meet environmental goals. Many manufacturers certify their EPA-compliant blowing agents as not having any CFCs or HCFCs to meet zero ozone depletion potential (ODP) roof insulation standards.

As an example, the R-value of a foil-faced polyisocyanurate panel produced with pentane ranges from an initial R-value of 6.8 per inch to 5.7 per inch as projected over time. Polyisocyanurate foam board roof insulation is one of the best roofing insulations and is one of the top choices for very high thermal efficiency. Polyiso is available for flat roof insulation systems, and it also can be supplied as tapered insulation board for flat roofs.

Designers who pursue certifications under green building rating systems, such as LEEDv4, would want to review adding insulation to a flat roof, such as nonhalogenated polyisocyanurate roof insulation. These formulations do not have flame-retardant chemicals that may adversely affect the environment.

Polyiso is the only foam plastic insulation product for direct application to steel decks to achieve FM Approval for Class 1 Roof Systems. It is also classified by UL for direct-to-steel deck insulation under both single-ply and asphaltic roof membranes.

Polyiso roof insulation is also produced at high density to become a much-specified cover board. Cover boards protect the roof assembly from roof traffic, mechanical abuse and hail. These roof cover boards are resilient and lightweight.

With an R-value of approximately 5 per inch, extruded polystyrene (XPS) is created utilizing an extrusion process to produce closed-cell rigid foam insulation with polystyrene polymer. Many manufacturers add a dye to the formulation to add a unique colouration, typically pink, green or blue to distinguish the product brand.

Most XPS is used for wall and below-grade applications. For commercial roofing, it is most commonly specified for Inverted Roof membrane Assembly (IRMA) or Protected Membrane Roof (PMR) systems. Extruded polystyrene is usually ranked in the middle of cost vs. R-value in terms of cost/benefit evaluation of flat roofing materials.

Expanded polystyrene (EPS) is made from the same base polystyrene resins as XPS, but the manufacturing process is different, resulting in beads that are formed and cut into different sizes and shapes or molded. High-density products have an approximate R-value of 4.6 as a relative comparison.

Contractors like to install EPS as it can be cut easily to meet site conditions, and it is lightweight. Theoretically, it has a more stable R-value over time as there are no chemical blowing agents that can outgas. There are no current studies that can confirm this attribute.

EPS has several disadvantages, however. It is combustible, will absorb water, can shrink and warp with high temperature exposure, and is not compatible with petroleum-based products, such as solvents, coatings and adhesives. In addition, foot traffic on the roof can damage the material.

The commercial roofing industry phased out rigid fiberglass two-by-four panels as single-ply roofing systems became a larger share of the market. Soft underfoot, fiberglass was not recommended for EPDM, PVC or TPO systems. Traditionally, fiberglass sheets were used as a substrate for hot roofing systems, and they had several advantages: They provided high fire resistance and flexibility, and they were chemically inert and resistant to mold growth.

There were few suppliers dedicated to market commercial roofing systems with the necessary resources to support various code testing in contemporary assemblies. Currently, fiberglass rolls can be purchased for specialized uses, such as metal building insulations. Additionally, there are formaldehyde-free mineral wool options for certain applications.

Perlite boards can rot when wet and need surface treatments or coatings to prevent excessive absorption of asphalt during installation. Again, with the advent of single-ply roofing systems and their increasing specifications, this product is losing favor as a choice in commercial roofing.

R-value is but one of many criteria in the selection of flat roofing insulations for commercial applications. Insulation is just one component in a full roofing assembly. Other determining factors can be:

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After creating a construction with EpConstruction, where can I find the R-value of it as a whole which the energyplus engine will use to do the simulation

As far as I concern, there are surface coefficients of heat transfer for both the outside and inside of the envelope construction, and there are correction factors for materials when they are used as different parts of the building. Should I calculate these corrections before build the materials to a construction or the energyplus will consider one construction with differently corrected R values by the catagories wall/roof/underground and so on?

Thank you for the reply!

So can I take it this way that if a construction for exterior wall has a R value 1(SI unit) at the output of Energyplus Construction component, the e+ will use the value 1+0.17=1.17 to run the simulation. The 0.17 comes from the HB_RValueWith AirFilms component after I compared the input and output.

Jolly,

You are talking about the U-factor alternative path to code compliance. For a fairly thorough discussion of this option, see "Three Code-Approved Tricks for Reducing Insulation Thickness." Here is the link:

-code-approved-tricks-for-reducing-insulation-thickness

Thanks, Martin.

I wasn't able to make the link work but found it with a quick search. In case anyone else comes across this comment string, here's the link.

-code-approved-tricks-for-reducing-insulation-thickness

This was very helpful.

The big challenge here is trying to maintain a fairly thin roof section, in line with the Mid-Century Modern aesthetic, especially the way it impacts the appearance of the eaves. That's why I looked into the U-value table, and was a bit surprised at how much the roof requirement, 0.024, differs so much from the R-value. I can't help wondering how the code committee came up with that.

To my ear it sounds like you are pushing the limits of the code book to build the house with the lowest R value allowed by law. If you take a step back, I do not really think that is what you want to build.

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