The Roofing Resource Blog

A Quick Guide to Polyiso Roofing Insulation

Wed, Mar 5, 2014 @ 04:00 PM | Perry

1. What is polyiso roofing insulation?

Polyisocyanurate  or Polyiso is a ridged insulation board made of closed cell foam. It is usually manufactured in either 4’ x 4’ or 4’ x 8’ boards and comes in a range of thicknesses from 1” up to 4”. Polyiso is a common commercial roofing product, used under roofing membranes to insulate the building or structure but it can also be use on walls.  

2. Are there different types roof insulation?

Polyiso comes in flat stock and tapered panels. Flat panels are typically installed on low slope roofs that slant towards a guttering system. Tapered polyiso panels are made with slope or an angle and are used to direct water to a drain or  to keep water from standing in corners or flat roofs. 

3. Why is this material used?

Because Polyiso board  insulation works, payback studies can clearly demonstrate positive ROI from reduced energy cost to structures when it is installed correctly and at proper thicknesses . And because polyiso has the highest R-value inch for inch of any other building insulation product available meaning it takes less of it to reach the desired R-value which also mean cost savings. 

4. How much does polyiso it cost?

The price will be driven by the type and thickness of polyiso you purchase but should average between .35 cents to .42 cents per sq. ft. per inch. To find out how much you are being charged per sq. ft. per inch simply take the price per board and divide it by the amount of sq. ft. in the board. 


One, 4 x 8 piece of polyiso 1” thick cost $13.10. There are 32 sq. ft. in one piece of polyiso because the board is 4’ x 8’. Take the price per board ($13.10) ÷  amount of sq. ft per board (32 sq. ft.) = .409375  or .41 cents per sq. ft. per inch. Hint: The price per Sq. Ft. you pay should decrease a little as you move up in thickness. For exact prices on flat and tapered polyiso stock consult with your roofing contractor or roofing supply yard. 

5. How much Polyiso do you need?

That will largely depend on whether your roofing project is new construction or a re-roof of an existing building. Consult with your local building inspector for specific R - Values required for new construction projects in your area. For re-roof, finding out the amount of insulation the building already has and if it can be saved is a good place to start. From there you can make a decision on how much additional polyiso to add. 

You can use a climate map to get a recommended amount Polyiso insulation but always  consult with your local building codes before starting your roofing project.  

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Topics: Flat Roofing, Commercial Roof