Whether you are replacing an existing flat roof or installing one on a new construction home, one essential component that you need to include is proper insulation.
Insulating a flat roof is a completely different process with different materials, than one used for a sloped roof.
However, its equally as important.
Quality insulation that is properly installed will greatly improve your roof’s ventilation, eliminate drastic temperature fluctuations inside the home, and protect it from leaks.
Here are the most common flat roof insulation options along with material and installation costs, to help you choose the product that works best for your house and budget.
How Much Does Flat Roof Insulation Cost?
The most popular and efficient insulation used on flat roofs is rigid foam board (Polyiso); it accounts for over 60% of all installs.
ISO is available in a range of thicknesses, that come in increments of 1 inch. The thinnest board (1 inch), which has a 6 R value costs $55/100 sq.ft..
Consequently if you are looking to increase your R value, you will need to get a thicker board, which will cost more. For example, to get an 18 R Value, you will need a 3 inch ISO board, which will cost $150-160/100 sq.ft.
R-value is extremely important, as it determines the board’s resistance to heat flow, and reduces the heat loss in your house (you want to have a high R-value). Its best to discuss with your contractor the most appropriate insulation thickness for your flat roof. This will depend on its overall construction, and the climate you live in.
Keep in mind that for new construction homes or commercial buildings, the minimum R-value requirement is 20 R, or 3.3 inch thick ISO board.
Installing PolyIso Rigid Foam Insulation
Its installed using mechanically attached or glue down method. ISO insulation has a fiberglass mat on either side, so the roof system can be installed using either glue down or mechanically attached assembly. Typical mechanical attachment requires 8 fasteners per 4×8 board. However, in high wind areas, you may need as many as 12 fasteners per board.
Resolving Flat Roof Drainage Issues with Tapered Insulation
One of the biggest issues with flat roofs and the one that causes the most leaks, is ponding water (huge pools of water on your roof). A properly designed flat roof must had a positive slope, means at least 1/4 inch rise per 1 foot run. This allows water to run off toward roof drains or gutters and not sit on the roof, causing leaks and even potential collapse.
Creating positive roof slope can be achieved inexpensively using tapered insulation. It comes in 4×4 foot sections with one end being 1.5 inch thick, and the other end being 0.5 inch thick. Using these sections, you can create roof slope either in one direction or in multiple directions, or even center the water flow toward a roof drain.
Because tapered insulation is a specialty product, it costs about 60-80% more than a flat insulation board of the same thickness. Thus, a 0.5 – 1.5 inch tapered insulation would cost about $80-85/100 sq.ft.
Standard tapered insulation is typically an ISO board with fiberglass mats on both sides. Tapered insulation can be made using EPS rigid or high density styrofoam. However, these are not widely used, because they require an additional separation layer between the roof and insulation.
The reasons for this are:
1. You can’t glue single ply membranes or built up roofs to either EPS or styrofoam. So you have to use mechanical attachment.
2. When mechanically attached, a single ply membrane with expand and contract at a different rate than the insulation, and will rub against the soft foam causing its top layer to eventually turn into dust.
Therefore, ISO insulation is often the most versatile system, because it has the separation layer already built in (fiberglass mat), and does not require using a costly separation layer.
Flat Roof Insulation Materials and Costs
In addition to rigid foam insulation, there are a number of other materials to consider.
EPS (pink/blue/green) Rigid Foam Insulation
This material is made using expanded polystyrene (EPS foam). EPS can have a plastic backing, but not fiberglass. It can be used with a variety of different flat roof covers. Similar to rigid foam insulation, EPS is also available in a range of thicknesses and R values.
The biggest advantage of this insulation is that on large commercial roofs with 3-4 inch insulation thickness, savings can be significant; as much as 20-25% over PolyIso. However, on small residential roofs, EPS is not cost efficient, because you will need to use a separation barrier between roof membrane and insulation. which will add at least 10 cents/sq.ft.
Cost: Roofing suppliers don’t quote EPS for small jobs. However, Home Depot and Lowe’s sells EPS insulation in 2×8 foot boards with different thicknesses. Those are about 30% more expensive than ISO insulation sold by roofing suppliers.
Low Density Styrofoam
This insulation can be used in either roofing or siding applications. It usually comes in a fan-fold package with each sheet being 2×4 feet, making a bundle of 24 pieces attached together. This insulation typically has a clear plastic on one side and foil backing on the other to improve thermal resistance. It comes in 0.5 inch to 1 inch thicknesses. One bundle covers almost 200 sq.ft.
Low density styrofoam has an R-value of 4 per 1 inch thickness. Styrofoam insulation is typically used in lay-over jobs; when a new single ply roof is being installed over old tar and gravel/built-up roof without removing the old materials.
This type of insulation is not used as thermal insulation. Its primary purpose its to separate a single ply membrane from the rough surface of built-up roofs. Styrofoam insulation should not be stacked to increase R-value.
Cost: Styrofoam insulation costs about $20/100 sq.ft. for 0.5 inch thickness. One inch thickness costs double, or about $45/100 sq.ft. and does not come in fan-fold (only as 4×8 sheets).
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