When it comes to flat roofing, one of the most important and often misunderstood components of the system is the insulation.
There are many different insulation materials and installation methods out there, along with a lot misinformation about which products to use to achieve the greatest energy savings.
Here, we debunk the most common myths about flat roof insulation, and offer our professional take on what actually works, how much it costs and the benefits you get as a homeowner.
Alternative insulation methods that we DON’T recommend
Some so called “experts” on line may recommend the following, claiming to achieve a greater R-value at a lower cost.
Myth 1: How to Save on Insulation
Add an additional layer of OSB over the roof sheathing. It should be installed on tapered 2x furring strips, in order to slope the surface of the roof to drains. Then, install R-30 fiberglass batts between the rafters.
Our Take: Framing this out and buying 30-R fiberglass insulation will cost you about 50% more than just buying 5 inches of ISO insulation (equivalent of 30-R). Because it will take a lot of time to actually install all of this, you are risking major leaks, if it starts to rain before your roof assembly is complete. Also, if a roof leaks and fiberglass insulation gets wet, it will loose most of its insulating properties. However, ISO insulation absorbs considerably less water, therefore if it gets wet, it looses only a small fraction of its insulating properties.
While expensive, PolyIso rigid foam insulation is the best quality and the most cost effective product for your flat roof. You can expect to pay about $55/100 sq.ft. for 1 inch insulation board (equivalent of 6 R-Value). We recommend having at least 20 R-value worth of insulation for optimal energy efficiency.
Myth 2: Increase Total R-Value by Combining Different Insulation Products
Install 4 in. of rigid foam insulation on top of the deck, 2 in. of closed cell foam under it, as an air barrier. Then, R-30 batts can be added for additional insulation at a lower price. This option can give you a total R-value of about 62.
Our Take: The extra labor required for getting inside a typically inaccessible crawl spaces under flat roofs will more than negate potential savings of using 30-R fiberglass batts. If on top of this you have to hire a spray foam specialist to install only 2 inches of spray foam, your costs will go through the roof. Once again, increasing the thickness of ISO insulation will give you a higher R-value at a much lower total cost. If you can actually get into the crawl space, its much more effective to install blown-in cellulose or fiberglass insulation.
Myth 3: Cost-efficient Insulation Option
First, install sloped decking as described in the first option. Then, do 2 in. of closed-cell foam insulation and fill out the remaining 10 in. of the roof cavity with dense-packed cellulose. With this combo, you can achieve a total R-value of about 47.
Our Take: Framing out a new sloped roof while making sure that your old flat roof does not leak during this process will cost you about double of adding a quality PVC roof and 6 inches of ISO or high density styrofoam.
Myth 4: Fiberboard Is an Insulation Material
Many budget roofers, those who typically install EDPM Rubber may try to convince you that fiberboard is an insulation. Do not believe this!
Fiberboard has only 1-R value per inch and is always installed in 0.5 inch thickness. Therefore, you get practically 0 R-value out of it. Additionally, attic condensation and/roof leaks will make fiberboard wet, and the moisture will be trapped inside the fiberboard, causing mold growth and further reducing its R-value.
Fiberboard is intended to be used as a glue-down surface on roofs above none-living spaces, such as garages or porches, where the R-value is not needed. Living spaces should have real insulation and the more of it the better.
If your roofer suggests using fiberboard, we recommend that yoo fire them immediately, because they don’t care about your roof or you, and will likely do a very poor installation job.
Why Its Important to Insulate Your Flat Roof
While flat roof insulation is expensive and can account for as much as 25% of the total installation cost, there are a number of important benefits to having it in place.
Reduce extreme temperature fluctuations inside your home
When you have an un-insulated roof, you have to live with rather uncomfortable room temperatures inside the home. During the cold months, warm air will quickly rise up and escape through the roof, leaving the rooms very chilly. The opposite happens in the summer: hot air gets trapped inside, making the room unbearably hot and sticky (this is particularly true on roofs that are dark and have a low R-value insulation)
Improve energy efficiency
A roof without good insulation is a great contributor to energy loss. This is bad for the environment and for your wallet, as you end up spending a lot more on your electric bills. Proper insulation can significantly reduce your energy consumption (heat in the winter, AC in the summer) and reduce your energy costs by hundreds of dollars a year.
A high R-value roof insulation (20 or more) combined with well insulated walls and energy efficient windows can reduce your heating and cooling expenses by as much 30% every year. Therefore, it will pay for itself within 4-6 years in energy savings alone. If you have oil heat, the pay-off will be even faster, and you will be saving money for many years there after.
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