“My Cabin Doesn’t Leak When it Doesn’t Rain,” Part 1 – Cost Analysis

Part 1 – Cost Analysis

In spring 2018 some shingles blew off of our roof in a bad storm, and we got a leak that made its way through our attic and down to our living room. This was not too surprising, given that our asphalt roof (generally considered to have a 20-year life) was at least 20 years old when Christian bought the house 10 years prior. He had a stack of extra shingles that came from the last roof replacement and had been using them to patch and perform roof maintenance as needed, but that supply finally ran out. Long story short, it was time to shop for a new roof, and I began exploring options.

At the time, I wrote a blog post based on some cursory research of roofing materials and investment value of asphalt, metal, and the new Tesla solar shingles.[1] Unfortunately, much in the tradition “The Arkansas Traveler,” [2] a song that was sung to me as a child (and clearly had an impact), “my cabin doesn’t leak when it doesn’t rain,” meaning Christian’s patch let us put that decision on the back burner while I did some more research. Nearly two years later, that research still hadn’t happened, and it was time for the house to remind us of our responsibility… on Christmas Eve.

We spent the holiday emptying pans of water in the attic and calling roofing contractors for quotes. I spent the week between Christmas and New Year’s diving into materials research, product warranties, and recycling options to build on my original blog post, which was only focused on financial investment, not product lifecycle.

Upfront Cost

My original cost calculations involved a lot of guesswork and questionably objective information from suppliers’ (particularly metal roof suppliers’) websites. I assumed a 50-year lifespan for metal (based on listed warranties, though they can last longer) and 30-year lifespan for asphalt (based on the probable age of our roof, though listed life was generally 20). Despite those assumptions, which leaned in favor of asphalt, it still meant that we would theoretically have to buy twice as many asphalt roofs over the time period in question, putting total cost for each option in the same ballpark.

However, skipping ahead to the end of 2020, our two leading asphalt roof contractors offered a 50-year materials warranty, which changed things. We also had specific estimates, rather than my original educated guesses on probable price range. Based on this new information, I could now confirm that the total cost for metal roofing would be over twice the cost of asphalt roofing over the same 50-year period.

Material and labor costs aren’t the only financial factors, though. One of the oft-touted benefits of a metal roof is the associated energy savings from absorbing less heat from the sun than asphalt does, particularly in the hot summer months. Some sources say a metal roof can save you up to 40% on your energy costs.[4] I had factored energy savings into my calculations when writing the original blog post, but kept the factor at 10% for reasons I will explain in more detail later, related to our electricity bills and attic insulation.

Energy Savings of Metal

Even when you can find studies that aren’t funded or conducted by the metal roof industry, a lot of them focus on low-slope, non-residential roofs (which presumably don’t have an insulated attic between the roof and conditioned living space). Given that, I’m not even sure I can apply that information to a residential setting… but I’m going to try.

Oak Ridge National Laboratory ran a test with multiple types of roofing material installed on an 18-degree (very shallow) roof slope, with ridge and soffit vents included for airflow below the decking.[5] While there were a lot of parameters related to color and reflectivity of materials, the study indicated that summertime heat flows through the decking material about the same for asphalt shingles and cool color metal shingles when the metal shingles are mounted directly to the deck. The cooling benefit (and therefore energy savings benefit) of metal appears to come into play when shingles are mounted with an air gap, or “Above-Sheathing Ventilation” (ASV) between the decking and the shingles. The gap does not need to be large; the study examined .75″ and 1.5″ installations.

Above-Sheathing Ventilation can keep cool air flowing underneath the metal, preventing the decking material (and therefore the air underneath the decking, in the attic) from heating up. Hot air in that gap will rise and flow out the top, pulling cool air in through the bottom. However, this convective current becomes less pronounced when the roof pitch is shallower (in which case reflective materials that bounce, rather than absorb, the sun’s light become more effective than ASV at keeping the roof cool).

The same ORNL study compared heat flows in a Direct-to-Decking (DtD) installation and an ASV (air-gapped) installation, both using metal shingles. I mentioned above that the study also found the similar heat flows for DtD metal shingles and traditional asphalt shingles. Because of that, I’m going to assume that the black DtD curve in Figure 6 looks something like what we might see from asphalt shingles.

Comparing the temperature flux curves of the two options in Figure 6, it does appear that that the ASV installation has about 40% lower BTUs/hour/square foot than the DtD installation (and, presumably, traditional asphalt as well), which may be where that “40% energy savings” claim originated. But about that claim…

Spurious Assumptions

Just because my assumptions above indicate that an air-gapped metal roof could be 40% “cooler” than traditional asphalt, that doesn’t translate directly to 40% lower energy bills. Most modern homes have some ventilated attic or other air space between the roof and the living space (which they replicated in the study), and in most of those cases, there is some level of insulation (which they did not). The attic and insulation are there to, well, insulate the living space from the heat of the roof in the summer. The better the insulation and airflow through the attic, the less that heat will impact the temperature inside the living space. (Our attic is poorly insulated – we currently have about half of what we should, per code.)

Additionally, any savings realized from a metal roof would impact cooling costs, but not baseline electricity use (e.g. lights, washing machines, etc.), at least not in any meaningful way. Based on our electricity bills, there are only three months of the year that we even run the air conditioning; our usage is pretty flat for the rest of the year, and our usage can be about double in those three months. Percentage savings would be applied to energy used for cooling, not energy used overall.

Finally, this study was run in July, and the “energy savings” frequently discussed refer to a reduction in cooling costs in the summer, with the implication of general savings year-round. A metal roof that adds a cooling factor in the summer will also add a cooling factor in the winter, meaning there may be increased heating costs associated with a metal roof in cold weather. Such information has not been forthcoming in my research, except for one metal roof supplier that cited the ORNL study as saying that metal roofs are warmer in the winter. The study said nothing of the sort, meaning that this supplier either cited the wrong source, betrayed a lack of understanding of basic thermodynamics, or lied.

In any case, because we have about half the insulation we should in the attic, and our cooling costs make up about half of our electricity bill for three months of the year, my original guess of 10% rather than 40% energy savings (for those three months) still seems to be a reasonable assumption, at least for our house. (1/2 x 1/2 = 1/4, 1/4 x 40% = 10%)

An Alternate Option

At this point, even factoring in the (completely unrealistic) full 40% reduction on our total energy costs for three months out of the year, the metal roof still came out to be more expensive than our highest asphalt roofing quote ($15k) over a 50-year period. Given that I was prepared to spend more than twice the cost of asphalt on a metal roof simply because of the touted environmental benefits, we figured there was something more effective to do with that money instead: weatherize.

Having spent a decade in the energy efficiency industry, and half of that time specifically in home weatherization, I am the first person to preach that the cheapest, cleanest kilowatt-hour is the one that isn’t used. I also know how much energy (and therefore money) can be saved simply by insulating and air-sealing an attic, closing up drafts with caulking, and sealing leaky duct work. And that is where my attention went next.

~

Next week we’ll look at how supplemental measures like weatherization can factor into a roof’s energy savings. Have you been through the roof-buying process yourself? What factors played into your final decision?
Thanks for reading!

Keep Reading –>

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[1] https://radicalmoderate.online/sustainable-roofing-materials/

[2] https://www.bluegrasslyrics.com/song/arkansas-traveler/

[3] https://www.bluefoxroofing.com/roofing-basics/roofing-system/

[4] https://www.metalroofing.com/residential-metal-roofing/benefits/energy-savings/

[5] https://web.ornl.gov/sci/buildings/conf-archive/2013%20B12%20papers/162_Kriner.pdf

[6] https://web.ornl.gov/sci/buildings/conf-archive/2013%20B12%20papers/162_Kriner.pdf

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