What Do You Really Know About Metal Roofs?

Metal roofs are efficient, cost effective, attractive and durable – but thousands of home and business owners shy away from installing one. As a matter of fact, simple misinformation is behind the reluctance of many who fail to turn to metal roofing, and the following four myths are among the most tenacious and most fallible of all.

  1. Metal roofing is too expensive to be practical. Untrue! While the up front cost of a high quality metal roof is higher than that of an asphalt shingle roof, a metal roof could be that last one you’ll ever have to buy – and a properly installed metal roof can even pay for itself by reducing energy bills by as much as 35% every summer.
  2. Metal roofing is insanely loud when it rains. Untrue! While a barn or warehouse with an uninsulated metal roof can mimic rapid gunfire during even a light shower, a modern metal roof installed over solid sheathing and roofing underlayment will produce no more noise in a storm than a traditional asphalt shingled roof.
  3. A metal roof is more likely to be struck by lightening. Untrue! Metal roofs do not attract lightening. However, if lightening did strike your house, the metal would quickly dissipate the charge; and it’s non-combustible, which means less chance of fire.
  4. Metal roofs are easily damaged. Totally untrue! Metal roofing has a special zinc or aluminum based compound bonded to the surface, and is then covered with special paint designed specifically to resist the elements and stay looking new for years. Metal roofs are also extremely hail resistant and cannot be destroyed by high winds as shingled roofs can.

A metal roof can last a lifetime, protect your home from the elements, and even save you money. The next time your roof needs a makeover, considering making it your last.

Why Choose Metal Roofing?

One of the most popular and effective metal roofs on the market is the galvanized metal roof. The galvanized metal roof has several significant advantages over other varieties that many owners find compelling.

Galvanized metal is created by treating metal with liquefied zinc which is what forms a coat over the surface of the metal. Below that layer is a combination of zinc and metal, and then the core layer is strictly metal. Since galvanized metal is extremely durable, owners of galvanized metal roofs can expect the following benefits:

  1. Because galvanized metal roofs are lightweight, there is little stress on the underlying roof foundation.
  2. Galvanized metal roofs are maintenance free. Aside from cleaning it up with a pressure washer now and then, there is no need for scheduled chemical treatments to protect the integrity of the roof.
  3. Galvanized metal roofing is not only extremely durable against snow, hail, rainstorm, heat and a variety of extreme weather conditions, it is also resistant to rust and corrosion.
  4. Galvanized metal roofs reflect exterior heat rather than absorbing it which translates into a cooler interior. This factor alone is an effective means of reducing energy costs associated to running air conditioning systems.
  5. Galvanized metal roofing is fire resistant and non-combustible, something traditional roofs cannot boast.
  6. In the event that you have extra galvanized metal roofing material at the end of your building project, it is not likely to be wasted. As galvanized metal roof material is very durable and lightweight, it can easily be used for any number of future projects.

When you consider durability, flexibility, aesthetics, the ability to get very creative using metal roofing and the cost efficiency of metal roofing, it is easy to understand why it is quickly becoming one of the most popular roofing materials for contemporary homes.

Prototype Insulated and Ventilated Roof Deck Cools the Entire Roof System

A new roof system design is being studied that is usable with almost all roofing products. The heart of the design is a profiled and foil-faced expanded polystyrene (EPS) insulation that fits over and between rafters in new construction (Figure 1) or can be attached on top of an existing shingle roof system. The EPS insulation is profiled to form a 1-inch (0.0254-m) air space between rafters to promote thermally induced convective flows that carry some of the heat penetrating the deck toward a ridge vent and away from the attic. The top and bottom sides of the EPS are foil-faced; the top side acts as a radiation shield in the inclined air space and the bottom-side foil performs as a radiant barrier in the attic (Figure 1). Ventilation is enhanced by cutting a slot near the eave just above the soffit to provide a source of makeup air from the soffit vent and attic. Buoyant air moves up the inclined air space and creates a negative pressure at the eave. Cool makeup air is pulled from the soffit and attic, which further enhances the temperature driving potential for natural convection heat transfer in the inclined air space. A ridge vent expels the heated air back to the outdoors.

This ventilation scheme keeps the air intake internal to the attic, which eliminates the intrusion of pests and any threat of burning embers entering the inclined space. The 1- inch (0.0254-m) of EPS insulation also serves to reduce the conduction heat transfer trying to penetrate into the attic. The lower deck flux results in a cooler radiant barrier temperature compared to conventional construction having an oriented strand board (OSB) deck with or without a foil faced radiant barrier. The reduced foil temperature of the EPS therefore further drops the radiation exchange between the roof deck and the attic floor.

As mentioned, the roof assembly can also be installed in retrofit applications provided the existing roof system can bear the added load. Furring strips are attached to the existing shingle roof and the EPS insulation mounted on top of the old shingles with a new OSB deck, weather-resistant sheathing and new layer of shingles.

Winter Field Tests of the Prototype Roof and Attics

During winter nights, field data revealed that night sky temperatures were much lower than the surface temperatures of the test roof systems, a situation that drives radiation heat loss to the sky. Good roof design should ideally limit heat gains during the summer while also limiting heat losses in the winter, which is why insulation works better than cool roof systems in cold weather.

Computer Simulations

ASTM C1340-04, “Standard Practice for Estimation of Heat Gain of Loss through Ceilings under Attics Containing Radiant Barriers by Use of a Computer Program,” [12] was benchmarked against field data for the insulated and ventilated shingle roof system (Figure 4 and 5).  Simulations were made for the hot climates of Miami; Austin, TX; Atlanta and the cold climate of Baltimore with and without air-conditioning ducts in the attic. An attic of 1539 square feet (143 m2) with a roof slope of 18 degrees was modeled with and without a cool color shingle roof; the cool color shingle’s solar reflectance was 0.25. The supply duct surface area was set at 304 square feet (28.7 m2). The return duct assumed 176 square feet (16.4 m2) of surface area exposed in the unconditioned attic. Energy Plus [13] estimated the hourly run times for an air-conditioner certified with a Seasonal Energy Efficiency Ratio (SEER) of 13 and for an 85 percent efficient gas furnace that heated the home. The hourly indoor air temperature for the house and the run time for the HVAC were estimated by Energy Plus and read by AtticSim to better estimate the roof and attic load as coupled to the building envelope.

Building Practices to Mull Over for New Homes

All too often HVAC ducts are in an attic, and the ducts are poorly insulated and are not well sealed so they leak conditioned air into the attic. Simulation results indicate that homeowners typically pay an added $100 to $300 more per year because of leaky and poorly insulated air conditioning ducts operating in an unconditioned attic. Energy costs are also increased if the attic floor leaks air to or from the home. Duct location and sealing the attic floor are of paramount importance and should take precedence over all other energy efficient roof system and attic strategies. The simulation results of Figure 6 illustrates why these practices should be a priority component of a building program. The dark blue bars represent a dark heat absorbing roof and attic that contains poorly insulated and leaky ducts and a leaky attic floor. The orange bars show energy use where the practitioner repaired the leaks in the attic and sealed and rewrapped the ducts in RUS-8 (RSI- 1.4) insulation. The light green bars show the benefit of moving the ducts into the conditioned space. The light blue bars are for the new-design ventilated and insulated roof deck with the attic floor repaired for leakage and the ducts moved to the conditioned space.

Figure 6.  Comparison of energy effects of leaky ducts in attic space, sealing attic floors, insulating attic floors, and eliminating energy losses from HVAC ducts in unconditioned attics.

The heat gains and losses from leaky ducts and a leaky attic floor are double if not triple the heat gains and losses crossing the attic floor. Adding insulation to the attic helps but the heat transfer tends to level off for floor insulations exceeding RUS-30 (RSI-5.3) because losses from the ducts predominate. Adding insulation does reduce the energy bill for all assemblies represented in Figure 6, but if all on does is add insulation then the energy consumed due to the roof system and attic is not minimized as clearly seen in Figure 6.  Sealing all duct joints with mastic and wrapping the ducts with RUS-8 (RSI-1.4) insulation drops the energy losses for ducts in the attic by roughly 40 percent (Fig. 6 dark blue bars compared to orange bars). However, more savings can be achieved if the ductwork is simply kept out of the attic. An attic with RUS-60 (RSI-10.6) floor insulation but with leaky ducts and a leaky floor (Fig. 6 dark blue bar) has 30 percent greater heat energy losses than an attic with just RUS-5 (RSI-0.9) floor insulation but with no ducts and no air leakage across the attic floor (Fig. 6 green bars). In many homes, the ductwork increases air-conditioner energy use by roughly 18 percent for moderately leaky ducts in a well-insulated attic [14] and [15].

Pre 1990 homes were hopefully built to the presiding ASHRAE Standard 90-80 “Energy Efficient Design of Low-Rise Residential Buildings” [18]. Therefore the payback for adding insulation above the 1980 code level set at RUS-20 (RSI-3.52) was computed for an attic assembly with sealed attic floor and inspected ductwork3, Figure 7. Adding RUS-19 (RSI-3.3) of insulation pays for itself in about 35 years when added to an existing RUS-20 (RSI-3.52) batt. Increasing the ceiling insulation to RUS-60 (RSI-10.6) yields a 34- year payback if the basis of savings starts at RUS-20 (RSI-3.52) batt.

Building America (BA) has a residential benchmark [16] that calls for no ducts in the attic, sealing the attic floor and RUS-50 (RSI-8.8) insulation placed on the attic floor. The seasonal roof heat transfer was computed for the BA benchmark and for the new roof and attic design (Figure 7). Increasing the level of insulation on the attic floor from IECC [9] code level of RUS-30 (RSI-5.3) in Austin, TX to the BA benchmark of RUS-50 (RSI-8.8) lowers the ceiling heat transfer by 41.5 percent of that computed for the code level of insulation (view red squares in Figure 7). At RUS-50 (RSI-8.8) there is only 2,900 kBtu per year crossing the attic floor; however, the new attic design with RUS-30 (RSI-5.3) shows heat flow of about 3,000 kBtu per year. Therefore, the ventilated and insulated roof system performs as well as the BA benchmark while using only IECC code level of insulation (i.e., RUS-30 for insulated and ventilated roof deck; RUS-50 for BA benchmark).

Figure 7.  Building America Benchmark at RUS-50 (RSI-8.8) is compared to the new roof system design having an insulated and ventilated roof deck.

3 Simulated data is also represented by the green bars in Figure 6.

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