Insulated Metal Roofing – Helping Reduce Global Warming

November 27, 2008

Insulated Metal Roofing – Helping Reduce Global Warming
copyright © 2007
by James Dunn

All information is to be considered partial and experimental.

The roofs of most homes are usually a dark color that during the Summer, absorbs enough solar energy that it makes the attics quite hot; this reduces the temperature of the exposed roofing materials. A blanket of insulation between the attic and living spaces provides somewhat reduced costs for heating and air conditioning. The greater the insulating “R” factor, the greater the utility cost savings.

The cost savings of blanket or blown-in insulation in the attic is somewhat reduced by “insulation bypass”; these are conductive and convective thermal channels that get around and through insulating barriers: concrete supports, wood structures, metal reinforcements and brackets, wiring, columns of air-flow in the walls, condensation, …

However, from a Global Warming point of view, most rooftops are designed to amplify the effect of global warming.

* Solar energy is converted into low frequency infrared energy, re-radiated out into the local environment, and transmitted into convective heat in the attics; which is pumped out of the vents of attics into the surrounding environment; either through passive air-flow or by forced ventilation.
o These low infrared energy levels are not as easily re-radiated out into space, less so than the higher energy levels of infrared from the primary heated surfaces of the roof, had the roofing material not been allowed to cool as much.
o The primary surfaces have reduced temperatures from the heat conducting into the attic and therefore re-transmits infrared energy at a lower wavelength; providing greater heating of the local environment.

Sheet metal roofing has become a popular and robust method of roofing modern homes. As such, sheet metal roofing coupled with an insulating board, also provides a means of helping to suppress global warming in four ways:

1. Re-radiating all solar energy wavelengths back out into space, including infrared; especially if white.
2. Providing improved thermal insulation in rooftops.
3. Coupled with thermally controlled attic vents, optimizing beneficial thermal gradients.
4. Reducing the energy consumed via utility companies (reducing global warming trends).

When replacing your present roof, remove the shingles down to your plywood roof sheeting. Lay and secure a low-compressive insulating barrier sheeting onto the roof (extruded polystyrene is about R-9 per inch). Secure the metal roofing materials to this insulating barrier sheeting. An intermediate paneling or 1/4″ plywood sheeting may be necessary for some applications either for bonding adhesive compatibilities, or for controlling thermal gradients. If possible, use an adhesive system instead of screws or nails to prevent the heat from being transmitted into the attic through the metal of screws and nails.

Care should be taken in considering thermal expansion of the metal materials and providing the movement necessary to prevent buckling of the roofing materials. The raised ridges often provide the needed expansion space and spacing in the lateral dimension, but the vertical dimension is often neglected and crackling noises can be heard during times of expansion/contraction extremes. Slip joints are an often used construction technique. The adhesive must provide both flexibility and adhesion at higher than normal temperatures.

Make sure the metal roofing manufacturer approves of your method of providing an insulating barrier between the roof and their metal sheeting; so you don’t violate a warranty, and you do not have a premature roofing failure. The materials and construction techniques should be simple, but they should also be engineered and not just flippantly tried on ones own incentive.

This will maximize the metal temperatures of the roofing material, while helping to minimize the energy transmitted into the home. The volume of heat needing to be removed from the attic is significantly reduced. The much higher roofing metal temperature more readily re-transmits solar energy back out into space, and therefore helps to reduce global warming. Lighter colors reflect more energy, but even if you choose darker colors for aesthetic reasons, the energy re-transmitted back into space will be much higher than if not using the insulation barrier, or if using other roofing materials like tile, shingles, or tar/gravel.

Roofing materials are commonly aluminum and have very low heat capacity and will NOT store energy for any appreciable time. In only a few minutes under cloud cover, the metal will be at ambient temperatures.

Installing attic forced ventilation and shutters coupled with a differential thermostat can significantly contribute to utility savings.

If you use a Swamp Cooler (used in low humidity climates instead of refrigerated air conditioning; lower cost to run), you can vent your outlet air partly through your attic, as well as out partially opened windows, to significantly improve the cooling efficiency of your swamp cooler. Hot air rises, so more of the interior hot air will exhaust through the attic. While the attic is significantly warmer than any of the air inside the living spaces. Thus both cooling the interior more, and cooling the attic as well. Swamp Coolers run continuously at a small fraction of the cost as refrigerated air conditioning. Use a ceiling vent that has an automatic damper (cheap) so that you don’t lose heat through the vent in the winter months.

The differential thermostat is made by comparing the outside ambient temperature with the temperature of the attic. A Programmable Logic Controller (PLC) can be purchased with thermocouple inputs for less than $200. Additionally, the PLC can be used for a hundred other uses like controlling your Solar Collector system, building security, exterior lighting controls, sprinkler system control, … A fan with shutters is at one end of the house, with motorized shutters at the other end of the house. The forced air circulating fan and shutters are controlled by these temperature sensors:

Outside Temp

Attic Temp greater than Outside Temp

Interior Temp greater than Attic Temp

Fan & Motorized Shutter

> 10° F

Low Speed

>75° F

> 5° F

Low Speed

>75° F

> 10° F

High Speed

* The attic fan runs whenever the attic is significantly (> 10° F) cooler than the interior of the house. Humidity of the house will collect (dew point) in the attic whenever the temperature of the attic is significantly less than that of the house interior. This can reduce the insulating properties of the insulation and actual water can collect to damage building materials (ever notice water spots on people’s ceilings?).
* Whenever the Outside air Temperature is greater than 75° F “AND” the Attic Temperature is greater than the Outside air Temperature by 5° F, the shutters will open and the Attic Fan will run at low speed. This reduces the temperature gradient between the attic space and the living spaces, so as to conserve the need for air conditioning.
* Whenever the Outside air Temperature is greater than 75° F “AND” the Attic Temperature is greater than the Outside air Temperature by 10° F, the shutters will open and the Attic Fan will run at high speed. The cost of running a ventillation fan is far lower than running refrigerated air conditioning.

A simple air-flow switch-type sensor can be used to indicate when the system is operating as expected.

* I can provide the diagnostic electrical/electronic information if you are interested in automating diagnostics and trouble-shooting messages.

By minimizing the temperature difference between the attic and the inside living spaces during the Summer, the “insulation bypass” is minimized. The cooling load is minimized and utility usage similarly minimized.

If you have the resources, you can provide a number of upgrades to reduce heat transfer through your windows:

* extend the overhang of the roof so that the windows of the house reside in the roof overhang shadow for most of the day
* angle windows to prevent sunlight from entering directly, except during Winter months
* apply window glazings to reflect light at lower critical angles
* install infrared blocking windows
* install shutters that can be closed from inside the building
* grow small-limb shade trees to help shade the house
* install insulated draperies
* install infrared blocking polycarbonate windows
* install multi-layer window assemblies (thermo-pane windows)

The outlined method above can be expected to pay for itself in reduced utility costs; while helping to reduce Global Warming, which is priceless.


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