Tuesday, December 7, 2010

Green Roofs - Slow Absorption & Release Heat Sinks

We've compiled data over the last year on how an Extensive Vegetated Roof absorbed and released solar energy (heat) over a twenty four hour cycle.

The roof under study is not insulated.  It has a tongue and groove pine deck with asphalt roll paper and asphalt shingles.  The interior was non-temperature (no heat or HVAC) controlled during the study periods.

The temperature measuring points were the exposed roof tongue and groove decking surfaces and consisted of multiple point averages.  Readings were taken with an EXTECH IR AN200 Unit.

The following illustrations depict a trend we've seen over and over.

The extensive vegetated roof systems absorb solar heat slower than asphalt roofing.  The extensive vegetated roof systems also retain the heat longer and then release the absorbed solar heat slower than asphalt roofing.

The linear trendlines for the temperature curves tell an important story also.  The ambient air temperature and the decking under the non-insulated asphalt shingle roof temperature possess more significant vertical trendline movements.

The temperature swings of the decking under the green roof are much more linear - showing a narrower range in temperature swings.

The data shown here is representative of spring, summer, autumn and winter with summer and winter having the more pronounced curves.

Importantly, we believe extensive green roofs, such as the one we are studying in this case, moderate temperature swings.

The data also points to the fact that green roofs serve as a heat sink, slowly absorbing solar heat during the daylight hours then slowly releasing the heat during the evening dark periods.

During the winter, green roofs may help by moderating cold temperatures at night by releasing heat back into the structure.  However if insulation separated the green roof from the structure then little or no heat would be recaptured unless a heat capture coil or other mechanism existed.

The inverse holds true for the summer.  During the day the green roof would moderate the solar heat gain to the structure by intercepting the solar radiation and absorbing it.  However at night, the green roof would radiate heat back into the structure, slowing the cooling process.

Again, if the roofs are insulated from the green roofs then there is no real benefit from a linear trendline perspective of sink and release.

We believe maybe this data points to the need for a method of harvesting the solar heat captured in the green roof during the day.  Potential capture processes could include water or silicon filled coils or tubes interwoven into the vegetated roof or other similar mechanical systems.

Harvesting the absorbed heat would allow for a more managed use of the solar heat resource.

We do believe that the notion of green roofs working to cool buildings should be more appropriately described as a moderating effect of absorption and slow release of solar heat.

Check out the range bars on the temperature curve chart to the left.

The green roof decking temperature has a much narrower temperature range.

Data points to the fact that green roofs absorb heat and quite a bit of it.


We always welcome your comments - feel free to email us anytime here.

Green roofs offer the potential to collect and harvest solar energy.  They may not be the cooling system we sometimes represent them to be.

Happy Green Roofing!

Kevin

3 comments:

Roofers Bronx said...

Great post! Thanks for posting

محمد عبدالله said...
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محمد عبدالله said...
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