[Bldg-sim] Insulate the roof in summer without HVAC system yes or no? COOL ROOFS

[Jim Dirkes]

Graham,
I have only skimmed the paper so far, but the hypothesis is one makes a lot of common sense.  I never gave the roof micro-climate much thought while using EnergyPlus, but I have personal experience of roof-mounted cooling equipment under-performing due to the roof's elevated temperature in the vicinity of an air-cooled condenser.  Thanks for bring the two parts of my brain together!
p.s. a full reading of your paper will follow in the next few days.

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com<http://www.buildingperformanceteam.com/>
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Graham Carter & Megan Lyall
Sent: Friday, April 19, 2013 5:39 PM
To: Jeremiah Crossett
Cc: bldg-sim at lists.onebuilding.org; David GARCIA-SANCHEZ
Subject: Re: [Bldg-sim] Insulate the roof in summer without HVAC system yes or no? COOL ROOFS

Depending on your climate, you may find a CoolRoof may add more value than insulation.  There are some products that claim very high solar reflectance (90%+) combined with high long wave emissivity (.92+) which can keep the roof itself cool thus reducing the sub-roof temperature and under some conditions creating a negative heat pump effect due to sky cooling that occurs in the daytime.  There are a number of products out there that use a ceramic nanospehere particle additive to the roof paint that lifts solar reflectance by 15-20%.  So when applied to a TiO2 white you can get into the 90%'s.

If your climate is cooling dominated you may find this a useful strategy.  If you have a cold climate then this can work against you but it sounds like cooling is the problem.

Melbourne Airport has successfully used such a product so I don't believe there are any concerns with the high diffusing reflectance around airports.

Most software packages will model a truly cool roof properly including IES-VE and the DOE-2.2 / E+ engines.  The problem is they don't represent more conventional and absorptive roofs which create a roof micro-climate that is biased upwards from ambient.  I am digressing, but thought I would promote this issue a bit as I would love our industry software to recognise this microclimate effect so that we drive better roof designs.  And where we have cooling equipment and outdoor air intakes at roof level, that we have the mechanisms to account for the higher condensing temperatures and higher ventilation air temperatures.  The issues are particular to large roofs like shopping centres, airports, manufacturing facilities etc.  I wrote a paper on this (attached) to try and give this issue some visibility.  Please keep asking your software vendors for these issues to be represented in your software fully ...

Also with your hangar do you have the space stratified?  While you can stratify in IES-VE and get an indication of annual bulk conditions, it would be best tested in CFD using IES-VE to get your boundary conditions if you are interested in more than just every, i.e. comfort.

Regards,
Graham
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