Insulate the roof in summer without HVAC system yes or no?

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Hi all

I have a project where I need to model a maintenance hangar,

In this project we are evaluating the relevance of insulate the roof to
improve the comfort level in summer. The hangar has no air conditioned
system but we use natural ventilation which seems to work well in our
model (we use IES VE).

The problem came when we try to put some insulating material to the
actual roof (which is composed of only metal sheet of 2 cms). Unlike one
might expect the temperature in the hangar increases, as the roof
temperature.

This is to some extent logic, and I know it depends of the insulate type
and thickness. But we agree (I think) that at least one insulation with
les of 2cm to the actual roof should give an improvement in at least
roof or air temperature. I know that insulation can serve as a "trap" of
interior thermal conditions, so it's improve HVAC systems performance.
But also it serves to avoid the external conditions gets-in.

I have tried several configurations without success, internal, external
insulation.

And maybe some of you have already some experience in model this kind of
configuration. We agree (I think) that I can't recommend to my architect
that he shouldn't insulate its roof, but also I want to explain the
pertinence of this with a dynamic thermal modeling.

Thanks in advance for any advice on this,

David Garcia

David GARCIA-SANCHEZ's picture
Joined: 2012-11-23
Reputation: 0

Hi,

Thanks for your answer. Yes we have strong internal gains in this hangar (we suppose a high rate of lighting gain and a heavy occupant metabolism). But, for me is still very strange the fact that we supposed to have natural ventilation who it supposed to help control this strong gain.

Even if air temperature is high, the roof temperature is also higher with insulation that without insulation.

Maybe modeling results are "correct" but one might expect that the temperature of wall should get reduced by using a very thin film of insulation. Despite the fact of the strong internal gains which are the same in both examples,

This is a difficult case, where you wonder if the "dynamic model" is in a different sense of "reality"

Bes regards and thanks again,

David GARCIA-SANCHEZ

De : ?lvaro Lapa [mailto:ajlapa at gmail.com]
Envoy? : Friday, April 19, 2013 11:46 AM
? : David GARCIA-SANCHEZ
Objet : Re: [Bldg-sim] Insulate the roof in summer without HVAC system yes or no?

The increase in temperature (when isolated) could be caused by the internal gains. Try to confirm that.

2013/4/19 David GARCIA-SANCHEZ

Hi all

I have a project where I need to model a maintenance hangar,

In this project we are evaluating the relevance of insulate the roof to improve the comfort level in summer. The hangar has no air conditioned system but we use natural ventilation which seems to work well in our model (we use IES VE).

The problem came when we try to put some insulating material to the actual roof (which is composed of only metal sheet of 2 cms). Unlike one might expect the temperature in the hangar increases, as the roof temperature.

This is to some extent logic, and I know it depends of the insulate type and thickness. But we agree (I think) that at least one insulation with les of 2cm to the actual roof should give an improvement in at least roof or air temperature. I know that insulation can serve as a "trap" of interior thermal conditions, so it's improve HVAC systems performance. But also it serves to avoid the external conditions gets-in.

I have tried several configurations without success, internal, external insulation.

And maybe some of you have already some experience in model this kind of configuration. We agree (I think) that I can't recommend to my architect that he shouldn't insulate its roof, but also I want to explain the pertinence of this with a dynamic thermal modeling.

Thanks in advance for any advice on this,

David Garcia

David GARCIA-SANCHEZ's picture
Joined: 2012-11-23
Reputation: 0

Insulation does trap heat and this is a real issue that happens in the real
world and not just in models. There was a thread on this form last year
about it and papers posted. I would suggest you look into phase change
materials rather then insulation- they will absorb the energy and cool the
space down to enhance comfort during the day and release that heat into the
space at night to keep it warm. There is a video of a metal building
install at this link---> http://www.phasechange.com/index.php/en/install-portfolio/videos

In addition to PCM's I suggest you look into high emissivity / reflectivity
paints or coatings.

*Jeremiah D. Crossett

Jeremiah Crossett2's picture
Joined: 2012-12-14
Reputation: 0

David,

I must admitt that it?s really extrange what it's happening to you. But the
only explanation is the SRI that it may change when you change the roof
composition. A metal sheet can easy have a SRI 0.6 and cement unpainted 0.2.

In a way it?s true that when you have a building with high internal load
and no HVAC system when you increased the insulation level in the envelope
the interior temperature normally gets high. (In the same way that
insulation dont let external heat get in, it doesnt allow internal heat
gets out). But in this cases it?s always recomended to keep the roof
insolation high.

Good luck!

*Julia Chabrera*

Julia Chabrera's picture
Offline
Joined: 2013-04-19
Reputation: 0

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

hamnmegs at ozemail.com.au's picture
Joined: 2011-10-02
Reputation: 0

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

James V Dirkes II, PE's picture
Joined: 2011-10-02
Reputation: 203