Canopy above the roof of a house
Last Post 05 Mar 2012 07:42 AM by EnergyWiseBuilding. 14 Replies.
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HueffenhardtUser is Offline
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22 Jan 2012 08:56 PM
I am interested in your thoughts about an idea I have. I have performed a few internet searches and I can't find if anyone has ever done this before.

My idea is to build a large canopy, slightly larger than the footprint of the house, several feet above the entire roof of a house. The point would be to put the entire roof of the house in the shade to keep it cool in hot climates. Air would be allowed to move freely between the roof and the underside of the canopy. Under high wind conditions, one might experience lift on the roof and/or lift on the canopy, but some engineering solutions might be explored to see if there is a way to mitigate that effect.

I am completely open about what material would be best for the canopy, whether it be solid or perforated, metal or fabric, etc. The main thing is that it provides shade for the roof. I realize that there are many strategies to keeping living spaces cool that are used from the surface of the roof to the underside of the roof and/or to the attic floor, but this one would simply be one more option to consider, that might even work better and might be worth the money. It might look funny over some house styles, but creative architects might find a way to incorporate the canopy design into the overall style of some new houses they design.

So, what are your initial thoughts to the idea?
BigrigUser is Offline
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22 Jan 2012 09:38 PM
What you are describing sounds like an overly complicated "cold roof". Usually it is a fully ventilated layer between the roof surface and your insulation. Look for some details, it solves your support and uplift issues. A separate structure as you suggest sounds expensive, short-lived and prove to animal activity (birds nesting).
HueffenhardtUser is Offline
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23 Jan 2012 11:25 PM
Thank you. I did not know about that type of a cold roof before.

I still invite more comments from others.
RoberthUser is Offline
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24 Jan 2012 03:58 PM
If you are looking for energy efficiency then I think there are better uses of your money. I think steel roof with an air gap would be a good option. The other things are to plan an air tight and well insulated home. Use and energy heel truss to provide high R value over the outer walls. Another feature would be deep over hangs.

I believe This New House did a show on a Texas house that used a metal roof and deep overhangs.
Dana1User is Offline
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24 Jan 2012 05:05 PM
Using high solar reflectivity moderate to high IR emissivity mateirals the heat uptake of the roof can be cut by quite a bit. The Cool Roof Rating Council compiles third party test solar reflective index (SRI) data on products designed & marketed as cool-roofs, with both initial & aged values, and has a searchable index here:

http://www.coolroofs.org/products/search.php

(Higher SRI == better heat rejection )

The Lawrence Berkeley Nat'l Labs developed a (freebie, downloadable) spreadsheet model that calculates the roof temperature and determines the SRI under a standardized set of air temp, solar radiation and sky temperature conditions (ASTM E 1980) if you specify the solar reflectance and emissivity of the material:

http://coolcolors.lbl.gov/assets/docs/SRI%20Calculator/SRI-calc10.xls

In cooling dominated climates a high SRI roof is very cost-effective on new construction (or at time of roof-replacement/repair).

A high-mass roof (concrete, tile, etc) with or without high SRI also reduces & delays the solar gain through the roof, and moderates the interior temperarure. That would probably be less expensive than a separate canopy structure, and can serve to make the building less susceptible to wind & storm damage. But it has to be designed-in for strucural reasons, and may not always be practical as a retrofit.

Another approach with good return in some situations is to cover the roof with photovoltatic panels, which effectively become a back-ventilated shade- a shade that powers your air conditioning. Where the initial cost is subsidized this can be a real winner. But at the rate with which PV panel and system costs are falling ANY new construction should at least consider siting & orienting the building & roof for PV. Even if the economics of installing PV aren't compelling this year, they will become so in a decade or less. (Even pre-wiring for PV makes sense in many designs.) Even though the albedo of PV cells is much lower than a cool-roof material (it absorbs more heat than a cool-roof, re-radiating some of that heat toward the roof), the shading factor and convective cooling of the PV array does a good job of reducing the temperature at the roof underneath the panel.

See: http://www.ases.org/papers/227.pdf

Sure PV makes for an expensive shade, but the payback will be faster & greater than separate canopy.

jmagillUser is Offline
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26 Jan 2012 06:48 AM
http://www.apartmenttherapy.com/popup-house-119850

http://www.trendir.com/house-design/sustainable-desert-house-design---recycled-reused-and-naturally-cool.html
Dana1User is Offline
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26 Jan 2012 03:55 PM
Nicely done, but different in scale from the "..slightly larger than the footprint of the house..." concept.

Price/performance relative to a mop-on cool-roof coating for a cargo can it doesn't work, but in both of those they are utilizing the rest of the shaded space similarly as wraparound porch/covered-patio features, which is a whole 'nuther factor. The total canopy area is a far larger footprint than the house proper (more than 2x), not just a little bit larger.
HueffenhardtUser is Offline
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29 Jan 2012 01:22 PM
Posted By jmagill on 26 Jan 2012 06:48 AM
http://www.apartmenttherapy.com/popup-house-119850

http://www.trendir.com/house-design/sustainable-desert-house-design---recycled-reused-and-naturally-cool.html

Awesome! Yes, this is pretty much what I was thinking about. I can't wait to read more.

And a big thank you to the rest of you as well. I value your comments. It seems there are a few options out there, each with their pros and cons, but I am glad to see that at least some people are trying out the canopy concept.
RoberthUser is Offline
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31 Jan 2012 01:06 PM
I dont see too many places this type of construction would be beneficial. Correct me if I am wrong but I would think it only work in areas that dont get very warm but the homes heat up during the day. I am thinking it would not be cost effective on the desert SW when night time temps stay in the 90s and days are 100+.

I would think a more cost effective solution is to spend the money on shell improvements.
Dana1User is Offline
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01 Feb 2012 04:17 PM
Direct solar gain is usually a very significant fraction of the sensible cooling load. The insulation only "sees" the difference between the surface temperature and the interior temperature. Under the shade the walls etc are all running near the air temperature. On a 100F day a composite shingle roof can easily see 150F, and even the sun-exposed walls can hit 130F. And while on a clear-night the radiational cooling under a clear sky can sometimes take the roof 10F below the ambient air temp the amount of "free" cooling is pretty small, and the low temp limit of roof temp is determined by the dew point- it goes no further.

But bang/buck, yes, the money is probably better spent on optimizing & improving the details of the building envelope.
RoberthUser is Offline
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10 Feb 2012 01:24 AM
Dana

I hope we can agree that the starting pint for any new construction is something that is very tight and well insulated. In doing that the value of a radiant barrier is not worth the cost. A canopy is a very expensive radiant barrier.

The Florida Solar Energy Center has said that radiant barier walls systems are not woth the expense. Radiant barriers in the attic were shown to have very small benefit in attics with R38 insualtion. So low in fact that it wasnt worth the expense.

Pick a construction method that makes sense for your climate. In a hot southern location get the HVAC out of the attic or raise the insulation to the roof. Large overhangs and lanscaping can minimize radiant heating of walls.
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10 Feb 2012 05:23 PM
Posted By Roberth on 10 Feb 2012 01:24 AM
Dana

I hope we can agree that the starting pint for any new construction is something that is very tight and well insulated. In doing that the value of a radiant barrier is not worth the cost. A canopy is a very expensive radiant barrier.

The Florida Solar Energy Center has said that radiant barier walls systems are not woth the expense. Radiant barriers in the attic were shown to have very small benefit in attics with R38 insualtion. So low in fact that it wasnt worth the expense.

Pick a construction method that makes sense for your climate. In a hot southern location get the HVAC out of the attic or raise the insulation to the roof. Large overhangs and lanscaping can minimize radiant heating of walls.
I think we DO agree that tight and well insulated is the right starting point.  But a canopy isn't exactly a radiant barrier- it's more akin to a super-ventilated convection cooled attic plus porch-roof.  For the canopy to achieve RB status is has to be very low-E on the underside to limit re-radiation of the heat downward, and either a similarly low-E top surface (or better yet, high solar reflectance high-E "cool roof" top surface, which outperforms low-E roof coatings.)


The canopy is definitely not the most cost effective way to keep the interior of the house cool. It has other aesthetic and comfort aspects that might be worth paying for, but from a net-present-value on energy savings for cooling the interior it's not a winner.

Both the Florida Solar Energy Center and Oak Ridge National Labs, as well as Texas A & M and the Berkeley Nat'l Labs have studied the radiant barrier/cost-effectiveness thing to death, but that never seems to stop vendors from hawking the stuff with outlandish claims.  The only time it has any significant return on investment is in attic retrofits where the ducts & other mechanicals are in the attic (stupid for new construction, as you point out) and only in cooling dominated climates.  And even then it's not always the most cost-effective upgrade to be spending the money on.  Under California Title 24 RB is allowed in lieu of cool-roof or more insulation to meet the prescriptive energy efficiency standard.  Cool-roof materials meeting the minimum SRI (solar reflective index) spec for the application are cost-neutral most of the time, which is why it heads the prescriptive list, and not RB (which is always a cost-adder.)  People still opt for the RB approach for visual aesthetic reasons, but most of the time there's a satisfactory cool-roof approach.

RoberthUser is Offline
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13 Feb 2012 02:10 PM
I totally agree that is not a radiant barrier but I think the purpose of the canopy is to shade the house and thus prevent the sun's radiant heat from reaching the home. I was trying to point out that shell of the house will need to be well insualted and air sealed regardless of the shade. In doing so the benefit of the shade will be reduced. Lower cost more conventional construction could achieve the same goal of reducing energy use and keeping the home comfortable.

As part of building green the goal should be to reduce reduce of uneeded materials. The embodied energy of the canopy would surely exceed the savings, not an energy wise choice. I was looking at some data recently regarding insulation. Whether it is spray or rigid foam it takes 40 or more years to save the energy that is embodied in foam with an r value of 20 or more. Why use that much foam when another insualtion has much less embodied energy and a return in months vs years. I think the same thing applies to the OP and the use of a canopy.
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28 Feb 2012 06:53 PM
I have noticed the cooling effect of such a structure as illustrated at the links above whenever I stop to get gasoline in the middle of the summer at one of those stations with the giant wing above the whole structure. It can be brutally hot outside but once you get under that wing there is always a cool cool breeze.
EnergyWiseBuildingUser is Offline
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05 Mar 2012 07:42 AM
What climate are you in? What you are describing is very common in South America and works very well. It helps with the very heavy rain they can get and allows the roof top to be used for other things, from a deck to drying cloths.

I like the system for the right application.
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