Having a double roof system can help create a much cooler house. A double roof can significantly reduce the heat gain in your home through the roof. Hence, this lowers your indoor temperature and reduces the need for air conditioning.
(Note in this article, the term double roof refers to using two roof layers as a climatic response to hot climates. It does not discuss the roof structure type, also known as a double roof, where rafters are on intermediate purlins.)
What Is a Double Roof?
A double roof consists of two roofs with one directly over the other. The upper roof protects the lower one from direct sunlight.
This type of roof system is excellent in hot climates to reduce heat gain passing through the roof. Hence, creating cool house interiors.
Reasons For A Double Roof
The roof of your home is usually the area that receives the most direct sunlight.
Depending on the time of day or of the year determines how much direct sunlight a wall receives. Some walls can receive little or no sun because of their orientation. However, your roof receives direct sunlight for almost the entire day. This factor depends on the roof’s shape and style.
In the tropics, this is more evident since the sun is more overhead than in the regions further away from the equator. Because of this, roofs in tropical climates receive the most sunlight and the most intense heat.
Therefore, shading your roof from the direct sun is an excellent way to reduce the amount of heat that enters your home. Hence, it creates a house that is cooler and more comfortable.
How Does A Double Roof System Work?
A double roof system consists of two roof layers or skins. They are typically two separate structures.
The upper layer is usually lightweight. It can consist of a timber or metal frame with wood decking and a metal sheet covering. The roof below can be similar in construction and materials. However, they are often massing materials like concrete.
Each roof layer has an air gap between the two of them. This air space is critical as this is what prevents the sun’s heat from passing through to the inside. In addition, the air space between the two layers should have an air space for ventilation. Therefore, heat from the upper roof passes through to the air space below it. However, the hot air in the space can quickly dissipate before the heat reaches the lower roof level.
The space between the two roofs can be a few inches or a few feet. There only needs to be enough space and air movement to prevent the heat from the upper roof from reaching the lower one.
A study published by Elsevier shows that the double roof system reduced heat flux by approximately 50%. In other words, about half of the heat energy did not pass through to inside the building. In addition, they also show that when insulation is part of the lower roof, there is an 85% reduction of heat flux from the original roof structure.
Imagine how much cooler your tropical home could be with that level of heat reduction.
Advantages and Disadvantages Of A Double Roof
Makes Your House Cooler
In hot tropical climates, reducing the heat in your home or building is paramount. This focus ensures that you and the other people in the space are thermally comfortable in their surroundings.
Shading your roof from the direct sun is an effective strategy for reducing the heat in your home. Along with other shading strategies and passive ventilation, this will help you reach thermal comfort more easily.
This system can reduce or even eliminate the need for air conditioning. Hence, it provides the benefits of lowering your energy consumption and bill and being better for the environment.
Reduces Heat Gain On Thermal Mass Roofs
Using a double-skin roof construction is particularly useful where you want the strength and durability of a concrete roof. Protecting the concrete slab with a lightweight roof layer above will stop your concrete slab from collecting and storing heat during the day. Hence, it will not have heat to radiate into your home at night.
The P24 House in Bangkok, Thailand, uses this strategy. OFFICE AT, the architects for the house, uses this system to “cover the house from the tropical heat”. They do this while creating a stunning tropical modern home.
P24 House by OFFICE AT Co., Ltd.
Using Photovoltaic Panels To Provide Shade
In addition, though I do not see a lot of this occurring, I wonder if using a second roof framing structure above to accommodate photovoltaic (PV) panels could not be a helpful strategy. The PV panels can therefore provide shade to the roof level below.
Incorporating photovoltaics and other renewable energy strategies as part of your house design and architecture is ideal. Hence, I wonder if using PV panels in this way could have its benefits.
Cost Disadvantage
As you can imagine, having two roofs will likely increase your construction cost. Depending on the construction type, double the roof could possibly end up being almost the price of two roofs. As such, this needs careful consideration when choosing this system.
However, try considering ways to reduce the quantity of material as much as possible. In addition, think about how the offset of your energy bill factors in your budget.
A double roof system is worth considering if you are serious about making your home cooler. It is an ideal way to improve your home’s thermal comfort through tropical design.
Featured image: P24 House by OFFICE AT Co.
Photographer: Ketsiree Wongwan
hi there
your comments re solar design are helpful; however, it is very generic, and what is missing are any discussion of relevant specific design tools/techniques (except on double roofs), such as solar chimneys,stacked ventilation, passive & active ventilation, etc. even on double roofs, you do not discuss roof pitch, or additional active adjuncts.
Hi Dec, thanks for your comment. The article on double roofs was meant to overview the topic. In addition, some of the topics you mentioned, like stacked ventilation and passive ventilation, are covered in other articles. Pitched roofs are also covered in other articles.
However, I would be happy to cover the other topics you mentioned. I would also be happy to cover any other topic you think would be helpful to readers or address any other concerns you might have. Feel free to reach out to me at hholder@architropics.com. Thanks once again.
PS additionally, you state that having a double roof (skin) would “double costs”. this is manifestly untrue. it would not (being a single lightweight structure/skin), likely even be the same cost as the primary roof.
Hi Dec, you are correct that a double roof system will not literally double the cost of the roof. When I said, “double the roof could mean double the price, ” I did not intend to mean that it would literally double the roof cost. However, it would have been better for me to say that it could almost be the price of two roofs. As such, I have edited the article. Thanks for highlighting it to me. Greatly appreciated.
Thank you for presenting the genius of the double roof! Love your work.
The hottest topic in housing in the tropics is cool roofs. Sadly many are on the wrong track with expensive roof coatings.
The simplest, easiest to build and lowest cost is the double roof.
Build a free standing roof not connected to the home with 4, 6, 8 metal poles and a simple pitch roof design with metal, fibre concrete or synthetic resin sheets and build a simple box home below. The genius is very little sun will hit the home to heat it up. The home below can be made very cheapy because it doesn’t need protection from direct rain.
And if want to cool the home even more build it on stilts 1, 2 or 3 feet off the ground. The crawl space below means no need for slab foundation another cost saving, allows air to flow below and protects the home from floods.
Double roofs, simple, cheap genius solution so few think about.
Keep up the great work.
Warmest regards
James
Hi James, Thanks for your comment and kind words. I am also surprised that double-roof systems are not done more frequently. There are not many applications out there, and I think that is unfortunate.
As you mentioned, raising a home on stilts is also a good strategy with many benefits. It was seen in many traditional homes across tropical regions. But again, they are not a lot of contemporary examples today.
P.s. I like your use of “The hottest topic…” 🙂
indeed, James.
clever pun, but sadly inaccurate.
some additional points
1. i am not sure reflective roof coatings should be considered “expensive”
indeed, it seems obvious to me that both a double roof AND a reflective coating should be used together
(btw, digressing, what is another name for a ‘double roof’? i have a previous word in mind, but it escapes me.)
2. i think you greatly over-simplify the construction of the ‘over-roof’, and consider the inherent lifting wind forces under such a roof in times of storms and cyclones/typhoons.
3. Ditto the under-roof, albeit with different/reduced requirements
4. Not sure 1/2/3′ counts as ‘stilts’, but i may be pedantic 🙂 but the airflow under is useful in the climates we reference.
on this point, ‘proper’ stilts or pole construction at a 2.4m or such height (to meet building standards), or the equivalent construction adds two, no three, other significant advantages –
– highly valuable usable underneath space (of course this should not be built-in!)
– the additional height should/would mean increased wind flow through all three house levels
– the increased height would normally be expected to add to the outward aesthetics ie views
5. btw, i note in various research, that in this and in roof gaps and similar, eg trombe walls, a much smaller gap than you might think is most effective/efficient, basically because of an increased venturi effect i think.
and on this points, two additional points –
6. from memory, and from intuition, and related window size theory, i believe that the roof gap should be designed to maximise the venturi effect
7. in combination, with a point that seems to be sadly missed in these double roof designs – a top air vent to sent hot air AND in the process greatly increase air flow
8. there is also research that outlines the optimum/non-optimum roof angles to maximizes airflow
9. it seems to me that it is more optimal, if any roofing insulation is envisaged, that that should be on the lower roof, but i am open to persuasion to this, not having thought it through definitively. One factor, to my mind, is to enable the upper roof to be more light weight in construction.
10. unfortunately, it seems that any thought of using ‘thermal mass’ in these climates is of no applicable use. eg, i noted somewhere to get any geo-thermal cooling effect at all, you would have to go down more that 6′. from memory, there was negligible difference at 6′. And thermal mass inside the structure would be counter productive. So different to continental climates.
11. finally, it is source of constant frustration to me, here in Vietnam, that, almost without exception, Vietnamese houses slavishly follow generic French (European) house design, ie massive concrete and small windows. so eminently suitable for such a typical tropical climate. if i had another lifetime, my ambition would be change that.
anyway, just some random thoughts i threw together
kind regards
don
Hey Don,
Thanks for your comments about the double roofs. You have made some interesting and excellent points.
I am really happy for your feedback on the topic. The topic of double roofs is one that I do not hear many professionals in the industry talking about so I really appreciate the input from you guys.
Regarding your point 9, I would agree that if any insulation is necessary, it would be on the lower roof. The research that I looked at suggests this as well. However, my thinking is that it may not be necessary at all if the space between the two roofs is wide enough. If the upper roof of the double roof system does its job of protecting the lower roof from direct sunlight and there is a space between the two roofs to let any heat under the upper roof escape quickly, then not much heat should transfer to the lower roof.
I have recently been thinking that a lightweight roof as an upper roof is better. Additionally, I typically do not prefer concrete roofs because of their thermal massing properties. However, I think they would be fine once protected by an upper roof. This way the homeowner gets a cooler roof while benefitting from the strength of a concrete roof in strong tropical hurricanes/cyclones/typhoons. I think it could make a great double-roof assembly.
I am also concerned with homes in tropical regions that have the European style of large, unshaded concrete walls and small windows. It is a practice that our people have become familiar with. But I wonder if this is further reinforced by the idea that typical window styles and sizes are based on suppliers from temperate regions.
However, I really do appreciate your comments and the conversation. It is a topic that I am very keen on. Thanks once again.
Best regards,
Hugh
Interesting article.
While living in Thailand I became interested in building techniques utilized there that contribute to a cooler house.
While a stacked tower traditionally vents hot air it is expensive to construct. More commonly in the provinces you see a double roof with a large gap between the layers. The first roof shades a large area containing the house and yard. This works very well to provide cooler temps in and around the house. Usually it is made of steel. It can’t be too expensive or these farmers would not build this way.
Dan
Hi Dan,
Thanks for your comment about examples of double roofs in Thailand. I would love to see some examples if you have any images. Feel free to email me at hholder@architropics.com.
Regards,
Hugh
Don brought up a very interesting point that reflective roof coatings is cheap.
If this is the case then no need to build a double roof house. Cheaper and easier to build a single roof with reflective roof coatings that will be as cool as double roof house but with lesser cost and complications.
If a single roof home with an insulated roof is as cool as a double roof house I am not sure why Don wants to build a double roof house and insulate the lower roof.
Why the added cost and complication? That’s like wearing a belt and braces at the same time,
Don also brings up building a house in a cyclone/typhoon area. Well if someone lives in a cyclone/typhoon area if they wanted a cyclone/typhoon proof house they will have to spend a lot more whether they live in a single or double roof house.
So if they wanted a double roof house in a cyclone/typhoon area they would have to as Don correctly points out build a double roof that is cyclone/typhoon proof but the house below also has to be cyclone/typhoon proof. But since Don says reflective roof coatings are so cheap then why even bother thinking about a double roof? Surely its easier and cheaper to just build a well insulated cyclone/typhoon proof single roof house that is just as cool using cheap reflective roof coatings.
I was also saying raising a house 1,2 or 3 feet will allow air to flow below and keep the house cooler.
I use the term stilts but the correct term is actually pier construction as opposed to a slab foundation.
I guess pillar might also be a suitable word beside pier.
Also adding a central court yard in the middle of a house like an ancient roman villa will also keep a house cooler especially with a fountain in the middle.
The main point I was trying to make is you can have a very cool house if you throw a lot of money at it.
But a very simple cheap double roof yes Don because of you I will have to add if you don’t live in a cyclone/typhoon and/or earthquake and/or flood zone will help with a cooler house if budget is tight.
As Don will correctly point out if you live in a cyclone/typhoon and/or earthquake and/or flood zone then you will have to build a cyclone/typhoon and/or earthquake and/or flood proof double roof with all its inherent extra cost!
The simplest way to see the benefits of a cheap double roof construction is parking your car under the sun and parking your car under a car porch or shelter.
I guess if people prefer they can insulate their car roof and if they also wanted to insulate their car roof and also only ever park it in an underground concrete basement car park.