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I have a large backyard and it has a structure that's basically the size of a small home. It is made out of concrete blocks and cement floors. So it has poor insulation and it has a low, exposed roof. So the structure gets very very warm in the summer even when its reasonably cool outside. The temperature difference between the inside and outside can easily be 10 degrees or more.

What are some of the ways that I can cool/ventilate this type of structure? Is it an A/C unit? Or maybe some exhaust fans to take out the hot air? Or bring the cooler air in?

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    @Kevin solar gain can easily do that, especially with a thin roof – Chris H Aug 16 '16 at 13:31
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    Not worth a real answer, but if you need a 'quick' solution for one time use, saturate the bricks with a hose. Literally spray the walls with tap water. Cinder blocks are somewhat porous and will absorb a little water. As that water evaporates, it will cool the dwelling slightly. We used to do this in westerly facing rooms of our house on ridiculously hot days. The difference was small but noticeable, – Levi Aug 16 '16 at 17:54
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    Hmmm this answer says "concrete is ... a great heatsink." and this answer says "Concrete is a very good thermal buffer". I'd expect at least one of those to be factually incorrect. ref – chux - Reinstate Monica Aug 16 '16 at 18:13
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    @chux functionally I think both're true. Concrete is a poor conductor of heat making it effective as a thermal buffer; but if it's exposed to heat long enough for it to penetrate anyway the high mass of a concrete wall means that it will be holding a large amount of heat and take a long time to cool down again. The net effect is similar to a large body of water. Your confusion may be that this use of heat sink - something that can soak up and hold a large amount of heat - is somewhat different than the behavior of the computer cooling component that tries to radiate it way asap. – Dan Is Fiddling By Firelight Aug 16 '16 at 18:30
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    Both statements say the same thing, we're both saying concrete is quite good at storing heat. That's because it is bulky and has a lot of fairly dense packed atoms. – Harper - Reinstate Monica Aug 16 '16 at 18:36

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There are many options, but concrete is very dense and makes a great heatsink.

You can try insulating and air-conditioning the inside, but this approach is costly, energy-intensive and overall wasteful.

If you have room, I suggest planting a deciduous tree between the building and the path of the sun. During the warm season, the trees leaves will shade the building and keep it from warming during the day.

In winter, the tree will shed it leaves and allow the sun to heat the building naturally.

Great method, but not always practical due to lack of room, etc.

A third method would be to plant vines on the sun-ward side of the house. This will also keep the sun off the walls and help cut down on solar heating.

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    It might take a while for the tree to grow big enough – Chris H Aug 16 '16 at 13:32
  • Some vines can fill that gap. Vine's weakspot is they can act as a ladder for vermin. Have to keep them in check. Take 'em out when the tree(s) is/are big enough. – JS. Aug 16 '16 at 20:44
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    If the structure is concrete, there isn't much worry of vermin. If worried, climbing hydrangea is fast growing, and extremely gentle to the structure, supposed its even safe to grow over shingle roofs. – ench Aug 17 '16 at 5:19
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The first thing you should do is insulate the roof. Even just gluing sheets of Celotex or your local equivalent will make a huge difference, though installing it in a proper ceiling would be better. This will reduce the solar gain from the roof. Depdning on the material, painting the roof white could also help quite a bit.

The walls will also allow a fair bit of solar gain. Shade from hedges or similar has been suggested and could work quite well; white paint is cheap. Insulation could work here as well.

If the outside temperature is reasonable, ventilation is probably your next step. A ridge vent would do a lot of good.

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To be clear -- Concrete does not cause it to get warm. Concrete is a very good thermal buffer - it resists changes in temperature. This is a highly desirable trait in a passively heated and cooled home. Because if the home is cool, it will stay cool despite the sun. If the home is warm, it will stay warm despite the cold. Insulation on the outside of the concrete helps it do this more effectively.

Solar rise is what causes it to get warm.

The trick is to attract the sun when helpful and repel the sun when unhelpful, and JS goes into some techniques.

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    Is not repelling the sun a bit more costly than regular AC? I mean... the sun is HUGE... – T. Sar Aug 16 '16 at 11:27
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    I'm lost on how one attracts or repels the sun, but I am impressed by your flippant use of doomsday devices in order to solve HVAC problems. – Sidney Aug 16 '16 at 21:50
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    What? You use a common off-the-shelf repulsor beam. Obviously this takes several hours, because like you say, the sun is huge, but the sun WILL go away if you are patient. Of course then it comes back again, not sure why. – Harper - Reinstate Monica Aug 19 '16 at 3:36
  • Concrete is not a good thermal buffer. It conducts fairly well; an 8" concrete block wall only has an R value of 1.1 – Sean Nov 23 '16 at 16:11
  • @Sean when I say "buffer" I mean concrete is good at storing heat, also known as thermal mass. That is a totally different concept than conductivity (the inverse of insulation or R-value). For instance the two concepts can be used together in passive solar design. – Harper - Reinstate Monica Nov 23 '16 at 22:43
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Add overhangs to your roofline, the roof should extend nearly 3 feet out from the wall to properly shade the concrete block structure in warmer climates. These can be simple canvas awnings that you remove in winter, or you can extend the roofline using more traditional building techniques.

Shade every window. If possible, replace windows with low-e glass that reflects most heat energy. Add drapes to prevent additional heat from coming inside. Add awnings if direct sunlight comes in the window at any time during the day. Shades should be drawn closed during the day.

Ceilings or roofs should be insulated to at least R-30.

If the ceiling is insulated and the attic is not, the attic needs to be very well vented. Add ridgeline, soffit, gable, and dormer vents. Consider powered venting to ensure the attic stays at air temperature, despite heating from the sun. While there are many types of venting, and some may have particular advantages depending on your location and wind patterns, you almost cannot have too much attic venting.

Use highly reflective roofing. White shingles or roofing sheets will absorb less heat than darker colors.

Trade all incandescent bulbs for LED (or CFL if LED is too expensive). Incandescent bulbs emit a lot of heat. Locate the hot water heater outside the cooled portion of the house. Make sure the dryer vent is clear and sealed along its path to the outside.

If on a concrete slab, remove wood and fabric flooring, and replace it with tile or simply refinish the concrete. The concrete should act as a heat sink to the ground, and without the insulating effects of many types of flooring it should cool the building.

Insulate the outside of the concrete block wall. As others have pointed out, the concrete blocks store a significant amount of thermal energy. By preventing them from getting hot during the day (insulation, shading), then blowing cool night air through the house overnight, they will be able to maintain much lower temperatures than they are right now. It's a problem of averaging - they maintain the average temperature of the building, but right now being exposed to sunlight all day the average temperature is very warm. By reducing sun exposure you bring the average temperature down. You may need to use a dehumidifier during the transition from cool night air to warm humid daytime air to prevent condensation depending on conditions locally.

Make sure your bathroom fans operate and vent to the outside, and use them during and shortly after all showers.

These aren't in any particular order, but I'd put ceiling/attic insulation, roof overhang/awnings, and windows at the top of my list if I could only do a few things. The best way to avoid a hot house is to avoid the heat in the first place.

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I'm going to assume that there is no ventilation from the top of the building since you did not mention any. If the building is warmer than the outside, simply opening a hole at the top and one at the bottom will create a chimney effect as the hot air rises up through the top and draws air in from the bottom.

You can enhance this by building a cupola on the building which allows for a large opening without allowing rain in. If that's not enough you could add a whole house fan. I would think the passive ventilation will work if you have a large enough of an opening. Just keep in mind you just as much air coming in as you want going out of the top. If it's a peaked roof you might want to put in a ridge vent.

Your heat gain will come either from any windows or from the roof. You aren't going to get substantial solar gain through concrete. For windows, shades will make a big difference. You can even look into ones that will automatically close when the sun is on them. You say it's a low roof and if it's really hot and not insulated, it will radiate down on you. If you were to add insulation, I would start with the roof.

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    When outside hot air is humid, the chimney effect can bring in moist air that is cooled and creates inside dampness with its benefits/problems. – chux - Reinstate Monica Aug 16 '16 at 18:20
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    @chux if the air is cooler inside, the chimney effect will be less significant. The air has to be warmer to draw. In a multi-floor building where the hot air in the higher levels draws air into lower levels that are not warm, you might have an issue (I closed off vents to a crawlspace for this reason.) For many centuries, people lived in large buildings in very hot and humid climates with no air conditioning. I can guarantee you that they did not button up the house all summer. Copulas have long been used for cooling houses in the summer. – JimmyJames Aug 16 '16 at 19:35
  • @RedGrittyBrick There's an edit button if you notice typos. Just sayin' – JimmyJames Aug 17 '16 at 13:22
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If you're in an area with low relative humidity, evaporative cooling is the way to go, particularly with poor insulation or leaky structure, since evaporative cooling air is once-through anyway. The home depots in California tend to use this method, for example -- it works surprisingly well, and lets them leave their rollup doors open all day. It's also used quite a lot in greenhouses and barns, and gets a lot of residential use in extremely dry areas such as Arizona.

Get or build a unit which uses commercial-grade cooling media that is several inches thick. Google for "Celdek" to see some examples. (Don't waste time and money on the aspen pad units -- they don't have enough surface area.)

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Plant a few trees to provide shade.

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In order to turn this into a comfortable space, you will need to insulate the structure. The most cost effective way would be to insulate it from the outside, and then cover it up with siding.

To start, you can use sheets of XPS (expanded polystyrene) foam to cover the entire outside. Tongue and groove sheets would give the best fit since there are fewer gaps. The XPS sheets can be attached directly to the wall using a construction adhesive which is specifically made for foam. You can't use most other adhesives because they will have a chemical reaction to the foam and burn holes through it. The foam will take some time to adhere, so you will need to use temporary bracing to hold it tight.

After the insulation is installed, you would need to cover it all up with siding to protect it. The foam itself is not designed to be exposed to the elements. Exposure to UV rays will break it down very quickly. To install the siding, you will need to install furring strips to nail the siding to. It would look similar to the image below. You can look at this site on how to install vinyl siding.

Siding Prep

The roof is another concern, you may need to have an entire new roof put on that is properly vented. You should take a look at Adam Davis' answer for tips on roof design.

Another thing that you may want to consider is waterproofing the inside of the building. Concrete readily absorbs moisture, and keeping down the humidity would make it much more comfortable. Coat the walls with waterproofing paint, and use an epoxy floor paint to seal the floor. This should be done after the siding is installed to give adequate time for everything to dry out.

After everything is done, you should be able to heat the space with a space heater, which would be enough to at least take the chill off.

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+1 Harper.

Stone houses stay cold because they are very heavy. Concrete is, on the contrary, not dense enough (actually not thick enough). We tend to add weight to these type of houses on the inside to absorb calories and cool down air during summer. Both solutions offered by JS are good, either you protect the building for direct sunlight (cheapest, but longer to be effective). Or outside insulation (bale of straw gives excellent results). If you have the time and the necessary space, building an inside mass stove is another good option.

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  • "Stone houses stay cold because they are very heavy." --> I'd expect the thermal mass to inhibit temperature changes, not stay cold. – chux - Reinstate Monica Aug 16 '16 at 18:23
  • How do you come to the conclusion that concrete blocks are not dense enough or thick enough to absorb significant amounts of heat. This shows concrete to have a higher thermal density than stone and I see nothing about the thickness of the blocks in the original question. – JimmyJames Aug 16 '16 at 19:52
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I didn't see anyone recommend Reflectix. This stuff is great at reducing radiant/solar heat gain. Some equivalent is in every commercial RV, as well as the van I converted. I think it helps a lot.

Since this is a building made of concrete, maybe aesthetics aren't your highest priority? If so, slap that stuff right on the roof and on the south-facing wall and see if it makes a difference. It's pretty cheap.

A vent in the roof is probably a great idea, too.

Have electrical out there? I'd try an upwards venting fan on your new roof vent with a window opened on the shady side of the structure, again to enhance the chimney effect.

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Cover the roof with solar panels. Keeps the heat out and produces electricity too. You could use it to run the AC in other buildings.

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