Home Reviews

Cooling Off A Hot Attic

Cooling a hot attic
Your attic is hot! Here’s how to cool it off

Most of the U.S. is broiling under a heat wave at the moment. From the Midwest to the East Coast a sweltering parade of 100-degree days seems to be marching across the country without end.  Severe weather is the norm these days, and when times are hot like they are right now, it’s nice to have a cool home to retreat into. But if the interior of your house is roasting too, you’ll be glad to learn there are some hacks that can help with cooling it down. Let’s start with cooling down your attic.

What is an attic?

An attic is an uninhabitable space in your home that lies directly below your roof and above the ceiling framing of the top floor. On homes with tall, steep-pitched roofs, the attic spaces can be very large and voluminous. Homes with low pitched roofs will have very small and cramped attics.

Attics spaces are there to provide access to these areas for inspection, maintenance and service. Building codes require access to attic spaces but deem them uninhabitable because they lack things like insulation, floors, escape windows, climate control, fire resistance and other basic elements needed for safe human occupancy.

Conditions

Cooling a hot attic
A clean, empty attic space

Since attic spaces are not inhabitable, they are usually left rough and unfinished. Attics spaces contain exposed framing, insulation, roof nails, electrical wiring, plumbing pipes, and heating ducts. On rare occasions a conscientious builder might have installed a bare bulb work light and some planking to traverse across the ceiling joists safely.  That’s about it for attic amenities.

Attics are required by code to be passively ventilated to the outdoors, meaning outside air is encouraged to flow through the attic freely. If properly designed, fresh air will move in through screened openings around the eaves at the perimeter, then out at the upper areas of the roof through the natural movement of convection. Convection basically means hot air rises.

Since air flows freely through the attic, pollens, dust, smoke and any other pollutants in the air migrate in as well and accumulate onto the interior surfaces. Any insects and animals able to find their way through the screened eave vents or any openings in the siding will also take up residence there. Because of all this, an attic space is a place most people prefer to avoid.

Heat generation

Under ideal conditions and with perfect ventilation, air would naturally flow through the attic at a pace that would keep the space very close to the same temperature as the outside air. This would be similar to how your house would feel if you kept the windows and doors open all the time. On hot days the rooms would be hot and on cold days the rooms would be cold.

In reality the attic space is much different than rooms in your home.

Inside your home, the outside walls and ceilings of the rooms are insulated, and the wood framing is covered with drywall. In the attic there is no insulation from the exterior (roof) and no drywall on the framing. This allows much more heat gain in the attic as compared to the interior of your home. The reason is that heat is transferred into the attic from the sun as well as from the temperature of the outside air.

Radiant heat from the sun

It works like this. The attic starts out the day with a volume of air that is about the same temperature as the outside air. As the day progresses, sunlight radiates down on the home heating up the surface of the roof.  As the roof surface temperature rises, this heat transfers (conducts) down through the sheathing and rafter framing.

Thermodynamics and heat gain

Now the roof structural members and sheathing in the attic space are hot and they begin to transfer heat to their surroundings. They do this in a couple of ways. 1) heat radiates from the roof framing down through the attic and heats up anything it comes in contact with including stored boxes, the ceiling framing, furnace ducts, wiring, ceiling insulation, dust, dead rodents, etc.  2) All the hot items in the attic (framing, sheathing, toys, boxes, insulation, dust etc.) now begin to conduct (or transfer) heat to the surrounding air molecules that come in contact with it. This all happens fairly quickly.

The process described above produces heated air much faster than it can be exchanged through ventilation. The result is an extremely hot attic.

The sun’s radiant heat is the engine for all this, and the process continues as long as the sun is shining on the structure. Even after the sun goes down, the hot items in the attic will continue to radiate and conduct heat keeping the air inside warm long after the outside temperature has cooled down.

Convection

Once the air in the attic has become hot, another thermodynamic process called convection occurs.  Convection is the characteristic that makes hot air rise and cool air sink ,and it is at this point where a good ventilation system comes into play. As the air inside the attic becomes hot, it will rise to the highest point in the roof and escape through any vents placed there for that purpose. (Continuous ridge vents are good for allowing hot air to escape at the place it collects.) As the hot air escapes out the top, cool air will be sucked in at the lower eave level to replace it.

Unfortunately, radiant heat causes the temperature of the attic to rise faster than the air can be exchanged through passive ventilation (convection).  Large mechanical fans aiding the ventilation process may help some (by moving the hottest air out faster), but that will not prevent the radiant process from continuing to elevate the temperature of the entire attic structure and all its contents.  In other words, faster air exchange (convection) may reduce the temperature of the attic air, but that will do little or nothing to cool the structural elements and contents because they are continually being heated by the radiant process.

So what’s wrong with a hot attic?  

A hot attic it makes for a hotter house. This results in higher cooling bills and the need for thicker insulation above the ceiling. Another problem is any ducting in the attic will be heated and the heat will transfer into any the duct and warm any cool air moving through it and into the home. Also, any equipment in the hot attic will have to work harder to do its job and thus the life will be shortened. No heat sensitive items can be stored in a hot attic.  You will be tempted to purchase an attic fan.

So how can you make a cooler attic?

Step one: Ventilation

Thanks to the radiant heat affect, attic temperatures on a 100-degree day can reach 170 degrees or more. That is certainly too hot, but how hot is OK? At the very best, conventionally ventilated attics can never be cooler than the outside air temperature. This is because the attic is essentially open to the outside air.  If the air outside is 100 degrees in the shade, your attic cannot ever be cooler than that. But even 100 degrees in your attic would be much better than 155, so how can we move the temperature in that direction?

Passive venting

The easiest way to start is to maximize the passive ventilation in your attic. Make sure you have the most ventilation your home can handle. Code requires 1 square foot of vent opening to 150 square feet of attic space. I personally don’t feel that is near enough because the total volume of air that needs to be exchanged can vary quite a bit with the pitch of the roof. In addition, locating the vents is just as important. You will need as much square footage of vent at the apex of the roof as at the perimeter eaves. Put in as much as you can.

Mechanical fans

Cooling a hot attic
Attic fan

If you plan to use an attic fan to exhaust hot air out of the attic, make sure you have enough incoming air vents around the perimeter to accommodate the extra flow. Otherwise you can create negative pressure in the attic that will suck expensive conditioned air out of your living area.

Good ventilation improves the convection process. It helps to move some of the overheated attic air out and replace it with fresh “outside” temperature air.  But this is not enough. Now we have to deal with the cause of the heat gain.

Step two: Radiant barriers

Once you have mastered the ventilation problem, the next step is to install radiant heat barriers. These barriers will help to reduce the heat gain created by the sun and prevent it from radiating from the roof throughout your attic. Radiant barriers are very effective in hot climates and especially effective when heating/cooling air ducts are located in the attic. Studies have shown these barriers will reduce a/c cooling costs 5% to 10% in hot, sunny climates. The reduced heat gain may even allow for a smaller air conditioning system.

Installation

Installation is fairly easy when building a new home. Simply drape the barrier between the roof rafters. Placing the material just under the sheathing will minimize dust accumulation on the reflective faces. Retrofitting barriers later can be more difficult. Space is tight when working in the attic and there will likely be framing, braces, wiring, plumbing ducts and other obstructions to work around.

It’s important to allow an inch or so of air space clearance between the attachment points and the bottom of the roof. The radiant foil will heat up, and the air space will allow the heat to migrate up and ventilate out. A couple of safety notes: Radiant foil will conduct electricity, so avoid making contact with bare electrical wiring. The foil may also interfere with cell phone reception. Please refer to the reflective insulation trade association as they offer other information and excellent installation tips.

Radiant barriers create invisible shade

What the reflective barriers do is halt the heat from the roof area from radiating down to the other interior areas and contents of the attic. The heat stops at the upper reaches of the roof framing. The barriers shield the area below that would normally themselves become heat sources (ceiling joists, cross braces, equipment, boxes with Christmas decorations and ducting etc.) that would then conduct heat to the air and make the attic hot.

cool a hot attic
Attic radiant heat barriers installed

As discussed above, these solutions work. Radiant barriers will immediately reduce temperatures in your attic. The improvements will be significant, perhaps 20-25% lower than temperatures prior, but as mentioned earlier the temperature in your attic using these methods will never be lower than the outside air temperature. If the outside temperature is 110 degrees in the shade, your attic will always be hotter than that.

 

Cooling your attic
A very hot 170 degree attic

A conditioned-space attic…the ultimate solution

If you want a truly cool attic, you can convert it to conditioned space. I have used this option personally several times and highly recommend it. It requires some work and has a cost but it is worth it.  A conditioned-space attic will keep the entire home cooler in summer, warmer in winter, while prolonging the life of any attic mounted equipment, and improving air quality in the home.

Creating a conditioned space attic means sealing up all eave/roof ventilation openings, removing the insulation between the ceiling/attic space then adding foam insulation to the underside of the entire roof system down to the wall plates.  This turns your roof and attic into a kind of thermos and the air in the attic area becomes (conditioned) the same temperature as the interior of the home.  This idea is not new but some building departments are not yet aware. Check the building codes in your area.

The key to a successful conditioned-space attic is to heavily insulate the roof diaphragm and completely seal it.  Any air allowed to migrate will eventually find a cold place on which to condense and cause mold/dryrot. This sounds simple enough, but the frame of a highly complex roof has many places that might allow air to pass through.

High-tech foam

The solution that makes the conditioned space attic possible is spray foam insulation. Foam insulation is a chemical product created by mixing isocyanate and polyol resin. When these chemicals come together they form millions of tiny air bubbles and quickly expand up to 60x. The bubbles in the foam create extremely high insulation qualities per inch and the finished solution blocks the three primary forms of heat transfer, including radiant heat. Simultaneously it fills and seals any voids in and around the framing and allows no air/moisture filtration. This is a perfect storm solution for roof applications. 

cool a hot attic
Spray foam insulation

On my projects, I like to use a closed-cell foam product blown on by hydrofluoroolefin (HFO) to avoid any environmental or global warming impacts. Closed cellfoam provides a higher R-value per inch and is completely air/moisture tight.

With a conditioned-space attic there are no vents to the outside air. The removal of screened vent openings eliminates the infiltration of dust, smoke, pollens, insects, rodents and birds as well as heat and cold. Eliminating dirty air migration from the attic greatly improves air quality in the home.

Summary

Designing a cool attic can be tricky and requires a basic understanding of thermodynamics and how air and moisture migrate through a home. The combination of sufficient ventilation and the ability to control radiant heat gain are keys to success. Before you go out and spend money on fans or barriers take time to study your roof and attic to come up with a reasonable plan. Using this guide will help you have a cooler summer.

Why attic fans don’t work well and what to do instead!

With the summer season upon us, now is a good time to review an uncomfortable topic. Hot attics, and why traditional exhaust fans don’t work well enough to cool them off!

A very hot problem

Last week we had some warm weather for the first time this year. One of our clients called to say that their second floor was extremely hot and requested we install a larger and more powerful attic fan. The client had decided a new fan was needed to replace a solar unit they had installed several years ago. The solar fan  “didn’t seem to be working as it should.” Compared to the rest of the house, the second-floor rooms were extremely hot and impossible to live in during the afternoon and evening.

Attic construction

Why attic exhaust fans do not work
Attic exhaust fan

On inspection, I found the villain in the attic exactly as the customer described.  A single, roof mounted, solar powered attic exhaust fan. It was working.

There was no other ducting or HVAC equipment in the attic. Insulation was a 6”-8” layer of blown-in fiberglass on the top of the ceiling, plus random pieces of R-19 fiberglass batts tossed askew atop the blown-in material, but not fully covering the space.

The 2×6 roof rafters with 4” skip sheathing, and ½” plywood shear was all fully exposed to view. 6”x24” eave vents were installed around the perimeter every 8’ and vents were clear and unblocked by the insulation. The temperature of the roof sheathing inside the attic was 147 degrees. The 45’x24’ (approx. 4500 cubic foot) attic space was very hot.

I explained to our client that the idea of attic fans to remove hot air from the attic area might seem logical. But they actually might need a bigger and better solution

Anatomy of attic heat

Imagine yourself laying in the sun on a beach in Cancun. All is good for about 5 minutes then you start to feel hot. You go and get a big fan and set it up to blow air over you. Ah, feels better for about another 5 minutes. Soon even the blowing air feels hot, so you exchange the fan for an air conditioner and let that blow cold air on you. Feels great now so you stay out in the sun till you notice that your skin is red and blistered and burned to a crisp.

What happened? You got zapped by UV rays, a form of invisible radiant heat! It’s why you can still get a terrible sunburn even on a cold or cloudy day. Radiant heat zips right through cold air without any affect and cooks your skin. So, what does that have to do with your house?

Attic exhaust fans
Radiant heat transfer

The roof structure of your home is like your skin. Radiation from the sun heats the surface of the roof. From there, the entire mass of the roof (roofing, roof paper, nails, sheathing, rafters) warms up through conduction. Soon the roof’s structural mass is so hot that it will radiate heat on its own (like the sun). This radiant heat passes down through the attic space and hits the material on the surface of the ceiling structure (insulation, wood joists, drywall, ducting etc.). Very quickly the entire mass of the ceiling structure also becomes a giant heat radiator. This heat moves back up towards the roof again and will continue to radiate heat well after the sun goes down. If your ceiling is not sealed airtight and extremely well-insulated, much of this heat is going to radiate downward into your home too.

Hot air

Air in the attic that comes in contact with the surfaces of the hot framing, will become heated through conduction. This creates a kind of hot air sandwich formed between the roof and ceiling structures. The warming of all these surfaces is why the attic interior gets so much hotter than the exterior temperature.

Ventilation

Trapped air and heat in the attic would cause problems for some roofing materials. It could also cause  condensation leading to mold issues. This is why attic spaces are required to have ventilation built into the structure. Air openings are placed along the lower eave areas and the upper roof ridge or gable end. These openings allow fresher and cooler outside air to circulate naturally via convection up from the eaves and out at the gable or ridge vents.

Mechanical fans

This is where an attic fan can help. A mechanical fan working in an attic space equipped with adequate ventilation openings can move hot air out and increase the flow of air through the attic. This will indeed help reduce the temperature in the attic, but only marginally unless the incoming air from outside is substantially colder (not likely in the summer till nightfall) You also need to exchange the air quickly. For instance, if the attic is 150 degrees and you circulate air through it that is coming in at 110 degrees from the outside, then you can expect that the cooling effect may bring the attic temperature down to perhaps 125-130 degrees. The faster the air changes, the closer you can get to the incoming air temperature. But you cannot get lower than 110. There are formulas for this that engineers use in determining heat transfer and the proper amounts of air flow needed.

The problem with attic fans is in how they are marketed. Homeowners need to know that fans can only be effective if sized properly and mated with the proper amounts and placements of ventilation openings. Even then, they will not “cool” a home in the way most homeowners understand the term. Fans will only lower temperatures a bit in the attic, and will require some power to do so. Still, a little improvement may be better than nothing. If you consider using an attic fan, you will need to also decide (as with any solution suggested here) if the benefits are worth the cost in installation and operation.  This will be a topic for another article.

Equipment and ducting suffer greatly in a hot attic

A hot attic space is bad enough. But if you have equipment, ducting or piping up there, working in elevated temperatures, you can expect a stressful and likely shortened life for those items.

If you have ductwork, furnaces, air conditioners, water heaters, water piping etc. in your attic, it would be a good consideration to:

  1. Move the ducting and equipment to the crawlspace or the interior of your home.
  2. Meticulously seal seams and add reflective insulation around your ductwork
  3. Convert your attic to a conditioned space. Do this by moving a radiant barrier and insulation  to the sloped roof assembly. Spray foam insulation makes this possible but you will need to create and follow protocols to avoid trapped moisture and ventilation issues with the existing roof framing. 

The point here is that if you have a hot attic space, it is going to shorten the life of any equipment residing there.

Fan problems

Even though an attic exhaust fan can incrementally lower the temperature of a very hot attic, using a fan does not stop the source-radiant heat. During the day, any cooler air brought in by the fan will be heated up immediately by the surrounding structure. Most fans cannot keep up. At night, after the sun’s radiation source halts, the structure will continue to be hot for some time.  Any cooler air brought in from outside will eventually lower the the attic structure temperature, but that will happen VERY slowly. As soon as the sun rises in the morning, the radiant heating process will start again.  

In the example of our client’s solar fan above, the unit was way too small to have an effect. It was only rated at 1000 CFM meaning it would take forty five minutes to replace all the hot attic air with outside air just once. It could not keep up with the radiant heat gain.

In some worst-case instances, attic fans can actually create more problems than they solve.

  • If there are not enough soffit, gable or ridge vents, a powerful attic exhaust fan can pull the air from your home through the ceiling if it is not perfectly sealed off.
  • Strong attic fans can actually back draft furnaces or water heaters by pulling combustion gases out of their burners and into the home.
  • Good attic ventilation is excellent for preventing moisture and condensation but it is usually not enough for cooling in the summertime.

A retrofit example that works

I believe the best solution may be a combination of radiant barrier to stop the sun’s heat from warming up the attic structure to begin with along with proper ventilation, attic exhaust fan(s), insulation and air sealing.

1. Seal off any air leaks at the ceiling. This prevents air/dust movement from living space to attic. This will also prevent excessive moisture from migrating in and out of your attic.

2. Install additional insulation to bring the total to over R-30 everywhere-even over the access hatch. Cover the tops of any wood ceiling joists by at least 3 inches.

3. Make sure the eave, gable and ridge vents are wide open and unobstructed.

4. Install radiant barrier foil between the rafters. You will need to allow the foil to “sag” at least an inch below the sheathing to allow an air space. You can get radiant barrier foil from Amazon here.

A reader Ken from Tennessee sent in his experience:

On my attached pictures I’m showing around 30 degrees on the sunny side and only 3 degrees on the shaded side.. I have a ridge vent and roof edge vents, because the contractor forgot to put soffet vents onto the house. Pictures were taken on December 12 2015, in eastern Tennessee. No attic fan.

Southwest “sunny” side of roof no insulation 105 degrees 12/15
1 rafter bay over with radiant barrier 71.9 degrees 12/15

On the shady side of the roof, the results were as follows:

Northeast “shady” side of roof 78.9 degrees
Northeast side with radiant barrier 74 degrees

Summary

If you your house has a hot attic and ceilings during the summer, the solution is a system and not simply a powered attic exhaust fan or ventilator. To eliminate a hot attic you should consider a plan that includes radiant barriers and proper ventilation This is always best done when the home is under construction. Once constructed it may be difficult or impossible to retrofit this without major work to the structure. 

What you must do is prevent heat from migrating down into the home. Your plan will likely require a completely sealed ceiling, a very thick layer of insulation, radiant barriers (reflective foil layers) above the insulation (preferably between the rafters) to block the radiation and isolate your hot attic from your cool house, additional ventilation openings and possibly a powered attic fan to remove warm air from the attic at the proper exchange rate.

Consider another, even better solution, the conditioned-space attic. Our client from above eventually opted to create a conditioned-space in their attic using a foam insulation applied to the pitched roof. This created a completely cool attic, dramatically reduced power bills and easy to maintain temperatures in the second floor rooms. Although their plan cost a bit more, this solution delivered a totally cool and comfortable second floor.  I will publish more details on this option later.


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