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Strategies for Managing Summer Attic Heat - Air Sealing and Insulation

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Join Matt Hoots of Sawhorse, Inc. and Amelia Godfrey, EarthCraft Program Manager for a conversation about attic heat, including why our attics get hot, why this matters, and strategies for managing air and energy leakage to keep homes and buildings cooler.  The two get to hang out in in Southface’s training cabin, which looks like a fantastic place to discuss building science!
Matt and Amelia discuss ways to think about energy leakage generally, but specifically in relation to attic heat - and hot temperatures outside.  When it’s 100 degrees outside, which is becoming more common in many places, it’s can easily be 120, 140 or even 160 degrees in the attic. These temperatures are of course uncomfortable for anyone who has to be in an attic, but they are also not good for our systems and not ideal for comfort in the rest of a building either.  



Why are attics hot? 

It’s important to understand that the old saying that that heat rises isn’t quite right, as building science experts seem to enjoy dispelling.   But we can say that hot air generally rises, as it’s less dense than cooler air, and gravity tends to pull cooler air down towards the earth.  
Air also moves from higher pressure to lower pressure. The process of air moving from outside of a building into the building at the lower levels and up through the roof is known as the Stack Effect, where air is pulled through cracks and crevices due to pressure differences between the outside and inside.  Wind also creates pressure differences and contributes to air movement into buildings. 
Air also moves from hot to cold, more than it moves up from down.  This movement is also a part of or at least related to conduction, a process thorugh which hear is transmitted due to a temperature difference where the material itself does not move.  As air moves along warmer surfaces to cooler surfaces, which are affected by conduction, this circular movement pattern is known as a convective loop.  Note than none of this air circulation means that the heat itself is rising, as heat is not an object as much as an amount of energy that can transfer to one object or another.  
All of these types of movement are part of what is known as convection, the process by which heat moves due to the movement of air or fluid.  Convection explains the transfer of heat where there is not movement.  

Radation also plays a significant role in why attics are hot.  Radiation occures when heat moves in form of energy waves, directly from the heat source to another materials.  It's worth knowing that all hotter objects radiate heat to cooler objects, even people.  Every person is literally affecting or is affected by their immediate surroundings.  In addition to rising and moving warm air and the pressure for the air to move out of the building, consider that the sun is baking onto most roofs, directly delivering heat to the framing and attic.  Most attics are vented passively, relying on hot air rising up through - but that movement of the hot air won’t provide cooling.  


Why is a hot attic a problem?

A hot attic is a challenge, because air moves, and it's a top goal to prevent envery transfers into and outside of the building envelope, as moving air means losing energy.  This includes air moving through any gaps in insulation into conditioned building space.  
Literally heating up heating and cooling equipment causes the systems to become hot and work less efficiently.  This equipment then costs more to operate and might easily have a shorter useful life.   



How does hot air move from the attic into the house ?

Air moves between spaces when there's a hole or gap to move through, and when there's a pressure difference which causes it to move from the area of higher pressure to the area of lower pressure, in search of balance.  Proper air sealing covers or "seals" potential crakcs and holes, giving air fewer or ideally no pathways to pass through.  
Air moving from high pressure to low pressure or from hot to cold will follow the path of least resistance, so gaps in sealing or insulation will likely lead to hot zones in the building.  But air sealing alone won't stop energy leakage challenges, as insulation is they key way we slow the heat transfer into our living spaces (or out of them in colder seasons).  
Top reasons for heat transfering (or hot air, shall we say, leaking) into conditioned or occupied spaces, include:
  • Poor Insulation
    • Insulation might be compressed, which lowers effectiveness, known as R Value
    • Insulation gaps let air right through 
  • Holes cut in the ceiling
    • Holes cut for recessed lights, ductwork, and ceiling fans allow air leakage movement by serving as a space for air to move through 
  • Unsealed or improperly sealed ductwork
    • Cracks in ductwork results in energy loss
    • Ductwork can also suck in hot air 


Hot Attic Solutions:


What can we do to reduce attic heat and protect HVAC equipment?  

  • For new construction, air sealing and insulation are essential from the start, at the roofline.  This means the conditioned, or controlled space will include the attic space, including any HVAC systems.   Even if it’s 120 degrees outside, it might be 80 degrees or so inside.  
  • insulation at the roofline is also a key consideration for existing buildings.  Insulation at the roofline may not be as easy as with new construction, but this retrofit project will slow the heat transfer “at the source.”  Roofline insulation means it’s less key to seal up all potential holes between the attic and the other floors, as these spaces will now share the same temperature, roughly.  
  • Some people also choose to “restart” and reapply insulation between the attic and conditioned space, and both insulate and seal at the same time. 
At a high and simplified level, of course we can’t say the same solution will work for everyone, as every home and building is different.  Even “insulation” as a solution varies effectiveness by the type of product selected.  All solutions will vary by priorities and budget, as Amelia reminds us, and it’s of course important to have a thorough understanding of the project and safety before undertaking any work yourself, and to work with a qualified professional when you’re hiring someone.  
It’s important to understand that options are available at different budget levels and for different project types for cooling attics and/or protection the energy efficiency and life of our HVAC equipment.  According to Green Builder Matt Hoots, if someone can try one project or strategy, air sealing delivers the best results for the effort and investment.  Air sealing and insulation can both provide a terrific return and don't have to invovle major construction.  That said, the work must be done properly.  If not, air will find a way to sneak around and through any gaps. 
It's worth noting that air leaks and heat transfers in both directions...  The same air selaing and insulation practices that help prevent heat loss in the winter can help with heat retention in the winter!
About Southface: 
Southface Institute is a sustainable building nonprofit that strengthens equity and the environment by transforming residential and commercial structures at every stage of the building life cycle. Southface is headquartered in Atlanta, Georgia, with a satellite location in Sarasota, Florida. Southface collaborates with other nonprofits, businesses, builders, developers, universities, government agencies, and communities to deliver practical solutions with tangible results for the planet and all its inhabitants. 
Check out Southface's awesome pre-recorded, FREE webinars on building science topics!




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