When using traditional insulation in an attic, ventilation is mandatory. An attic that lacks insulation will suck heat and AC out of a home, leading to condensation that helps promote mold growth among other ailments, while potentially causing damage to a roof. When applying spray foam in lieu of fiberglass or cellulose however, you’re creating what is known as a sealed attic space.
Ventilation is not needed in attics with spray foam insulation because sealed attics that have spray foam applied on the underside of the roof sheathing (the attic’s ceiling) should be airtight, ie they are thermally and physically closed off from the outside world.
There is still some confusion on why ventilation is bypassed when using spray foam for attic insulation. Homeowners have concerns about a lack of ventilation causing asphalt shingles to get too hot in the Summer or condensation forming when cold indoor air meets warm outdoor air (and vice versa). In actuality, there are more worries when you do ventilate a spray foamed attic then when you do not.
Why attic ventilation is not needed with spray foam insulation
The easiest way to understand why ventilation is not needed in a sealed attic is because the spray foam is applied on the ceiling of the attic instead of the floor. This insulates the attic from the outside weather while keeping the temperature inside the attic and the living areas in tandem. This negates any disadvantages of an otherwise unventilated attic.
When cellulose and fiber are installed on the floor of the attic, instead of under the roof sheathing, it creates a pocket between the cold air coming in from the roof and the heat coming up from the house.
The very term ‘sealed attic’ implies that there are no openings to the outside and therefore no ventilation. Not only that, since the foam is applied on the underside of the roof sheathing, it’s nearly impossible to ventilate. Ridge cap, roof vents, and whirlybirds push air out of that pocket where warm and cool air from the roof and house meet. With spray foam creating a barrier from the roof, the plan is that the two different temperatures of air never meet – and thus do not create condensation.
The first concern would be that unvented attics get incredibly hot during the Summer, as even spaces with ventilation are scorching during the warmer months. The opposite is actually true, a properly sealed spray foam attic repels the warmth of the sun, while at the same time keeps more cooled air in the area. Unvented attics are actually the more comfortable of the two, in colder months as well.
What is Spray Foam?
The success of a sealed attic relies on an almost impenetrable barrier under the roof. The only way hot and cold air never meet in an attic is if the thermal properties of the insulation are that sound.
Spray foam is created by combining isocyanate and polyol resin, which makes a polyurethane. This has been used as an insulation material since the 1970s, and is effective because the final foam product contains a low conductivity gas in its cells.
The makeup of those cells determines the specific insulating properties of the spray foam. Open cell spray foam will allow air into the cells, making it softer and less dense but also not as thermally resistant. Closed cell spray foam creates a true air seal, and has the best insulating properties of any material on the market.
Is Spray Foam That Impermeable?
The unvented attic simply would not be effective if the thermal properties of the spray foam were not the highest in the industry. If the spray foam doesn’t perform (or is installed incorrectly), condensation will occur and now there’s no natural ventilation to help circulate air through the space.
Spray foam does perform. Even open cell spray foam has an R-value of 3.7 per inch, which is higher than fiberglass, which is 2.2 per inch. Besides thermal properties, there are other advantages to using open cell spray foam over fiberglass or cellulose, mostly that the foam is less likely to settle over time, won’t get damaged by water, isn’t attractive to rodents, and is soundproof.
When opting for the sealed attic, it’s really the closed cell spray foam that is more desirable and nearly mandatory. The closed cells create an air impermeable membrane that serves as a vapor, air, and moisture barrier. Closed cell spray foam has an R-value approaching 6.5 per inch or higher.
Can Any Insulation Truly Be Air Tight?
When homeowners blow fiberglass or cellulose into their attic, they sometimes go 12” to 16” high depending on their climate. The problem with that insulation method is it’s very easy to miss spots or have low coverage in corners, around light and outlet openings, and on top of joists, which all lead to energy inefficiencies.
When properly installed, closed cell spray foam can truly make an attic air tight because it doesn’t compress over time like blown in insulation or batts, and when sprayed as a liquid before expanding, the foam can seep into cracks, seams, and other hard to reach places.
The ability of spray foam to fill cracks is an important asset, especially on a roof where it can actually help prevent damage from rainwater leaks. Closed cell foam is moisture resistant, pushing the water back onto the shingles instead of letting it into the attic.
Attic Integrity When There Is No Ventilation
The main question from homeowners asking about the safety of an unvented attic has to deal with the temperature. Attics can reach temps of 160 degrees on a hot Summer day, causing damage to items that may be stored in there and creating a ton of moisture from humidity. Even vents don’t do much to lower those temperatures without fans and other air circulation methods.
Contrary to some beliefs, the heat in a conventional attic (insulated on the floor) doesn’t come from warm air in the house rising, it’s actually from the roof passing intensity into the attic. That same premise holds true in Winter months too. The spray foam insulation is never going to push hot air back down into the home – but it’s also not going to let a cold roof suck warmth out of your property.
Any Risks to the Roof When There Is No Breathability?
Another thing that’s also interesting to note is that attic temperatures directly effect roof performance. A really warm attic will push hot air up to the roof, melting snow in the Winter. While that may sound like a good thing, that snow melts and the water runs to the eave outside of where the attic releases heat. That water then freezes again on this cold surface, creating ice dams and added weight to the roof structure.
It is very important to check with a shingle manufacturer’s warranty when considering an unvented attic with spray foam. Some shingle manufacturers may void the warranty if a steep slope roof is unvented, because there are usually so few problems with high-pitched roofs considering all the debris rolls of very quickly.
Do I Still Insulate the Attic Floor When Spray Foaming the Ceiling?
Even if a homeowner were to apply spray foam to a roof deck after the fact, the common conception might be that the insulation already on the attic floor, “doesn’t hurt anything.” While it would seem that the more thermal barrier in a house the better, leaving the attic floor insulation in may be doing more harm than good.
The reason for spray foaming the decking of the roof to create an impenetrable barrier from the inside and to the outside is to eliminate that pocket of different air temps in the attic. In an unvented attic, the air in that space is connected with the air in the house – unless you keep the floor insulation installed.
The attic will be colder than the outdoors, but warmer than the indoors in Summer if the floor insulation is left installed. What we’re striving for is the attic temps to be as close as possible to the rest of the house, which is why the floor insulation should be removed.
Final Benefits of an Unvented Attic with Spray Foam Insulation
Vents in a traditional attic (fiberglass, cellulose insulation) were a necessary evil to keep air flowing and moisture from forming. Those vents were also an inevitable loss of energy however as heat (or cooled) air would rise and eventually get sucked out through the lower R-value of attic floor insulation.
Eliminating these vents boosts energy usage right off the bat, as unvented attics are said to be about 20% more efficient than conventional layouts. Not having a vent on the roof that is accessible by wind-driven rain also helps eliminate moisture problems in the space. Unvented attics also help minimize the risk of fire from outside the home, not giving a vent for wind-driven embers to enter into and providing an added layer of resistance below the roof in wildfires and other outside sparks.
So ultimately, ventilation is not only not needed with spray foam insulation, but creating that extra source of air completely defeats the purpose of building a thermal barrier on the roof deck.