Baffles are important for attic insulation installation because they protect the ventilation of this moisture- and heat-vulnerable room in the house. However, there is a distinct difference between important and required by code.
The International Residential Code (IRC) mentions attic insulation baffles in two sections.
Baffles are required in vented attics containing air-permeable insulation. They are installed in the rafters to keep insulation away from the soffit vents and to channel air over the insulation. Baffles are also one of the options for protecting insulation around hatches and doors.
Eave Baffles Are Required By Code
According to Section N1102.2.3 of the IRC, eave baffles are required for air-permeable insulation that is installed in vented attics.
There are no exceptions to this in the code. However, there can be local adaptations or additions to the code, so be sure to research your local codes to find out if your area has differing regulations.
Air permeable insulation includes loose-fill and batt insulation.
Why Baffles Are Required
Baffles are relatively flexible boards. They have a shorter back panel that sits nearest the eaves and a longer top panel that runs along the rafters, creating a space behind and above the baffle.
The purpose is to stop insulation from falling all the way to the back of the attic, which would block the eave and soffit vents.
In addition, they help to ensure that the air is channeled up past the insulation as opposed to disappearing into the insulation.
The soffit and eave vents are what supply outside air to the attic, so without the use of baffles, the attic runs the risk of becoming hot, stuffy, and humid.
Moisture from a lack of air circulation can lead to rust, leaks, mold, and mildew. Each of these things can lead to the degradation of your roof.
Extreme heat buildup in the attic can also lead to damage to your roof and affect the temperature of your home despite the insulation.
Code-Required Baffle Location
The baffles must be installed between the rafters leading up from the soffit and eave vents and they must extend over the top of the insulation.
The extension helps to channel air up towards the ridge (or similar) vents, which is where it needs to go.
You don’t have to install the baffles all the way up along the length of the rafters. You just have to go up to the top of the insulation (which should not go too high).
In terms of how far it must extend in the opposite direction (towards the exterior wall), the code becomes more specific.
It must extend “to the outer edge of the exterior wall top plate”.
The reason for this is also included in the code. It’s to ensure that you get as much insulation in as possible. Otherwise, it would shrink the size thermal envelope.
Do All Rafters Need Baffles?
The last part of Section N1102.2.3 says:
“Where soffit venting is not continuous, baffles shall be installed continuously to prevent ventilation air in the eave soffit from bypassing the baffle.”
Now, there are people who would say that baffles are only required in the rafters that have intake vents at the soffit. Yet, this part of the code seems to contradict such a claim.
If you only have vents in every other or every couple of soffit panels, you still need baffles installed between every rafter in the attic.
Without this setup, Insulation can still fall into the soffit. Maybe not over the vents, but once inside the soffit, it can be picked up by the air flowing into the attic and buildup over the vents.
Moreover, as mentioned before, one of the purposes of baffles is to funnel air through the attic and up to the top vents as opposed to it getting stuck in the insulation.
Required Amount of Space Created By Baffles
The IRC states that baffles must have a net free area opening that is equal to the opening of the vent or larger.
When they talk about the net free area, they simply mean the overall open space; it does not have to be an uninterrupted area.
If the net free area of the baffle opening is smaller than the vent’s opening, then it can limit the amount of air entering the attic. It can also end up creating a vacuum effect as air is lost out the top vents faster than it can be replaced through the soffit vents.
Allowable Baffle Materials
In terms of the materials that are allowed by the IRC, there are very few parameters—in fact, there’s only one. The baffles must be made from a solid material.
Commonly used materials are plastic, metal, and cardboard.
The reason it has to be solid is that it must maintain its structure when suspended between the rafters. Any sagging can lead to insecure connections, gaps in the baffles, and compression of insulation.
Baffles That Contain Insulation Around Access Points
The IRC mentions baffles one more time in Section N1126.96.36.199. This section talks about installing and retaining insulation around access hatches and doors.
In these cases, baffles are only one of the options that can fulfill the code. In addition, these retaining structures are only required for loose-fill insulation. The purpose is to “provide a permanent means of maintaining the installed R-value of the loose-fill insulation”.
If you have loose bits of insulation around a hatch or door, when you open it up, it is easy for the particles and fibers to fall out.
Not only does this spread insulation around living areas, where it can cause health problems and annoyance, but it can also result in lowering the R-value of the insulation in the areas around the hatch, which are already areas more prone to heat loss because of the opening.
In addition, the door is not allowed to open onto and compress the insulation, which would also diminish its R-value.
The baffles must be installed over the insulation, but not compressing it, and should be able to withstand the weight of the access hatch or door if it opens horizontally.