Part IX: Keep your Attic Cool with Ventilation Baffles

Series: Installing Blown-in InsulationPermalink

Save money by adding blown-in insulation to your attic.

One task that needs to be completed before adding more blown-in insulation is to install attic ventilation baffles. Once finished we’ll be able to completely cover the attic floor, which will give us the best performance from the new insulation.

Currently we have a reasonable amount of old blown-in insulation that (mostly) covers the attic floor. An exception is along the external walls near the top plate, which is evident in the summer when that part of the ceiling is noticeably warmer than along the interior. (See my FLIR ONE for iOS post for proof!)

With the baffles installed we’ll have an even thicker layer of insulation with more consistent coverage. We also won’t need to worry about blocking the soffit vents so that our attic can passively vent. Hopefully the top floor of our house will stay warmer in the winter and cooler in the summer as a result.

Relevant Roof Vocabulary Permalink

For this post we’re only interested in discussing eaves, which are the edges of the roof that overhang the face of a wall, and soffits, which are the underside of the eaves.

One way to ventilate your attic is to use soffit vents. They allow cool air enter at the bottom of the attic and exit at the top via a roof vent. If your attic has enough vented area then you can vent passively, which is superior to running an attic ventilation fan.

The attic insulation baffles that we’ll be installing will make sure that there’s adequate air flow from the soffit vents. They will also allow us to maximize our blown-in insulation without worrying about obstructing the air flow. The baffles make sure that the blown-in insulation stays put, so there’s no wind washing.

Why is Attic Ventilation Important? Permalink

Oh let me count the ways:

  1. It helps prevent ice dams in the winter by keeping your roof colder
  2. It reduces moisture in the attic, which discourages mildew growth and roof rot
  3. It reduces cooling costs in the warm season by removing hot air
  4. In hot weather it extends the life of your shingles by keeping the roof cooler.

I should note that there’s healthy skepticism about the need for attic ventilation, just so long as your attic is properly insulated. Personally, I would prefer to err on the side of caution and make an honest attempt to ventilate my attic. At worst I waste my time and money, at best I have a more durable roof and more comfortable home.

What are Soffit Baffles? Permalink

Soffit baffles are made of rigid foam, cardboard, or plastic and are rectangular in shape. They are designed to channel air into the attic and are installed in rafter bays.

After some research I went ahead and purchased AccuVent baffles because I was confident that they’d stay secure once installed. The cardboard and foam baffles at my local big box store seemed too flimsy.

Relevant Framing Vocabulary Permalink

The above diagram is a nice cutaway view of the framing in an attic. We’re most concerned with the ceiling joists, rafters, and top plate because that’s where we’ll be installing the baffles.

Installation Difficulties, or the Problem with a Low Sloped Roof Permalink

Baffles are usually installed while building a house, although they are sometimes added later (even though it’s probably more difficult to do so). Two common impediments are a low sloped roof and old insulation. Together they make it both difficult and itchy to move around.

My house has a hip roof with a low slope, so there is very little room to maneuver in the attic, especially near the top plate of the exterior wall. The ceiling joists are 2×6’s and where they meet the top plate they have their top corner cut to fit under the roof deck. That leaves roughly five inches of space with which to install the baffles.

How to Install Attic Ventilation Baffles, Even if you Can’t Reach the Top Plate Permalink

My solution was to make bricks of rigid foam insulation, attach them to the baffles with double-sided tape, and then use a small rake to push them into place.

I secure the baffle by stapling it to the roof deck and then spray foam insulation into the cracks between the brick of insulation and the rafters.

Cut Large Sheets of Rigid Foam Insulation into Smaller Blocks Permalink

I bought four 2” thick sheets of Foamular 250 extruded rigid foam insulation from my local big box store. They didn’t fit in my car (which I knew ahead of time), so I cut them in half in the parking lot, which elicited a lot of looks.

I could have purchased the Foamular 150, which is less expensive, but decided against it out of ignorance. I later learned that the only difference between the two is the impact resistance rating. The Foamular 250 can take 25 psi, while the Foamular 150 can take 15 psi. I don’t have a sense for what that really means, but presumably the 250 stands up to the scoring and breaking better than the 150.

The easiest way to cut the rigid foam insulation is with a utility knife and a metal yardstick. A snap-off utility knife is best since it’s easily extended to a longer length. Score the insulation an inch deep and then hit it with your hand to break it along the score line.

My ceiling joists and rafters are both 16” on center, which should leave 14.5″ inches, but each rafter is paired with a ceiling joist that reaches to the top plate, so there’s actually 13”. Unfortunately my ceiling joists and rafters aren’t exactly 16” on center. Due to these irregularities I cut my blocks to be 11.75” in width, which leaves 1.5” of wriggle room.

The dimensions of the my blocks are 2” x 6” x 11.75”. I made them 6” deep to cover the top plate and then some.

Glue the Blocks Together Permalink

I took my foam blocks and glued them together, two-by-two, to make a height of four inches. There’s roughly five inches of space between the top plate and the roof deck, so that then leaves one inch for ventilation.

It’s important to get the right type of adhesive for the rigid insulation because it’s made of extruded polystyrene. Many types of adhesive will burn through it, so take care to buy the correct product for the job.

Trim the AccuVent Baffles to Fit Permalink

As I’ve previously noted, there’s roughly 13” of width available for my baffles in my rafter bays. The AccuVent baffles are designed for rafters set 16” on center and are thus 14.5” inches wide, so they should be cut to fit. I decided to trim them down to 12” at their base to allow a little room for my 11.75” insulation blocks.

I didn’t trim away the excess width equally on each side. This is due to the ceiling joist that overlaps the top plate in my rafter bay. On the ceiling joist side I cut away an inch for about one-third the length. On the opposite side I cut away the remaining 1.75”. See the photo above for a better view of what this looks like.

Affix the Insulation Block to the Baffle Permalink

I used double-sided carpet tape to affix the insulation block to the ventilation baffle. I am not sure if that’s the best choice, but I could find no adhesive that would stick to both extruded polystyrene and flexible PVC. I figured the double-sided tape doesn’t need to last forever, but just long enough for me to place the baffle in the rafter bay.

How to Position and Secure the Baffle Permalink

Once in the attic I use a small metal rake to move insulation and put the baffle in place. I pull the existing insulation several feet away from the top plate and then push my insulation mound over the neighboring ceiling joist.

With the insulation cleared away I push the baffle into the rafter bay so that it rests above the top plate. Sometimes I find a hole for electrical wiring, so I fill that up with a small amount of spray insulation. Also you may run into the top plate of an internal wall. In that case the 4” insulation brick may be too tall, so you’ll need to swap it out for a 3” brick. (I had extra 1” rigid foam scraps from an earlier project).

Once the baffle is in position I roll onto my back and put several staples into the top-most part of the baffle. This is the best that I can do staple-wise given the limited space. I tried using a hammer tacker for the extra reach it would afford me, but found that there wasn’t enough room to generate sufficient force to insert the staples.

Lastly I fill in the gaps with some spray foam insulation. I average about four baffles per can of insulation and I’m sure that I waste some of the insulation due to the time it takes me to install each baffle. It sets in the can while I am working away.

Unbeknownst to me, I mispositioned some of the baffles over the top plate. I was later afforded access from the outside when I increased the intake ventilation of my attic by removing the old wooden soffits. I took this opportunity to supplement the insulation with additional spray foam.

Note that I am usually on my stomach with at least one limb on a ceiling joist while I work. My weight is distributed enough that I haven’t had any issue falling through the ceiling. Crawl and contort your way through your attic at your own risk, obviously.

Also know that I compress a lot of the existing insulation with the weight of my body. This decreases the R-value, but so far as I am concerned it’s a cost of doing business in the attic. I will cover whatever remains of the existing insulation with a thick layer of new insulation.

Before and After Photos Permalink

In that first photo you can clearly see sunlight, which is a sign that there’s not enough insulation in the attic. I know that I’ve pulled the existing insulation away from the top plate, but I have many other rafter bays where the light is visible. Keep an eye out for this in your rafter bays because only you can prevent under insulated attics!

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