Part II: Increase Attic Intake Ventilation to Drastically Cut Cooling Costs

Series: Attic VentilationPermalink

Reduce cooling costs by increasing attic intake and exhaust ventilation.

Is the top floor of your house a little too hot during the summer? Ours was, due to an acute lack of attic ventilation. This caused a significant heat buildup during the day that our air conditioner just couldn’t keep up with.

The second step to improving our attic ventilation was to increase the intake ventilation under the eaves. This project was inexpensive and completed incrementally over the course of several weekends.

The results of the increased ventilation were immediately felt by human and cat alike in a cooler upstairs. We were also surprised by a 25% decrease in that month’s electric bill compared to the previous year, a savings that has continued over subsequent months of A/C usage.

Good Riddance Wooden Soffits Permalink

Unbeknownst to me, we had both aluminum and wooden soffits, the latter of which were original to the house. I didn’t realize this until I installed ventilation baffles. That led me to wondering if we had enough attic intake ventilation.

Our aluminum soffits are double six (the six refers to width and is in inches) and they alternate two full vented by two solid. The depth of the eave is two feet, so these alternating sections are two foot by two foot squares.

It seems like a decent volume of intake ventilation, at least until you consider that there’s plywood hidden behind the aluminum soffits. The plywood had holes cut into it for vents, but I’m skeptical that the intake air could easily flow into the attic. The plywood had to go!

Interestingly, the under eave soffit vents that were previously installed were roughly adequate. Each 16” x 8” vent had 56 square inches of Net Free Area (see definition below, sorry). The wooden soffits had ten vents total, so 560 square inches of NFA, which is just short of what’s required for my home.

So How Much Intake Ventilation Do I Need? Permalink

The ideal amount of attic ventilation is one square foot of vent space for every 150 square feet of attic area. This value is split evenly between intake and exhaust ventilation. In this post we cover the intake ventilation.

Attic length x Attic width ÷ 150 = total vent space or Net Free Area (NFA)

My attic has an area of 1,288 square feet, so my NFA is ~8.5 square feet. That is split into 4.25 square feet (or ~618 square inches) for both intake and exhaust. Ideally your intake ventilation equals your exhaust, but it’s likely your home is not balanced. Err on the side of having more intake than exhaust.

With a vapor barrier between the attic and conditioned space below you can halve the recommended Net Free Area. This results in using one square foot of vent space for every 300 square feet of attic area. Although my ceiling is air sealed, it’s not actual vapor barrier.

For a more thorough explanation, see How to Calculate Attic Vent Area Needed When Adding Soffit Vents.

How Much Intake Ventilation Do I Have? Permalink

My double 6” vented aluminum soffits have 11 square inches of NFA per square foot of area. Thankfully the alternating 4 square foot sections of vented and solid soffits make the math easier. I walked around the house and counted 43 vented sections. Together these sections have 172 square feet of vented area. With the 11 square inches of NFA per square foot we have a total intake NFA of 1,892 square inches.

That is triple the recommended value, but again, I would rather err on the side of having too much intake ventilation than not enough. It’s also possible that I’ll someday increase the exhaust ventilation with hip vents. Regardless, this project significantly reduced our summer electric bills, which is why I’m now so enthusiastic about attic ventilation!

Reach for the Eaves Permalink

Your eaves could be very far from the ground, depending on your style of home and how many floors you have. My house is a bi-level, so I was able to reach the soffits with my 22’ multi-purpose ladders and an extension plank. If my soffits were any higher I am not sure that I would have attempted this project.

I’m glad that I could do the work myself because the cost of the project would have been largely in labor. If you want something done well you should probably do it yourself! The materials I bought were inexpensive and I thankfully had all the tools that I needed for the job.

Gently Remove the Aluminum Soffits Permalink

The first step is to remove the existing aluminum soffits. You’ll want to read through an aluminum soffit installation guide (or two) to better understand how everything fits together. It’s likely that your soffits are securely installed, so you may feel frustrated at times during removal.

Basically, each section of soffit locks into its neighbor and the eave by two parallel channels, usually a J-channel or an F-channel (the names refers to the shape). The soffits are then stapled and/or nailed in-place.

The technique that I developed was to pull apart the soffits with a paint can opener (really). That usually revealed at least one staple or nail that I removed with a pair of needle nose pliers. Once the soffit had free lateral movement I would bend down the fascia-side J-channel and slide out the soffit.

I removed as many soffits as I could safely reach from my length of suspended plank. Sometimes I accidentally bent their edges, which I did my best to bend back before re-installation.

Hello Fiberglass, My Old Friend Permalink

One surprise was the volume of blown-in fiberglass that was resting above the wooden soffits. I collected as much of it as I could through the under eave vent holes. Some areas of soffit had more, while others had less. It was messy work collecting it.

Safety First! Permalink

There was a lot of blown-in fiberglass, sawdust, and asphalt shingle granules falling down on me during this project. I wore long sleeves, work gloves, a safety respirator, and alternated between safety glasses and goggles for eye protection.

It got pretty hot, so when I wore the goggles they would fog up. That’s when I switched to the safety glasses, although I didn’t always wear any eye protection if I weren’t cutting anything or removing the old blown-in fiberglass.

Tools for Removing Wooden Soffits Permalink

To remove the wooden soffits I held a circular saw above my head and cut away each of the sides. I contemplated pulling out all the nails that secured the soffit to the eave, but then I would have needed to remove the house-side J-channel (also known as receiving channel) and later reinstall it.

For my initial cuts I used a jigsaw, but it wasn’t powerful enough to quickly cut through the wood. I switched to a circular saw, which was heavier, but still somewhat comfortable while wielding above my head. This is when it started raining sawdust.

Anywhere the circular saw wouldn’t fit I used a Japanese-style double-edged hand saw. It was definitely the right tool for cutting through the old plywood.

After making the cuts with the circular saw and hand saw I pried away the plywood from the underside of the eave with my pry bar. Be alert to any nails that previously secured the wood to the eave.

How to Remove Wooden Soffits Permalink

Beware the Fiberglass Shower Permalink

Ideally you’ll collect as much old fiberglass as you can before it falls to the ground. I spent more time than I would have liked picking up blown-in fiberglass from my mulch beds.

Supplemental Attic Insulation Part I Permalink

When I removed the wooden soffits I was finally able to reach the outer edges of the attic. The previous summer I installed insulation baffles between the rafters, but there were some areas that I couldn’t get to due to the low slope of my roof. The corners of my attic were completely inaccessible because each side’s rafters meet at a right angle.

This was an opportunity to supplement my earlier insulation work. In the previously barren corners I added a few inches of loose fill fiberglass insulation with the aid of a small garden rake.

Supplemental Attic Insulation Part II Permalink

To make sure that the loose fill fiberglass insulation wouldn’t fall onto the newly liberated aluminum soffits, I built a rigid insulation (XPS) dam. I cleaned the top plate by blowing away any residual fiberglass or dust with my air compressor’s spray gun attachment. Then I secured the insulation into place with spray foam insulation.

Seal the Top Plate Too Permalink

I also supplemented the insulation that I had previously sprayed on the top plate to secure the ventilation baffles into place. It’s not much, but every bit helps.

Vacuum the Soffits Before Reuse Permalink

Given the amount of old fiberglass and asphalt shingle granules it made sense to vacuum the vented aluminum soffits. I’d hate for to impede airflow with vents clogged by roofing and attic detritus.

Obligatory Reinstalled Aluminum Soffits Photo Permalink

After the spray foam insulation cured I reinstalled the aluminum soffits, one section at a time. This was easy-going until the last piece, which was usually a bear to put back into place. I used a hammer to bend the fascia J-channel back into shape. As you can imagine, it doesn’t look so good up close.

In the time since I completed the project I have occasionally inspected my work to ensure that no soffits have come loose, either from inclement weather or the meddling of animals. Thus far it appears that all the pieces are adequately in place.

A World of Difference Permalink

In the nine months since increasing the attic’s intake ventilation we’ve had a marked decrease in attic heat as demonstrated by our smaller electric bills during the summer months. With the increased attic exhaust ventilation we’ve already broken even with the cost of both projects. We’ll enjoy these savings for the remainder of the time we live in this house, which is the best part. I’d definitely recommend that homeowners investigate their attic ventilation to evaluate whether it would be worthwhile to undertake such a project as this.

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