attic ventilation Archives - Charles Buell Consulting LLC

August 18, 2022 by Charles Buell

Attic Ventilation and Soffit Vents–getting it better, if not perfect

Why are soffit vents important?

There is perhaps no topic that gets many home inspectors more riled and confused than attic ventilation. If your inspector tries to tell you that mold on your roof sheathing is due to lack of ventilation, it might be best to find a different home inspector. As a consumer it is a question that could be used to vet home inspectors.

Mold or what looks like mold on roof sheathing

The primary function of attic ventilation is heat reduction.

And yes, incidental amounts of moisture will be vented out as well. But, if you have enough moisture that you are getting mold growth, we have to look elsewhere to find out why. There can be a plethora of sources of moisture. Too much moisture in crawl spaces is a big one. Moisture sources, or any combination of a dozen or more air bypasses from the house into the attic, can lead to moisture in the attic.

In older houses it can be very difficult to seal all these bypasses, but if it is not accomplished to a high degree, you can get mold growth in your attic. This can be exacerbated by climate as well.

Night-sky radiational cooling.

A more unusual source of moisture in attics shows up in houses that are VERY tight and super insulated. In these structures, the high-humidity winter air in maritime climates is drawn into the attic where it condenses on the cold roof where the cold night sky has suppressed the roof temperature below the dew point. Research is ongoing to determine how best to deal with this but reducing recommended amounts of ventilation at the ridge can help as less moisture is pulled into the attic.

The flow through the soffit vents can actually reverse as the cold attic air drops through the vents to the warmer outside ambient air. Generally speaking, the soffit vents are almost always under positive pressure as air pushes its way into the attic.

Quite some time ago, I insulated my attic with 12-14 inches of cellulose fiber. I installed soffit baffles all around the edge at that time assuming that one day I would install the vents. Not wanting to pass up an opportunity to experiment, I deliberately never vented the closed soffits. I wanted to see if having only ridge vents and a couple of gable vents would result in moisture issues in the attic or would that much cellulose prevent house humid air from finding its way into the attic.

It is almost embarrassing to confess it had been this way for 20 years. Time flies when you are busy ignoring things. Regardless the attic suffered no adverse effects from my experiment.

Welcome to the Heat Dome of 2021.

I noticed that when it was very hot outside (for Seattle that is) the top corners of the outside walls of the rooms got VERY warm compared to the ceiling and walls generally. Remember earlier I said the primary function of ventilation is heat reduction? Well not venting the attic created a little “soffit oven” that heated up the corners of the rooms—all around the house. This was exacerbated by some settlement of insulation and missing insulation above window headers. As you can see in this first drawing, the red areas represent the closed soffit with no vents installed.

This next drawing shows how the heat builds up in the corners and moves to the cooler interior.

The wall and ceiling at the corner heats to the interior

This picture shows the thermal image of the ceiling corner adjacent to the soffit and how much it heated up on a 90°-day midafternoon.

Corner at ceiling overheating from the hot soffit

This next picture is the thermal image of the same area the next day after vents were added and the outdoor temps were slightly warmer midafternoon.

Lower temperature after venting the soffit

When I was drilling the holes, it was early in the morning when it was about 65 degrees ambient, and I noticed cool air was dumping out of the new drilled holes. The air temp measured 58° F at the vents inside the soffit and the roof surface temp measured 51°F. These two thermal images show the cool air pushing its way out of the attic and hitting the frieze board under the soffit–directed there by the louvers in the vents.

What is the takeaway? We need good attic ventilation, neither too much nor too little and in correct proportions. Code guidelines are for the most part adequate if we can get air sealing right. Some adjustments to these recommendations may be warranted for night sky radiational cooling.

Seal that ceiling between conditioned space and the attic!!

Also, looks like I am going to have to get around to top off that insulation–hopefully not 20 years.

One possible solution to the inherently poor insulation in these areas would be to retrofit the interior ceiling all around the room with insulated soffits. This soffit detail (uninsulated) was a common detail in homes in this area in the late 1950’s.

By Charles Buell, Real Estate Inspections in Seattle

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January 25, 2019 by Charles Buell

Confused About Attic Ventilation?

Builders, roofers, insulation contractors, homeowners and home inspectors are seemingly dazed-n-confused about attic ventilation.

Lets first discuss what the purpose of ventilating the roof is. The primary purpose is heat reduction. With the ventilation necessary to accomplish this it will also remove very minor amounts of moisture that may find its way into the space as well. Its overall purpose is NOT TO REMOVE MOISTURE. We want to reduce heat to reduce cooling loads and improve indoor comfort.

One can find an endless number of articles on the web about the solution to moisture issues in attics being to add more ventilation.

While I think a lot of these answers are only accidentally correct, when someone says moisture issues in the attic are a “ventilation issue,” I think they are missing the big picture and may result in the wrong solution to the problem. These solutions too often make things worse.

Sure the ventilation can be wrong in terms of “amounts,” or it can even be completely missing. It is interesting, because attic ventilation can actually be completely missing and the issue might still not happen at all.

I think it is safe to say, having too much upper ventilation would be worse than none at all, because it will increase depressurization of the lid. This will lead to even more house air (which is at higher pressure) pushing its way into the attic.

So ventilation may need more balancing of intake and exhaust, but that alone will not correct the issue.

What the house needs is proper air sealing more than ventilation correction.

Of course in the context of any air sealing the attic, it would be a good idea to adjust the ventilation to industry standards. This is typically 50% upper and 50% lower spread around the sides. As long as that upper level does not go above 50% it is fine. 40% upper and 60% lower is fine as well.

You can get ventilation horribly wrong per code recommendations, and if you get air sealing correct, the attic will behave itself.

This attic had 15% upper ventilation and 85% at the eaves and this is what it looked like 33 years later. (R-60 cellulose, raised heel trusses, and who even knew about spray foam in a can back then?) (As a side note: the shingles on this roof were white 3-tab, over no underlayment and lasted 30 years with no sheathing damage—even at the eaves.)

You can get ventilation perfect per code recommendations, and if you get air sealing wrong, the attic can go horribly wrong.

This is a 15 year old attic with perfect 50/50 ventilation. (R-38 white fluffy, conventional framing)

It is more accurate to say this is an air bypass issue than a ventilation issue because the issue cannot be corrected by just focusing on ventilation.

This is where power vents become “snake oil” and can make the condition radically worse.

To illustrate the problem, I pose a question? How much do I need to increase attic ventilation to fix a roof leak?

I think most would agree you fix a roof leak by fixing the roof.

The same is true for too much moisture in the attic from indoor sources–you exclude the moisture as necessary–you fix the moisture bypasses. This is accomplished with proper, adequate air sealing and in some areas this will include vapor barriers. The vast majority of moisture finding its way into attics is from air bypasses—not vapor diffusion.

This air sealing can be difficult in older homes and should be automatic in newer homes.

One of the biggest hurdles to overcome is that roofers are not air sealing experts and many insulation contractors are also not on board with the science of it all. As a homeowner, you need to make sure the roofer and builder and insulator you hire knows how these systems play together. You will also need to find a home inspector or building performance professional to help you sort it all out.

Charles Buell

Real Estate Inspections in Seattle

October 30, 2017 by Charles Buell

Is your insulation doing what you think it is doing?

I do not want to tackle the huge ugly topic of whether fiberglass insulation should be used at all–as it frequently is in attics.

For today, I just want to talk about one obvious issue with fiberglass insulation that prevents it from performing as expected.

The short version of what is wrong with fiberglass insulation is that it is not an air barrier, therefor, if it is not encapsulated and air sealed on all six sides its performance suffers. In an attic, at most, only five sides is likely to be sealed leaving the entire top not sealed.

This post is about the sides, which can and must be air sealed. The vented sides are typically not adequately air sealed, especially along the eaves. Insulation baffles, designed to keep insulation out of the lower roof venting and to allow for air flow into the attic, rarely gets adequately sealed.

Because the baffle/top plate connection is not air sealed, and because fiberglass insulation represents very little resistance to air flow, air pushes its way into the attic right through the insulation. As it does this, it either cools the ceiling in the area, or warms the ceiling in the area depending on the season and/or side of the house.

In the winter and/or the north side of the home, the air will tend to make the ceiling cooler in the area of the vent. In the summer, especially on the sun side, the air will tend to warm the ceiling in the area of the vent. This will increase both heating and cooling loads of the home.

This next picture shows what that area looks like at the interior ceiling with Infrared camera on the South side of the home. Warm air is moving through the insulation and warming the ceiling.

In the same house on the North side we can see how the ceiling area near the vent, as indicated by infrared camera, is “cooler.” In the actual picture we can see the fungal growth present because this vent happens to be in the area of the bathroom. The moisture in the bathroom condenses on the cooler surface creating a perfect environment for mold growth.

The only real repair for this condition is to pull back the insulation and properly air seal the gaps where the insulation baffle and the house framing meet.

Spray foams are good for sealing the areas where the baffle makes contact with the framing as indicated in the circle in the picture above. Of course, in a perfect world, we would not use fiberglass insulation at all, and instead use types of insulation that are much better at stopping the flow of air. Cellulose fiber insulation can do a much better job at this.

Charles Buell, Real Estate Inspections in Seattle