1. Myths of Attic Ventilation & An Intro to Night Sky Cooling
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Charles Buell
Part 1 of 7
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

2. The Moving Target Welcome.
I know you will have questions, but please lets wait until the end. You will notice each slide is numbered so you can refer to the slide number with your question. Holding your questions will allow me to get through the huge amount of info I have for you today. This will be a sprint to get through it all.
HOWEVER! If you are going to have an actual coronary if you do not get your question answered, I will consider taking your question.
You will notice some of the slide numbers are blue. This is to remind me there is a lot of stuff I need to read from my notes as well as where you will need to pay more attention. Some of this is complicated enough that in order for me to fully convey everything I want to convey, it will require considerable reading from my notes.
In many ways what we are going to discuss today is a “moving target”
Do not believe what I say will always be accurate for your area.
The science principles are the same---it is their application to different climates that changes.
Much of the science is well established while other areas there is still much more to understand---or at least how to apply to different conditions.

3. In achieving our ventilation goals there are…..
Materials limitations
Workmanship limitations
Cost/Benefit limitations
Codes ahead of the curve
Codes behind the curve
Read from slide
Which brings us to Myth #1 of 10 or so myths.

4. Myth #1: What works in one area will work in all areas. Myth #1
The goal of this presentation is to get to a place where we can all understand how different “solutions” to ventilation—and even not venting--can be around the country.

5. Myth #2: There is only one way to do something in any given area.
This is sort of a Sister to #1, but by the end of the presentation we will see that most areas can achieve the desired goals in multiple ways.

6. Myths of Attic Ventilation & An Intro to Night Sky Cooling
The title of the presentation makes it sound like the second part of this presentation is: “An Intro to Night Sky Cooling,” however, we are going to get it out of the way before we get into the “Myths.”
For this part, I am going to have to put on my tin foil hat.
How many of you have heard of Night Sky Radiational Cooling?
When I first heard about this phenomenon, I wondered how I could possibly have lived my whole life and not heard of it.
If you live in an area where it occurs, you likely blamed what you saw on something else.
But surely we have all noticed how much colder it feels on a clear night than it does on a cloudy night.
Night sky radiational cooling also accounts for black ice.
It accounts for why you feel colder when you are near a big cold window.
It also accounts for why desserts get colder at night as the earth gives up its heat to the night sky.
This effect is not that the cold of space is making the roof cold (or that the window is making you cold) as much as the roof is giving up its heat to the cold of space (and your body is giving up its heat to the to the cold window).
In some of the hottest parts of the world people sleep on their flat roofs at night to take advantage of this effect.
It is happening under the principle that “heat flows from hot to cold,” or more accurately from warmer to cooler. It is “relative” as we will see in a moment.
While it has always been present in the Northwest--and a part of whatever happens to roof structures--it only made itself visible and distinct from other factors that masked it, when we started to tighten up our houses. The tighter and more well insulated the house, the more likely you will be able to attribute the adverse conditions your are seeing to see the effects of night sky-radiation.
These areas are mostly maritime climates--and “maybe” in areas near large bodies of water or other means of keeping air moist when it is cold outside.
The rainy Pacific Northwest is perfect for it.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

7. Who has seen frost patterns like this on roofs before?
This picture was taken in the Fall of 2015.
What was the ambient temp at the time of inspection?
43° Fahrenheit
What was the overnight low temp?
38°
What was the relative Humidity?
Greater than 90%
Who can tell me the temperature of Space?
It is close to absolute 0° Kelvin or about -460° Fahrenheit.
The fact Heat Pumps can function at lower and lower temps is because there is so much heat energy in the air even below 0° Fahrenheit. Extracting that heat is a technology issue.
On a clear cloudless night the roof can “see” that cold of space and give up its heat even to the point that the roof surface drops below freezing—even though the ambient air temperature is not below 32° F.
Now if we bring that cold wet air into our perfectly vented attic, what happens to that moisture?
It can condense on the underside of the cold roof---just like it is doing on the outside. Obviously if we heated our attics, which we essentially used to do, it would not form on the inside.
Night sky cooling is a result of the cold, night sky absorbing infrared radiation emitted by the roof.
The degree of cooling of a roof depends on several factors, including the presence of cloud cover, roof material properties, humidity and the roof’s slope.
This effect cannot be improved with additional venting. Additional venting would just draw in more of the cold wet air.
The effect is exacerbated by OSB roof sheathing but will also occur on newer plywood, but less so on plywood from older growth trees and less on sawn lumber in general--especially old growth lumber in older homes.
The denser the wood, the less storage capacity. The more storage capacity the more moisture there will be to feed fungal growth.
Older homes also faired better because roof sheathing and underlayment materials were more vapor permeable and wood moisture content did not tend to build up as much. They also benefitted greatly from attic space being essentially semi-conditioned by air bypasses and lack of insulation.
Fall 9/29/2015
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

8. This is the air temp and relative humidity of the preceding picture.
Those of you not in areas that see frost like this can get a sense of how it works by going outside on a very clear night, early in the morning, and see how the roof surface temperature compares to air temperature.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

9. This picture was taken in the Spring of 2017.
Notice how similar it was to the picture taken more than a year and a half earlier?
If you live in a humid hot climate, you may see very little reduction in roof surface temperature over night, and the roof surface may never drop below ambient.
This can be due to the heat storage capacity of the structure---how much solar loading it has. The heat migrates to the exterior too slow to lower surface temperature before it starts to heat up again.
If you are inspecting an attic in the morning and it is still warm from the day before, you are in such an area.
Spring 4/9/2017
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

10. I cannot tell the “exact” temperature of this roof in the previous picture, due to the limitations of the camera---but it is well below 32 degrees F.
Shooting the roof at this angle, from distance, and that the roof is white with frost, the camera is not adjusted for getting an accurate reading.
It does not have to get very much warmer at high humidity for this effect to not show on the exterior.
At 45 degrees F, and 87% humidity, there will likely be no frost on the roof on a clear night.
It is a temperature/humidity window that is common day after day for just a few weeks in the spring and fall in the Northwest---well at least on the nights that are clear.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

11. Frost Due to Night Sky Radiational Cooling
You can have frost “outside” standard frost expectation dates. (Shoreline, WA) 26
Frost Guaranteed 6 7
Frost is a Possibility 25
Frost Due to Night Sky Radiational Cooling 26 2
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

12. This picture is just to show that the roof does indeed sometimes frost over more uniformly when ambient temperature is actually below freezing.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

13. As you can see with this ice cube, it appears quite cold against the warm floor background.
Compared to the warm wood floor, the ice cube is “relatively” cool.
The little pyramid with the exclamation mark next to the temperature is just the FLIR C2’s reminder of its capabilities---it cannot “see” the actual temperature---it is somewhat lower. Its temperature range is from 14 to 302 degrees Fahrenheit.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

14. Compared to the night sky the same ice cube is “relatively” warm
Compared to the night sky the same ice cube is “relatively” warm. And there is that pesky pyramid again.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

15. In this picture you should be able to see the overall darker discoloration of the roof sheathing except for a triangular area that is protected by the overlying roof that runs to the house.
What makes this a great example of how night sky radiation works, is because there is no house structure to complicate the issue.
There are no indoor air bypasses that could be causing the condition.
There is only the wet ambient air to condense on the underside of the cold roof.
The area of the triangle simply does not get cold enough---it is protected from the night sky by the overlying roof.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

16. Photo curtesy of Jonathan Quigley, All Island Home Inspections
This is a good example of how the softer grain of the plywood is able to store more water resulting in more staining and possible fungal growth. It has less “drying potential.”
Photo curtesy of Jonathan Quigley, All Island Home Inspections
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

17. Discoloration and fungal growth due to night sky radiation will be more uniform and heavier on north facing slopes.
While night sky radiation will create this condition in a very uniform pattern across the underside of the roof, it may vary if parts of the roof are shaded in some way as in the previous picture.
However other inputs have to be eliminated before night sky radiation should be considered the primary cause of the adverse condition.
Flooded crawl spaces or a history of very high indoor moisture levels, as well as dozens of other factors, will result in similar appearances.
It is these other possibilities of excess moisture that leads us naturally to “Attic Ventilation”
Note the condition does not go behind the truss top chords. This is consistent with this not being a condition that was on the wood prior to installation.
Night sky radiational cooling should not be considered the primary cause of attic staining except in VERY tight construction--homes that meet or exceed modern energy code requirements for air sealing. This is generally going to be homes below 3 or 4 air changes per hour tested with a blower door at 50 pascals and insulated more than R-38.
Why is it that this problem only shows itself in very well insulated houses and perhaps not at all in houses with NO insulation?
Prior to the days of adding insulation, attics were essentially semi-conditioned spaces.
They stayed too warm for moisture to condense on surfaces.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

18. So why does mold growth tend to show up more on the North side of the roof than the South side of the roof?
So why does mold growth tend to show up more on the North side of the roof than the South side of the roof?
Mostly it happens on the north side of East/West oriented roofs because this is the side with the least drying potential. Solar loading of the South side dries out the wood, lowering the moisture “stored” in the wood. Where moisture is stored more efficiently, the mold has more of a source of moisture. It is a condition well suited to most OSB.
North/South oriented roofs will not so easily favor one side over the other, unless affected by trees or other obstructions.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

19. And the solution is? ?
And the solution is?
That is a good question.
Currently however, there is no real good answer. Well there probably is an answer—we just have not figured it out yet.
One thing that IS clear though, increasing the amount of ventilation is NOT the answer. There is more evidence that decreasing ventilation can help when there is too much.
Night sky Radiational Cooling is an oddity that most of the country does not have to think about but it is still useful to think about because, any adverse moisture condition in an attic, from interior bypasses, will result in fungal growth, and night sky Radiational cooling of the roof will be working to encourage that growth.
Some ideas that have been tried or proposed are: (read slide)
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

20. Treating the surface with wood preservatives
Insulation on top of the roof sheathing
Hot roofs (insulation directly applied to the underside of roof sheathing).
Research is ongoing--stay tuned.
What you DO NOT want to do:
Increase ventilation beyond what is absolutely necessary for heating and/or cooling reduction.

21. Why Wood Frame Attics Get Wet & Moldy in the Pacific Northwest
If you want to learn more about how night sky radiational cooling works, this is the major treatise on the subject:
Why Wood Frame Attics Get Wet & Moldy in the Pacific Northwest
If you want to learn more about how night sky radiational cooling works, this is the major treatise on the subject:
Just google it.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc