Roof Ventilation

[kbs2244]

I know this gets discussed a lot here.

This is a good explanation of the basics of how and why.

https://www.proremodeler.com/roof-ventilation
When properly detailed, roof ventilation keeps roofing materials cooler and prevents ice dams

 

[matt_i]

Good stuff, I installed 100% punched vinyl soffit on my build rather than the 1pc punched then 2pc non-vented that is on my house. The typical recommendation is to match the square inch area in the ridge vent with the same calculation in the soffit but I chose to over-size the soffit from this perspective.

The only downside I could see is that the additional airflow could present more convection in the attic in the winter if strong winds are cross-communicating inside the roof (e.g. from one soffit over to the other) but one side is ~50% consumed by one roof peak intersecting the flat plane and so the soffits are more limited on that side.

 

[yeldogt]

Except -- most times it does not work. I stopped ventilating roofs 20+ years ago.

Study after study shows that roof shingles to be maybe 10 degrees lower with a full second roof -- 10 degrees is nothing.

The key is to keep the heat from getting past the top ceiling -- or to insulate the roof. When you live in an area with any humidity -- that humidity come into the attic.

 

[ddawg16]

Except -- most times it does not work. I stopped ventilating roofs 20+ years ago.

Study after study shows that roof shingles to be maybe 10 degrees lower with a full second roof -- 10 degrees is nothing.

The key is to keep the heat from getting past the top ceiling -- or to insulate the roof. When you live in an area with any humidity -- that humidity come into the attic.

Go ahead and keep doing what you're doing. Just stay away from my house. I don't think you have any real clue what you're talking about.

 

[CNGsaves]

Good reading . . . thanks for posting OP.

Only in very UNIQUE circumstances should "No Venting" apply . . . like super cold environment where some "extra" heat would be a good thing !

 

[DC73]

When properly detailed, roof ventilation keeps roofing materials cooler and prevents ice dams

While proper ventilation helps with ice dams, it is equally important to stop the flow of heat AND air from the interior of the home into the attic. Insulate well and spray foam all penetrations that could allow air to pass from the interior to the attic.

Go ahead and keep doing what you're doing. Just stay away from my house. I don't think you have any real clue what you're talking about.

Actually he does. Go to BuildingScience.com and GreenBuildingAdvisor.com and read up on un-vented attics. It's becoming more and more the preferred method of construction. I talked to a builder last week who has made the switch from vented to un-vented attics and is completely sold on the concept. In some areas prone to forest fires, an un-vented attic is now required.

DC

 

[yeldogt]

Go ahead and keep doing what you're doing. Just stay away from my house. I don't think you have any real clue what you're talking about.

Actually I do --- do some reading on it.


In the USA -- we like to ventilate because we have this notion that it gets too hot in the attic ( and for the roof shingles) ..... there are many places in the world that get hot. And they don't ventilate that way ... I'm fine with manual ventilation that you close in the afternoon.

The problem is not what comes out -- it's what is brought in and where it come from .... normally more humid air from along the wall of the house. Power roof fans are particularly bad... and were promoted for 40 years ... same with using plastic for vapor barrier.

I foam my projects -- the whole thing .. and attic is part of the inside.

 

[ddawg16]

..........
Actually he does. Go to BuildingScience.com and GreenBuildingAdvisor.com and read up on un-vented attics. It's becoming more and more the preferred method of construction. I talked to a builder last week who has made the switch from vented to un-vented attics and is completely sold on the concept. In some areas prone to forest fires, an un-vented attic is now required.

DC

BuildingScience says otherwise....

Actually I do --- do some reading on it.


In the USA -- we like to ventilate because we have this notion that it gets too hot in the attic ( and for the roof shingles) ..... there are many places in the world that get hot. And they don't ventilate that way ... I'm fine with manual ventilation that you close in the afternoon.

The problem is not what comes out -- it's what is brought in and where it come from .... normally more humid air from along the wall of the house. Power roof fans are particularly bad... and were promoted for 40 years ... same with using plastic for vapor barrier.

I foam my projects -- the whole thing .. and attic is part of the inside.

Vaulted ceiling? Yes....but otherwise...vent.

https://buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting/view

Excerpt from the above article....

Understand when to vent your roof and when not to, and how to execute each approach successfully. Reprinted with permission from Fine Homebuilding, Aug/Sept 2011, pages 68-72.

So much information has been devoted to the subject of roof venting that it's easy to become confused and to lose focus. So I'll start by saying something that might sound controversial, but really isn't: A vented attic, where insulation is placed on an air-sealed attic floor, is one of the most underappreciated building assemblies that we have in the history of building science. It's hard to screw up this approach. A vented attic works in hot climates, mixed climates, and cold climates. It works in the Arctic and in the Amazon. It works absolutely everywhere—when executed properly.

Unfortunately, we manage to screw it up again and again, and a poorly constructed attic or roof assembly can lead to excessive energy losses, ice dams, mold, rot, and lots of unnecessary homeowner angst.

Here, I'll explain how to construct a vented attic properly. I'll also explain when it makes sense to move the thermal, moisture, and air-control layers to the roof plane, and how to detail vented and unvented roofs correctly.

 

[DC73]

BuildingScience says otherwise....

Actually, it doesn't. That article is over 5 years old and actually came from Fine Home Building magazine. Most of the BuildingScience un-vented attic articles are newer. BuildingScience tells you how to build properly vented attics as well as how to properly build un-vented attics. Nothing wrong with either approach in most cases. Both are proven to work. The various myths about why un-vented attics won't work have been proven wrong.

There is nothing to dispute about the information you bolded. The last sentence you bolded equally applies to un-vented attics. The key is the phrase "when executed properly".

DC

 

[EOC_Jason]

I foam my projects -- the whole thing .. and attic is part of the inside.

I went in a friend's new house in Houston a couple years ago where they spray-foamed the rafters... OMG wow... The attic was barely warmer than the rest of the house, no humidity, and also quieter to the outside noise.

It was so weird not seeing all the insulation along the ground and instead just seeing the exposed joists & sheetrock...

In Houston that is a very nice upgrade because of the crazy humidity and extreme heat, which obviously beats up whatever is stored in a traditional attic.

 

[yeldogt]

drawg16: I never said they could not be done .. and work. The question is at what cost -- doing a roof over roof with air gap will work ... but why? And the cost.

I think it's better to use rigid foam and sheathing -- IE ...an insulated roof. Vs trying to get all the vents correct -- if you look at what's needed for proper venting -- nobody does it. People think they have a vented roof and don't.

Show me a vented flat roof --- (they do exist .. but very rare) Why is it OK with a cathedral?

I enjoy building science ........but they made mistakes. Look how long they held onto the plastic vapor barrier --- I think we are going to have problems with zip walls.

Walls need to dry to the inside and to the outside -- stop air flow -- keep heat contained.

 

[Orionrising]

Timely subject just added Gable vents to one end of my garage attic yesterday. Have to finish the ceiling and insulate and see how it goes... Really don't want to have to add them on. The other end.. 22 feet up a ladder

Sent from my Moto G (4) using Tapatalk

 

[Falcon67]

Except -- most times it does not work. I stopped ventilating roofs 20+ years ago.

Study after study shows that roof shingles to be maybe 10 degrees lower with a full second roof -- 10 degrees is nothing.

The key is to keep the heat from getting past the top ceiling -- or to insulate the roof. When you live in an area with any humidity -- that humidity come into the attic.

Not in Texas - our roof wasn't vented, they just put in the eve vents for some reason. House was built in 2001, we got in in 2010. Underside deck temps were 150~160F and our 3 tab are about 10 years older than they should be. After venting with 5 9" passive vents up near the peaks the under deck temps run 120~130F in summer. We're probably within 5 years of tear off. Have had to deal with tabs breaking off over the last couple of years. Waiting on a hail storm - we are past due for a beater. Last ones were 1999 and 2006, so well over due. So yes, we vent here.

 

[forAK]

Not in Texas - our roof wasn't vented, they just put in the eve vents for some reason. House was built in 2001, we got in in 2010. Underside deck temps were 150~160F and our 3 tab are about 10 years older than they should be. After venting with 5 9" passive vents up near the peaks the under deck temps run 120~130F in summer. We're probably within 5 years of tear off. Have had to deal with tabs breaking off over the last couple of years. Waiting on a hail storm - we are past due for a beater. Last ones were 1999 and 2006, so well over due. So yes, we vent here.

As noted in post 10, the Houston house was built as yeldogt had said he does. I have seen this starting up here also.

That being said, I'm confused by yeldogt's stating that vapor barriers are not needed. Do you have specific links? I'm looking into this notion also.

 

[DC73]

Not in Texas - our roof wasn't vented, they just put in the eve vents for some reason.

There are plenty of builders in Texas building un-vented roofs including builders in Houston and West Texas that I'm personally aware of. Your roof wasn't an un-vented roof. It was a poorly vented roof. There's a huge difference between what you had and a properly built un-vented roof.

Studies have now proven that shingle color plays a greater role in shingle temperature than does whether or not the roof is vented.

When un-vented attics were first being constructed, shingle manufacturers were wary and there was talk they were voiding warranties on shingles over un-vented roofs. That's in the past. Most shingle manufacturers are now on board with un-vented roofs.

One thing I like about un-vented roofs is that there is no longer an energy penalty for putting HVAC equipment and duct work in the attic.

Important Note: When I and yeldogt use the term un-vented roof in this discussion, we do not mean a standard attic with no ventilation and with insulation at the ceiling level. We mean a properly constructed un-vented attic with insulation above and/or below the roof deck so that the attic technically becomes a "conditioned" space.

DC

 

[DC73]

That being said, I'm confused by yeldogt's stating that vapor barriers are not needed. Do you have specific links? I'm looking into this notion also.

In recent years, building scientists have pretty much stopped using the term "vapor barrier". Instead they use the phrase "vapor retarder". yeldogt is correct - walls must be able to breathe so they can dry out if the interior becomes wet.

Here's one good article: http://www.finehomebuilding.com/membership/pdf/174534/021264084-87-EnergySmart.pdf

You can find others on BuildingScience.com and GreenBuildingAdvisor.com.

DC

 

[lakeroadster]

Our home is in an arid climate... we have cactus in our yard. We have no vents on our roof at all. The home was built in 1978. The roofing plywood and trusses are like new.

My point.... the articles similar to that which the OP posted are too generic, as are building codes that apply to an entire continent.

What works in Florida may not work in Michigan. What may work in central Colorado may not work in Tombstone Arizona.

In our area I've learned:

A lot of roof / soffit vents in an area prone to forest fires means the roof may **** fire embers into the structure.

A lot of roof / soffit vents in an arid area during the winter time means no solar heating advantage from the attic space. If that same area has cool summers, the suns heat in the summer may not be an issue.

 

[yeldogt]

Not in Texas - our roof wasn't vented, they just put in the eve vents for some reason. House was built in 2001, we got in in 2010. Underside deck temps were 150~160F and our 3 tab are about 10 years older than they should be. After venting with 5 9" passive vents up near the peaks the under deck temps run 120~130F in summer. We're probably within 5 years of tear off. Have had to deal with tabs breaking off over the last couple of years. Waiting on a hail storm - we are past due for a beater. Last ones were 1999 and 2006, so well over due. So yes, we vent here.

House design plays a part .... in our quest to build fast, cheap and maximize square footage .. there are designs that just don't work well in some climates. But we build them anyway. Large ...low slope roofs .. on rectangle single floor dwellings -- aimed at the sun ... being a prime example.

Venting an attic of excess heat is not the same as a vented roof -- it all started out with bad design. The stack effect is often the driver of the drop -- as you exhaust hot air from the attic -- you **** conditioned air from the dwelling .

 

[Scott H in Wheaton]

The stack effect is often the driver of the drop -- as you exhaust hot air from the attic -- you **** conditioned air from the dwelling .

In this area the attics are typically sealed from the conditioned spaces, and the soffits provide the intake air to supply the exhaust venting at the top.
On a 95 degree day my attic is only 105 (I have a sensor in the attic and the readout is on my kitchen counter).
I have R49 insulation on the attic floor, and continuous soffit vents supplying air to a ridge vent. No more ice damning in the winter as the temperature is more balanced from the eave to the ridge.

 

[yeldogt]

As noted in post 10, the Houston house was built as yeldogt had said he does. I have seen this starting up here also.

That being said, I'm confused by yeldogt's stating that vapor barriers are not needed. Do you have specific links? I'm looking into this notion also.

It's the use of plastic -- that's the problem. Plastic creates a moisture barrier -- it also a condensing surface. In many climates during the summer the sun will drive moisture through the exterior siding -- it will condense on the cold plastic chilled by the AC on the sheetrock.

 

[yeldogt]

In this area the attics are typically sealed from the conditioned spaces, and the soffits provide the intake air to supply the exhaust venting at the top.
On a 95 degree day my attic is only 105 (I have a sensor in the attic and the readout is on my kitchen counter).
I have R49 insulation on the attic floor, and continuous soffit vents supplying air to a ridge vent. No more ice damning in the winter as the temperature is more balanced from the eave to the ridge.

Done correctly it will work on a hot day in the summer -- But, is it the best strategy? And what about on cold wet winter days? Those vents are bringing in moisture laden air ... even in the summer morning the wet area along the siding is where those vents get the air to cool the attic.

Most houses don't have sealed ceiling (piping/ can lights/ electrical/ chimney) -- that's why ice dams occur. Heat leaking into the attic warms the roof .. melting the snow. If you insulated the roof -- all problems are solved -- and you get to use the attic (if it's big enough)

There are climates where controlled ventilation in a closed space is advisable the hottest days. Controlled is the key.