5 Biggest MYTHS About Unvented Roofs | What You Need To Know
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- Опубликовано: 31 май 2024
- Unvented roofs, also known as "conditioned roofs," have a negative reputation in the building community due to the many issues that arise from poorly designed unvented roof assemblies. This has led many builders and design professionals to avoid them altogether. However, when correctly designed, unvented roofs can outperform vented roofs in all climates. In this video, we will debunk five of the biggest myths surrounding unvented roofs, explain how to design them properly to avoid moisture issues, and demonstrate how they can increase the durability of your home.
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0:00 Intro
0:29 Myth 1 Unvented Roofs Will Rot
2:49 Myth 2 You Must Use Spray Foam
5:34 Myth 3 Unvented Roofs Cause Ice Dams
7:40 Myth 4 Unvented Roofs Make Shingles Blow Off
8:20 Myth 5 Unvented Roofs Are Hotter
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0:29 Myth 1 Unvented Roofs Will Rot
2:49 Myth 2 You Must Use Spray Foam
5:34 Myth 3 Unvented Roofs Cause Ice Dams
7:40 Myth 4 Unvented Roofs Make Shingles Blow Off
8:20 Myth 5 Unvented Roofs Are Hotter
Are there calculations done on preventing the need for a vented over-roof by using more roof insulation and a colder attic space? How much fiberglass insulation and vapor barrier can you buy with the cost of an over-roof...
Great information. I had a tree fall on a home in 2020. The roof needed significant repair, and had zero insulation (in a cold climate where 3-4 feet of snow in the winter is normal). I could not find anyone willing to rebuild as you describe in option 1, so I did it myself, with a staggered 6" layer of polyiso (3"x2). I've had zero issues with moisture or ice dams since then and the house stays warm and comfortable. Everyone I talked to thought I was crazy... Thanks for spreading the word and hopefully more will begin to understand the science around this method.
So sorry to hear that! Glad the new roof assembly turned out well though. Thanks so much for watching!
Only thing I'd say about that is polyiso R values drop in the cold whereas EPS and XPS go up when the temps drop. But if it works, it works.
For Roof with exterior insulation, your better off just install SIP panels, since you basically hand building one. For SIPS use a high perm roofing membrane so there is a path for moisture to escape. Better would be a vented metal roof, which the metal roof panels are installed on perlings that have air channels from the eaves up to the ridge cap. This will also help reduce heat load during the summer months as it will create passive convevtion air flow as the hot air escapes from the ridge vent & pull in cooler air from the eaves. Also recommend have a 2 foot roof over hang which will protect the walls from weather, & reduce summer sun radiant heat from the walls and windows directly above the roof.
Wow. Great content. Please continue with this. It’s exactly what I’d been hunting around for. Bravo.
Great explanation on assembly s, on your condition attic with the sheathing separated and vented for cold climates. I think for air intake at eaves either vented drip edge or continuous soffit vent. For exhaust air continuous ridge vent. Important to have air flow under roof sheathing. No matter what type of assembly. As a carpenter who has first hand experience.
Unsurpassed !! Great video presentation. Great graphics, narration, nice quick pace, thorough discussion of the topics.
Thank you so much! Cheers!
Great video topic. While the geometry of a roof peak means it requires more insulation and other construction materials than an attic floor, it's not THAT much more when you consider the total cost of the home and the extra clean space provided. It only SEEMS inefficient on paper. In reality, how much would it cost to ADD that much extra space to a home after it's built? Exactly.
If you want to save money, save it somewhere else. Don't put a rat's nest on top of a new home if you can help it.
I am going with exposed rafters and roof board. Gonna insulate the outside of the roof. Doing a sealed shell with eaves on outside of shell. Going with mineral wool board on roof. Have a 6/12 pitch to help with snow sliding off.
Great video, as always! Coming from the PNW and cold climate design further up north, we exclusively use a vented roof assembly as per the BCBC. You can spray foam with closed cell, however, as you mentioned, it can introduce condensation risk through cracking over time. We'd prefer to avoid that. Generally, we only use spray foam for joist ends, if required.
Our go to system is a vented roof assembly which is trussed or hand framed with I-joist and 2X4 cross purlin, with a smart vapor barrier such as Siga Majrex. In marine environments we also spray the framing with a natural anti-mold agent. There's actually a good argument in those situations to go with an unvented roof assembly, however, it's not commonly practiced in our market and the cost would certainly exceed the anti-mold spray.
I built a non vented 2 story 2010 , using spray foam in the attic and batt insulation in the walls below in north central Fl.
I added a couple A/C outlet ducts and tapped into a return in the attic(easy) to condition it. Never had a problem.
Great video and pleased to see someone take on this topic.
My house is from 1920s and i noticed under the aluminum soffit covers there isnt any openings the roof comes flush to the plate…and theres no ridge vent
Great videos.
You could add that the ratio of exterior rigid to cavity insulation can change depending on climate zone. The ideal being that the sheathing remains warm over the whole year - this covers cold climates and hot humid climates. Building Science Co has tables for a general rule of thumb.
The other aspect you could talk about is having a vapour open ridge detail, allowing buoyant vapour to migrate up to the ridge over a diurnal cycle, of heating/vaporization and cooling and condensing, before venting out to the environment.
Cedar breather under metal roofing helps with redistributing localized water accumulation, but still requires vapour drive into the condition space to dry. I think a vapour oven ridge and an interior vapour throttle is the most general assembly.
Finally, unvented roofs are excellent where wildfires are a risk. There is no vent for the embers to get into the roof assembly.
Thank you so much for watching, great points. I actually put up an image of the required climate zone ratios at 1:23 per BSC. The vapor diffusion port is a great option for those warmer humid climates, but we just aren't sure how to get them to work in colder climates without a smart vapor retarder. Might need to make a separate video on these too.
@@ASIRIDesigns Nice, I missed that at 1:23.
In cold climates you either vent, or tend towards a vapour barrier right?
The diffusion vent would be a redundancy for any accidental moisture accumulation - being cheaper that vented over-roof.
If you're taking request: I think suspended ground floor assemblies need much more attention.
for the ice dams issue you can just build the roof tiles or metal roof with a vented space between the underside of the roofing material and the counter battens on top of the roof membrane or sarking board.
Specially in a gable roof, it has the perfect airflow for ventilation between the roofing materials and insulated structure it self.
of course before thinking about any of this you also have to make sure your structure is insulated up to standards, so make sure you have air exchange of less than 0.60 and a U value of less than 0.20 at the very least.
Could you provide a suggestion for myth #4 - open cell scenario? Is it as simple as inhibiting the moisture diffusion through the open cell, so adding the "taped smart vapor retarder" membrane on the face of the rafters under the strapping?
At about 4 minutes in - the smart vapor retarder - is that sort of a "one way" barrier? If so which way do you want the vapor to go? Or it doesn't matter? Thanks.
I’m a roofer in south Louisiana. Heat and humidity are the overwhelming factors to consider when building a home, with hurricane resilience a close runner-up. It’s so frustrating trying to explain to a homeowner that the spray foam in their attic is what is causing their asphalt shingles to peel right off the roof. “But I’m saving $$$ on my electric bill….”
Question: what is a “vapor permeable peel & stick underlayment”? All of the peel & stick underlayment I am familiar with act as a vapor barrier.
Hes probably referring to a product like Henrys Blue skin. Cheers from Montana
Adhero 3000 is one example of a self-adhered vapor permeable membrane that can be used as a roofing underlayment as well as a WRB. In Louisiana it makes sense to have a vapor impermeable underlayment in most cases due to all that warm humid outside air.
Is it possible to design a perfect wall/air tight house that can work without the need of mechanical ventilation? Or would you always run into moisture problems without active ventilation and dehumidification?
Why wouldn't you want ventilation or a fresh air system?
@@ASIRIDesigns For a structure that is air tight but won’t be occupied full time, i.e. a shed or a vacation cabin.
What is an example of the "taped smart vapor retarder?" Is this just housewrap that allows moisture out, but not in?
Intello and SIGA Majrex are the two big brands.
@@ASIRIDesignsthank you! I opted for r30 mineral wool in my attic because I didn’t like the idea of spray foam concealing a roof leak for a long time if one occurred. If I wrap the inside of the cathedral ceiling with either one of these products you mentioned, will they let water through if I get a leak?
@Incredibletinyhomes Is there a better alternative to spray foam???
Yes! Rigid insulation installed outboard with an air barrier membrane is significantly better.
Unvented roofs are basically the same as walls. The only issues that arise from unvented roofs is that the roofing and waterproofing degrades faster because of the heat not having a way to escape.
this is a American thing.
In Germany we have been building unvented roofs for many generations already and never had such problems in mass.
We build living spaces in the roof, so for us is quite normal not to leave it as a open space in contact with the outside, but rather as just another part of the house insulated just like any wall but with roofing membranes and sarking boards but sheathing boards as well depending on the design.
You have a different climate and different building materials. A lot of your building code is written in stone and has not changed for 50 years. In America, cheap is king when it comes to the majority of our new construction.
@@SteveThompson-li2fc
I live around Munich in the south of Germany and my climate is much of the same as your's in the US.
In winter I get -5 to -10 Celsius degrees and in summer I consistently get 30 to 33 Celsius degrees.
Not sure about "writing in stone" and "not changed in 50 years", it has changed a lot in the last decade alone, especially in regards to energy efficiency, but the building practices themselves truly don't change much.
a lot the detailing around houses has been an evolution of old practices to fit new building products and standards, however most buildings still adhere to the basics of vapour permeability, health living without hazardous materials and use of traditional styles and materials such as bricks, klinkers (lime bricks or paver bricks), plaster and use Walmdach and Krüppelwalmdach roofs that you know as hip and jerkinhead roofs.
nobody builds houses anymore out of bricks, instead most buildings are made out of single leaf solid clay block walls made out of concrete columns/beams with clay blocks in-between or just clay blocks if its a small structure.
The exterior is usually finished with continuous insulation in the exterior or with bricks for the facade finished look.
If if has bricks of the outside or not depends if the clay blocks have enough insulation within than.
And as I said on my fist comment, the common practice is to construct a unvented steeply pitched roof to server as a additional living space in what you would call a attic.
Your climate is not the same as south Louisiana. @@user-wy5ch2xc8z
Great video, I’m not hating, but… It completely skips non-spray foam options for trussed roofs. Dense pack will settle and you’ll be left a pocket at the ridge in which humidity will collect. If you use blown fiberglass the pocket will be even bigger. No one is doing blown mineral wool right now, and mineral wool bats create a ton of cold spots. That only leaves a vented over roof assembly, which is hilariously expensive in terms of labor and materials. You can do LVL rafters but that is also super expensive. BCI is better but mineral wool leaves a lot of voids with that too. Now we’re back to foam. I like Zip-R 9 🤫, with two inches of closed cell sprayed over the truss members and plussed up with open cell, then covered by a clay based fire retardant paint I’m not trying to pretend I’m a building science expert, but I have a JLC subscription and I think about this problem ALL the time.
Properly installed dense-pack cellulose does not settle. It's blown in at twice its settled density, so there's simply no room for it to settle. You might be thinking of standard blown cellulose used in attics, which does settle because it's just loosely blown. There's also damp-spray cellulose, though, which I have in the walls and under the floor of my house. it adheres to the substrate and builds up into a spongy mass. I haven't seen it used in roof assemblies, but I'm not aware of any reason it couldn't be blown that way just like it is for walls. I have 6+ inches of the stuff between my floor joists directly exposed to humid central TX outside air, and after 15 years, it's still adhered just as well as the day it was applied.
I’ve never remodeled a house with damp blown, but I worked in San Antonio for 4 years and dense packed definitely settled in areas above showers, and other places with high moisture exposure.
WHAT ???
Myth 3 Unvented Roofs Cause Ice Dams. Solution: build a vented roof over top of the unvented roof. Ooof!
Who has the money for that?..sure you can do it...but...
$100k roof defeated by mice.
Myth #3, just fix your unvented roof by making it a vented roof, myth busted? Worst ice dams I've ever seen were in Washington DC, not just a cold climate issue if you don't want water running into your house.
9:06 "negative pressure", as far as I understand, the code requires at least half the venting to be soffit vents, ridge vent is optional, soffit vent is required, for this very reason. The outdoor air will be warmer, more humid and less dense than the indoor air to lessen the amount of air being pulled through the ceiling by ridge vents.
Number 3, by your own admission, is not a myth. You solved the problem by venting the roof.
You're venting the sheathing/roofing finish, not the actual roof itself.
@@ASIRIDesigns, that's not a real distinction. You're making the distinction because it fits your argument, but it's not a distinction that is made in the real world. If you were making the argument that the roof structure doesn't need to be vented, then sure, but you aren't.
You've offered a presentation that gets all the details right, and then makes an overbroad conclusion. You're sacrificing accuracy and confusing people to make yourself sound like some sort of revolutionary. The reality is you are saying the same thing that anyone who has a basic understanding of building science would say: You can get away with an unvented roof in many scenarios, in an area with high snow loads, you need a vented roof.
So you're venting about venting. Ok.
It's like I always said....idk.
I said a lot of stuff. Age 67.
Technically great presentation. However finding a competent contractor to do the work is a big challenge. Also an inspector should be checking on the contractor to make sure they are following the specifications as the project moves forward.