Advantages of a Vented Over Roof
Welcome back to Fine Homebuilding House Kansas where today we are standing on our air barrier. We are excited about this detail because it’s going to solve a lot of problems down the road. The vented over-roof, or back-vented roof, has become a key feature of the build because it’s helping us solve some problems such as the cantilevered exposed rafter tails that make up our overhangs. The rafter tails are continuous to our interior and we’re accepting that thermal bridge sacrifice to keep the tongue-and-groove aesthetic on the back of the overhang.
Choosing insulation for a vaulted ceiling inside
In order to have the vaulted ceiling inside and avoid using spray foam, we like working with Rockwool Comfortbatt. On past projects in our market, we’ve seen improper and incomplete spray-foam applications with voids, shrinkage, and delimitation that could create pathways for moisture-laden air to reach the back of roof where it could condense into liquid water that would stain the ceiling and potentially create far worse problems like mold or rot.
In comparison, the vapor-open Rockwool allows drying if moisture-laden air ever did enter the system. That coupled with the high density of the product for ease of cutting and slightly compressed installation mean we will achieve contact on all six sides of the cavity to eliminate any convective looping. Since these vaulted ceilings share the structural member with the roof and we aren’t venting that cavity we want to be certain that we allow for the maximum drying potential should any water vapor enter the system from the interior.
In order to have a vaulted ceiling, we really need a way to vent the roof above it. So on top of our air barrier, we install furring strips on top of every rafter and add an entire second layer of decking to separate high temperatures in the attic from the back side of that first layer of asphalt roof.
We have to account for the difference in the thickness from the 1/2-in. ZIP System roof sheathing to the 3/4-in. tongue-and-groove so that our roof will plane out. In order to do that, we used AdvanTech 1-1/8-in. subfloor rips over the tongue-and-groove soffit and then transitioned to 1-1/2-in. furring strips over the ZIP roof decking. (We added Huber’s peel and stick underlayment over the tongue-and-groove to protect the wood, which is why we used 1-1/8-in. furring rather than 1-1/4 in.).
One of the reasons for building an over-roof is that it will provide a longer life for our shingles. In the Midwest we have to deal with hail and other strong weather conditions; you can’t out-guess natural disasters, but this roof will be well-ventilated and should last our homeowner a really long time. The same way a rainscreen extends the life of the paint job on your siding, this back-vented situation does a big service to your house.
For our subfascia and fascia, we ran the tongue-and- groove an inch back to create a vent space. We then have a furring strip on top of that 3/4-in. material, as well as a bug screen in the ventilated intake shoot to keep out any insects in this space.
This space will allow the air to flow free all the way up to the ridge, which will help reduce the temperature of this upper roof deck.
If this were a typical hot roof, this roof deck compared to the interior temperature on a hot summer day could be a 50°F temperature difference: 70°F degrees inside and 120°F on the roof in the sun. That’s a big difference to make up in that cavity. So by cooling this space, the cavity may only see 90°, which means we’re only dealing with a 20° delta T at that interior ceiling.
On the back side of the roof we can see the exposed framing of our screened porch; this is not covered with ZIP System sheathing because it’s going to have the same material as our soffit and we want it to look from underneath like finished wood. One of the benefits of our over-roof strategy is that we don’t have to resolve for the roofing nails coming through that material because we’ve introduced that gap for our ventilation.
Because the underside of the porch ceiling is that same 1×6 tongue-and-groove as the rafter tails, the under-roof plane is actually 3/4 in. thick whereas the main house under-roof deck is 1/2-in. thick. We make up that difference, once again, with a 1-1/8-in. furring strip on top of peel and stick underlayment that protects the wood until we get a chance to cover it with roofing.
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Пікірлер: 83
1:00 Would you not be better off staggering the seams of the rockwool? It seems odd to go to so much effort in double layering to not then do a "bonded brick" layering to reduce gaps at joins.
@ronaldoleksy8264
Жыл бұрын
Agree.Why not. Take it to the next level
I did this to our 1913 bungalow. Made a huge difference in the indoor temperature during the summer.
@markprimiano
9 ай бұрын
hey did you use regular lumber for the furring above or pressure treated?
So, extra zip roof sheathing and the labor to install it, just so you can have venting just so you can have exposed cantilevered beams just so you can have vaulted ceilings? Homeowner must have money to burn. I would love for these content creators to show us the numbers like ROI and any long term savings projections along with these innovative build solutions.
@crabkilla
Жыл бұрын
In the world of homebuilding, houses are like cars. You have cheap ones, mid-range ones, and high-end ones. You can do the cheap details all you want and save your money so you can put frivolous ornamentation on your Ford truck that has no ROI or long-term savings. Same vibe.
@spencer476
Жыл бұрын
Some of us will pay extra for quality. It is not all about the lowest cost. I am about to start a home build in the mountains of North Carolina. I have no desire to throw money away. However, I want to build a house that is worthy of passing down to the next generation or two. I cannot control whether they will want to keep it, but I can control whether it is worth keeping. Price is a factor, but it not the only factor.
@crabkilla
Жыл бұрын
@@spencer476 Sure . . . everyone's equasion for what to spend money on is different. Some geezer jumping on and saying "I have been doing it this way for 40 years, let me see the ROI for that new way", is ignorant at best. Nothing would advance in the world otherwise.
@dlorien7306
Жыл бұрын
Wow, all the pushback for a great point. I think the onus is on the ones doing the new, very expensive thing to justify, otherwise it very much is "just throwing money away" Builders will gripe all day about how anything less than their max profit is a "donation" but then all these buyers don't seek max value. So only buyers can make donations, they don't have a business, just lots and lots of money to burn
@josephhuether1184
Жыл бұрын
Skip that one Xmas Caribbean vacation.
With the furring strips, you had the perfect opportunity to do standing seam metal, and yet on such a nice house you went with the cheap option in shingles. I'm disappointed.
@tt7hvn
Жыл бұрын
Between the cost of the second layer of sheeting, and the shingles themselves, they probably would have been close to a standing seam roof. The performance of a metal roof would've been better overall since it's not a thermal mass nearly as much as the shingles are. The temperature in that gap would've been much lower depending on the color of the roof they chose.
@michiganengineer8621
Жыл бұрын
It might be an HOA requirement, I didn't notice any standing seam roofs in any of the background shots.
@ouimetco
Жыл бұрын
I agree but not everyone likes standing seem or the cost associated. But yes your right.
Wouldn't a metal standing seam roof be cheaper than the 2nd layer of ZIP + shingles? And it would last longer and look better too while keeping the benefits of a vented space underneath.
@vince9486
Жыл бұрын
Yes, great question and also why not use regular OSB or plywood on the 1st layer? Serious overkill and even if a high dollar house it is wasteful. Matt Risinger on his channel has done this with metal as the top layer and it looked a lot better.
@MattSapp
Жыл бұрын
Still need the Zip + tape on the first layer for air sealing. Notice they filled in all the spaces between the rafter tails with zip, and in another video they tape and liquid flash all those joints around the tails and tape up onto the first roof layer. I think an easier argument would be to use cheaper regular OSB + a synthetic underlayment for the upper deck below the shingles
@mr.g937
Жыл бұрын
@@MattSapp You need even more than that - standing seam warranty is void without a high temp peel and stick underneath
@MattSapp
Жыл бұрын
@@mr.g937 That must vary by manufacturer or product line. The vendor I'm most familiar with says this in their install guide for their snap-lock standing seam: Note on Underlayment Not all conditions require an underlayment, However metal roofing is susceptible to Condensation and it is recommended that an appropriate Roofing underlayment be use on all wood substrates to protect the structure during installation. For added protection from rain and snow, it is recommended to use rubberized Ice and water shield on eaves, valleys, and areas of roof penetrations.
@mr.g937
Жыл бұрын
@@MattSapp Does it say anything different on their warranty details manual? Curious, what manufacturer is this? All of the ones I've seen specify the high temp ice and water
The furring strips run to the ridge but I can't yet see any ridge vent to allow flow and exit of the heated air. Maybe that's coming in the next episode?
During winter months in Kansas, that sheathing under the vented roof is going to be cold (20F during many nights). By installing (vapor open) mineral wool behind the vaulted ceiling (0:50), won't vapor pass through, hit that underside of the cold sheathing, and condense? I'd expect this to occur even if humidity is well-controlled, e.g. 40% at 70F has a dew point of 45F. Maybe it won't be awful, but no matter the condition, it's going to be hard to see if a problem is forming. Reference that comes to mind: Building Science Insights 016 ("Ping Pong Water").
seems like a good application for midwest cathedral roof w/ overhangs.
I literally send Matt Risinger an email a couple years ago about his thoughts on this. I called it the double decker roof system. The idea is a ventilated channel on top of a sealed roof deck. I like it. I’d probably do some zip r on the trusses then a 1.5” gap with some commodity OSB with a few layers of 60lbs paper and some architectural shingles on top.
@DrWarBear
Жыл бұрын
Curious what Matt’s thoughts were when you emailed. I’m a big follower of his and I’m looking to build a house soon with a conditioned attic. Curious if he felt it was necessary to vent the roof deck, especially if you’re using closed cell spray foam. Or if this is only something you do when using insulation that is susceptible to water vapor
@mylifeintexas
Жыл бұрын
@@DrWarBear He never returned my email so I’m not 100% sure what he thinks on this. I just think the idea of having an umbrella over your roof is a good idea. Like I said, I’d probably use a Zip R 3.6 panel on top of the trusses then 1.5” gap with commodity osb over that. Of course all the details done correctly. Should technically be able to keep the attic air temperature at a very comfortable temperature with very little effort.
@CodyBrannan
Жыл бұрын
He has a video on metal roof details where he goes over adding furring strips or matting between the underlayment and roof material for the purpose of ventilating.
@mylifeintexas
Жыл бұрын
@@CodyBrannan That’s a common detail for metal roofs. It’s not common for shingles or other types of roofs though
@donlourie769
Ай бұрын
I built a house with a similar vented roof. The space was 1" with an OSB decking above, covered with a standing seam metal roof. Utility bills savings more than paid for the extra expense and labor. The occupants averaged less than $100 a month for electricity on a total electric house. 2200 square feet and 7 children. This type of roof is a great idea and I am remodeling a house for myself using the same details.
Wow, nicely done!!! I wish Huber or LP would make a panel with WRB on one side and radiant barrier on the other. Imagine your outter-most layer of roof decking with a radiant barrier. Thanks for sharing
@Sheets213424234
Жыл бұрын
Huber used to have one and took it out of the market for some reason...
@JoshuaRes
Жыл бұрын
Radiant barriers don’t do all that much when you have adequate insulation. Check out the building science corporation articles on that topic, it’s pretty interesting.
@treystills
Жыл бұрын
@@JoshuaRes I'll look into it. Thanks!
@treystills
Жыл бұрын
@@JoshuaRes was unable to find anything on radiant barriers. I'm sure Joe has published on barriers before. What specific article are you referring to?
@JoshuaRes
Жыл бұрын
@@treystills BSI-136 though in reflecting, I wonder if the roof deck is the one place a radiant barrier could still have value… to reflect heat absorbed by the roof from the sun back out. But in that case, maybe venting and adequate insulation still solve the problem. See what you think after reading :)
Any available documentation on this? I would love to do this next reroof.
In Colorado they called it a cold roof so ice dams don't start from warm air melting snow. I wouldn't use zip for the first layer.
I've always wanted to vent in a similar way. My dig question is about condensation. Where will it appear?
Great video. Lots of information in just a few minutes. FYI- some serious issues with safety though (no harnessing, walking right next to a massive hole, man on ladder seems to be on the top step and leaning over. Call 911.)
Not sure why 1.5 inches. Is that enough? Any why not a metal roof over a bigger space over peel and stick water and ice barrier
What your doing is creating an air break the same principle as an insulated double wall mug but I have a question do you not have carpenter bees where you are or how do you keep them out
@JoshuaRes
Жыл бұрын
Usually core-o-vent or some similar material.
Cold weather application?
Why do u put a ice and water shield over the zip roof? Isn’t the sheeting supposed to take the place of felt paper or an ice and water shield. That’s a new detail I have been seeing lately( or maybe it is old but I just started seeing it) but what is the benefits to it? 👍
@finehomebuildingmagazine
Жыл бұрын
We used the Ice and Water Shield for an additional measure of protection beyond the taped and rolled Zip System sheathing. It allowed us to protect the Tongue and Grooved soffit material from water staining prior to roofing at the perimeter as well as at the back porch where there was no zip. It’s also common for many shingle manufacturers and therefore some building code enforcement officials to require the ice and water shield, felt or other additional underlayment which is likely an extension of the Ice Barrier requirement (R905.2.7.1) 2’ past the interior plane. We also used the Zip Peel and Stick underlayment to block water vapor exfiltration through the sheathing.
@hammertime7349
Жыл бұрын
@@finehomebuildingmagazine thanks for the reply.
Isn’t the effect on the shingle life from not having a vented roof affected only negligible? A few years maybe?
@ronaldoleksy8264
Жыл бұрын
Agree heat does kill a roof.which brings me to why reproof over old asphalt. Besides the weight. You will create a thermal mass.
where is your air control layers in this assembly?
Why not put a vapor-open radiant barrier on the bottom layer of sheathing to reflect most of the radiant heat away?
@zefrum3
Жыл бұрын
OSB RB are BS for cooling; they may (may) aid in retaining heat. Why I say BS? Because, heat radiates from the sun to the roof shingles, where it then conducts through the decking and framing. The heat is already in the house. If now there is a reflective material, all this does is to bounce, or emit, it back to the roof deck. But being that heat travels from hot to cold....it goes right back to through the decking and yea...like I said BS
@finehomebuildingmagazine
Жыл бұрын
Radiant barrier can make a lot of sense in many warmer climate zones but not necessarily at our location in CZ-4 just 50 miles from CZ-5 where we still have more heating degree days than cooling. A vented system such as ours outperforms a radiant barrier system since the reflection of the radiation in radiant barrier systems offers no convective drying, only reduced conduction.
@mmroofs
13 күн бұрын
@@finehomebuildingmagazineUnfortunately by not installing a Radiant Barrier OSB backing you missed a great opportunity to reduce your heating in that cavity by up to 30%. But it still works ok without it, congratulations on the venting, assuming your installing a continuous ridge vent? Nice work!
Zip ice and water shield is made by grace.
Seems like a cheaper and easier option would be to put Rockwool between the roof deck and a standing seam roof. (Would look better and last longer too.)
How does the gutter system work?
@baxtronx5972
Жыл бұрын
The cantilever is the gutter. See Japan for reference.
Pretty talented worker at 4:20
What does "We don't have to resolve for our roofing nails" mean? It's at the 3:50 mark.
@CybekCusal
Жыл бұрын
It's just a bunch of word salad because he's trying to sound smart but isn't.
@drummereef
Жыл бұрын
"find a solution to..."
@travisburch4342
Жыл бұрын
@@drummereef I’m aware of that, just curious about why you need a solution to roofing nails.
@vince9486
Жыл бұрын
He means with the air gap over the T&G and second layer of Zip the roof nails will not go through and be visible from the porch below.
@ouimetco
Жыл бұрын
The roofing nails would protrude through the underside soffit. Unless they used specialty nails, like 3/4 which would the cause issue with high winds. 1-1/4 standard length although we use 1-3/4 for the capping around here.
👍👍
Come on guys 2 layers of Zip is ridiculous.
@michiganengineer8621
Жыл бұрын
Hell, if I could afford to build a new house I'd be doing the same thing for the roof system except for one thing. Even though Huber hasn't given their seal of approval, I'd be using Zip-R6 for the inner level of the decking and the regular Zip for the layer immediately under the shingles or solar roof.
This house has an excessive carbon footprint and is bad for the environment.
@donlourie769
Ай бұрын
We need more carbon in the air. That is what plants use to grow.
Complete waste of money and time. Spray foam the rafters you will most likely get a better R factor. Saving tens of thousands on all the labor and material, literally adding a second roof.
This is great for you rich guys to throw you your money at. the rest of us would be happy to afford just a re roof that didn't cost 15 thousand dollars.