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Lstiburek’s Rules for Venting Roofs

You need an airtight ceiling, lots of air flow, plenty of soffit vents, and deep insulation at the attic perimeter

Posted on Jul 24 2011 by Joe Lstiburek

Building Science Fundamentals: Roof, Part 1: Ventilation

By Dr. Joseph Lstiburek

Dr. Joseph Lstiburek talks about the not-so-controversial ways to maximize the efficiency and airflow of your roof and attic.

Video Transcript:
There’s been so much stuff said about roofs that you sometimes lose perspective. I’m going to start off by saying what might seem controversial but really shouldn’t be.

This is a vented attic, and it’s probably one of the most unappreciated building assemblies we have in the history of building science. It’s beautiful. It’s hard to screw this up.

For 20% of the effort, it gets us to 80% of optimal performance, and it works in hot climates, in mixed climates, the Arctic, the Antarctic, the Amazonian rain forest — it works absolutely everywhere. The value proposition of a vented attic, meaning the money that you invest in building one of them — it’s hard to argue with the benefits. But for all kinds of reasons, we manage to screw it up.

The single most important thing you have to remember about a vented attic is that the ceiling plane — the gypsum board layer, the drywall layer — needs to be airtight.

1) The ceiling plane MUST be airtight
Absolutely airtight. Above the airtight ceiling plane, the only thing that should be seen is insulation and air, nothing else. Not last year’s Christmas decorations, not your high school prom dress, not the tuxedo you were married in and can no longer fit in. Nothing but lots of insulation and air. Just an airtight ceiling and nothing else.

2) If you’re going to vent the roof, then VENT THE ROOF
If you’re actually going to vent the roof, let’s be serious about venting the roof. Wash the underside of the roof deck with air. That means the entire perimeter of the roof needs to have air inlets, meaning continuous soffit ventilation. It’s dumb to have baffles every third or fourth bay, the entire underside of the roof deck should be washed. Where the air leaves isn’t as important — whether it’s a ridge vent, or mushroom caps, or gables. What’s important is that you have continuous air entry at the perimeter of the roof down low.

3) Put more vents down low than up high
This is where the code tends to have it wrong. You want more entry points at the perimeter than exit points at the top.

People say you want to balance the lower down ventilation with the upper ventilation, and a lot of people interpret the codes to say that if you get it unbalanced you want more ventilation up high. That is absolutely wrong; you don’t want more places for the air to get out than to get in. The reason is, if you construct a house with a leaky attic ceiling and you have lots of ridge vents or you have lots of vents up high, the makeup air is going to be pulled from the house rather than being pulled from the outside. That scenario is a disaster.

Attics should be ventilated with air from the outside, not the inside. That’s why I hate these whirligig turbine vents — because they depressurize the attic, and if your attic ceiling isn’t perfectly airtight, you suck air conditioned air or heated air out of the house.

It’s even crazier when the powered attic fans can actually suck on the roof and they’re controlled by a thermostat. How stupid is that? Of course the attic is going to be hot. You turn them on and they suck all the air conditioned air out.

No powered attic ventilation; more vents down low than up high; wash the entire underside of the roof deck. But all of that is secondary to having the ceiling plane airtight.

This last tip is more important in cold climates than anywhere else. But where the ceiling insulation hits the perimeter wall, you don’t want the amount of ceiling insulation on the top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate. to ever be less than the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. in the wall itself.

4) Put more insulation on top of the wall than inside it.
In other words, if you have an R-20 wall, you want at least R-20 on your top plate. A higher R-value is better, but a lower R-value is not. If you have an R-30 wall you want at least R-30 on top of your top plate.

A reasonable rule of thumb is: Thou shalt never, according to Joe’s Rule of Thumb, have less R-value on the top of your top plate than in the wall. It would be nice to have even more, but not less.

Notice: nowhere in this discussion did the term “vapor barrier” come up. If you really want to have a vapor barrier in the ceiling, limit it to climate zone 6 or higher, but that’s really not important compared to the airtightness of that ceiling plane.

The building code calls for a vapor retarder in climate zone 6 or higher. It’s okay to put one in, but if you don’t, take a Valium and relax. You don’t want to go through a lot of brain damage in a renovated house to try and add a vapor barrier underneath insulation in an attic. What you really want to do is make that ceiling plane airtight, make it airtight, declare victory and be done. Don’t mess around with permeability’s and calculations and whatever.

To recap, airtightness on the ceiling; washing the underside of the roof deck; unbalanced ventilation should be in favor of the lower vents because you don’t want to depressurize the attic; no to powered attic ventilation or the whirligigs; and you don’t want to squeeze the insulation at the perimeter so it’s less than the R-value of the wall.

That’s it. You can build that everywhere in the world and life is good.

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Jul 24, 2011 10:08 AM ET

Good to be reminded
by James Morgan

of the basics once in a while. And even more so of the underlying principles that go with them.

Jul 24, 2011 10:29 AM ET

Service Core & Airtight Sheathing
by John Brooks

You say that "the drywall layer needs to be airtight" ...why the drywall?
Why not create a 4" to 12" deep service core for can lights, small ducts & wires and then install continuous airtight sheathing on top?


Jul 24, 2011 10:57 AM ET

Edited Jul 24, 2011 11:02 AM ET.

Vented roofs
by Doug McEvers

Dr. Joe,

Thank you for a nice piece on venting roofs. I like your point about venting between each rafter and washing under the decking. Yea for energy heel trusses, this, along with air sealing will eliminate ice dams in new housing.

In MN we get attic frost buildup in homes with poor roof venting and leaky ceiling planes, as soon as the first warm day comes, water drips through ceilings and folks think the roof is leaking. This is due to lack of venting and attic bypasses.

Jul 24, 2011 12:12 PM ET

Edited Jul 24, 2011 12:13 PM ET.

Service core & airtight sheathing
by James Morgan

John B's notion of a ceiling service core below the air barrier is a great one - future-proofs the installation by allowing easy electrical remodels below below the air-seal layer. Not sure about the truss though. Here's a close-to-standard stick-frame alternative which meets Joes' requirement for depth of insulation at the attic edge above the wall but which avoids the structural redundancy of the truss above the encapsulated ceiling joist. Just have to figure out how to tell the client they're going to to pay for a fully-sheathed attic floor but they can't store anything up there.

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Jul 24, 2011 12:27 PM ET

In a renovation
by 5C8rvfuWev

Joe L. mentions renovation, but I've wondered --

if the original envelope lacks an exterior air barrier, does installing one (non-continuous obviously) at the attic (like John or James describe) create any problems? I can't see why it would but wonder if there are consequences I'm not understanding.

Joe W

Jul 24, 2011 12:31 PM ET

Edited Jul 24, 2011 12:36 PM ET.

I agree with the stick frame option
by John Brooks

James, I envisioned the concept being used with stick framed roofs as well.
The cladding structure can be trusses or stick framed
It is very similar to what Thorsten Chlupp is doing on the SunRise Home


Jul 24, 2011 12:51 PM ET

Edited Jul 24, 2011 1:01 PM ET.

Another one:
by James Morgan

A double or triple plate over the attic sheathing may be needed to lift the rafter high enough to clear a thick layer of insulated sheathing on the outside of the wall. Alternatively I suppose you could make the seat cut longer on the rafter notch. A baffle of board insulation above the air barrier will eliminate thermal bridging at the raised plate.

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Jul 24, 2011 1:04 PM ET

offsetting the cost of the sheathing
by John Brooks

James, my thinking would be to ask the homeowner to pay for the sheathing instead of paying for sprayfoam.

I would try to convince the framing crew that they would save time with a method like this by having a SAFE stage from which to build the roof.
I also think the house could be "dried in" quickly and tested for airtightness.

The tape on the deck would not be subject to a lot of stress because the deck and the tape would be close to the interior temperature.... and if the tape did fail in 50 years .. it could be redone.

Jul 24, 2011 1:29 PM ET

Edited Jul 24, 2011 1:33 PM ET.

Reply to John B:
by James Morgan

All good points.

I very much like the baseline idea of the service core within the air barrier as I have seen so many attics where an otherwise satisfactory depth of insulation has been kicked out of the way for later electrical work and never replaced, and where can lights have been cut in without any attempt at air-sealing. This may prove to be a more robust setup. Dare I say more idiot-proof? I've long advocated for the attic as a storage-free zone with a tightly-sealed inspection scuttle rather than the standard pull-down stair. This looks to be another good step in that direction.

By the way, I never specify spray foam anyway, at least if there's any other reasonable option.

Jul 24, 2011 4:54 PM ET

Edited Jul 24, 2011 5:52 PM ET.

Service Core
by josef chalat

Why not make the walls 6" higher and fur down with 2x6s under a continuous gypsum board ceiling that provides the air barrier. You could use a truss with a raised heel and you would not need to frame an attic floor. You could omit the furring in places to create a tray ceiling.

Jul 24, 2011 5:56 PM ET

Edited Jul 24, 2011 6:00 PM ET.

John,A way do it is to
by Armando Cobo

A way do it is to install furred down hallways to reach rooms and blow the air through the rooms. In larger rooms you need to build a coffer ceiling or in the case of a vaulted ceiling, I’ve designed a duct chase at the bottom of the vaults. You can install ambient lighting on the coffer ceilings and above the chase, that creates an impressive lighting effect washing out the entire ceiling or vaulted ceiling.
Some of the problems with furred down ceiling are bringing the framer for a second time to close the fur downs, and a close attention to the air sealing detailing in the air barrier and penetrations.

Fur Down 1.jpg Duct Seal 1.JPG Coffer Ceiling 1.JPG Duct Chase 1.JPG

Jul 24, 2011 8:32 PM ET

The problem with a drywall air barrier above furring:
by James Morgan

two visits by a sheetrock crew. Or am I missing something?

Jul 24, 2011 10:00 PM ET

Roof Thrust Connections
by Robert Riversong

One problem with some of the built up plate options is the elimination of the ceiling joist as the rafter tie, which would be particularly problematic in high wind or hurricane zones. The toe-nails from the rafters to the plates are insufficient. Metal hurricane ties would be necessary and these would create thermal bridges.

Jul 24, 2011 10:09 PM ET

Roof Washing
by Robert Riversong

Joe somewhat contradicts himself. He's emphatic about the need to wash every rafter bay with ventilation air but then doesn't care whether the exhaust vents are continuous ridge, "mushrooms" or gable vents. Only continuous soffit and continuous ridge vents will provide complete washing of the underside of the roof deck.

And most of the ridge vents on the market, especially the most popular ones, are inadequate for their purpose. Unless they have external wind baffles, they will allow entry of wind-driven rain and snow into the attic - undermining one of the primary purposes of roof venting.

Now Dr. Joe might not like the fact that externally-baffled ridge vents also have the property of increasing negative pressure as the wind is lifted over them by the baffles, since he doesn't want any negative pressure to suck air from the conditioned space. But, if the ceiling air barrier is intact (no can lights, and no unsealed attic hatch, roof flashing on all plumbing vents at the ceiling), then this is not a problem.

I do not use attic ceiling hatches. Instead, I build a weather-sealed "hay-loft" door in at least one gable for service entry from a ladder. Since no attic storage is possible, there's no reason for an inside hatch, which has to be perfectly both air-sealed and as well insulated as the rest of the ceiling.

Jul 25, 2011 9:42 AM ET

Hurricane ties
by James Morgan

Hey Robert, welcome back.
In the zones you mention hurricane ties will probably be necessary anyway for reasons of wind uplift, especially with a wide eave. Thermal bridging will be minimal and easily managed with exterior insulation - yes I know your preferred techniques like to avoid foamboard at all costs but many of us are OK with thoughtful and limited use of this material. And rafter spread can always be handled with collar ties.

I do like your idea of exterior attic access though. Makes a lot of sense in appropriate situations.

Jul 25, 2011 9:44 AM ET

Reply to Riversong
by Joe Lstiburek

Continuous soffit vents are significantly more important than continuous ridge vents in washing the underside of roof decks. Are
continuous ridge vents better than button vents and gable vents?
Yes. Does that matter much with continuous soffit vents? No. So do I care much? No. That was pretty clear in the presentation. The reasons for all this are pretty obvious.

Do externally baffled ridge vents and the venturi effect create
suction at the ridge when the wind is blowing? Yes. Do I like that?
Yes? Is that a problem with continuous soffit vents where there is more soffit vent area than ridge vent area? No. Is it a problem when there is more ridge venting than soffit venting? Yes, if the ceiling is not airtight. The reasons for all this are also pretty obvious.

Jul 25, 2011 9:45 AM ET

Response to JoeW
by Joe Lstiburek

Yes there can be some problems with installing an effective air control layer in an attic - mostly with combustion safety and air change. See the links below for more information and how to stay out of trouble.

Jul 25, 2011 9:46 AM ET

Reply to John Brooks
by Joe Lstiburek

I have no problem with your approach. The air control layer does not need to be drywall. And I certainly would never argue against service cores. Good luck with winning that argument with a production builder or most architects.

Jul 25, 2011 9:47 AM ET

Response to James
by Joe Lstiburek

Yes, this is a nice detail. See my earlier comment to John. Well done, both of you. But good luck convincing other folks besides the clearly smart and handsome ones like yourselves.

Jul 25, 2011 10:25 AM ET

What's the cost?
by Gerard Celentano

If some is good; then more is better. We’re so focused on saving a roof that we're willing to sacrifice livability, lifestyle and the planet.
It is wise for builder to ignore that there is a cost associated with not being able to use the dry, secure, unused attic for storage?
In a cold climate, ventilating a roof more than needed requires more energy to heat the house. That costs the homeowner more money and contributes a little bit more than necessary to global warming.
Any time the wind is blowing, the house (and attic) are pressurized. Lookup Bernoulli's principle. As such, air will be escaping from inside the house, through any crack or vent (including the soffit vents) to the outdoors. It's wise to not pull air from the living space, but simply increasing ventilation is a costly oversimplification.
The proper amount of venting is a function of many variables including vent area, interior and exterior temperatures and relative humidities, construction and associated materials, wind speed and direction, etc. This obviously changes with the seasons, climate and lifestyles of the occupants. It's hard to imagine how any passive system can adjust to these dynamic variables. Smart ventilation seems to be the answer, but building codes won’t allow it (sigh).

Jul 25, 2011 11:53 AM ET

Edited Jul 25, 2011 12:55 PM ET.

Why can't the attic be used for storage?
by Ray Sten

As long as there is sufficient insulation on top of the ceiling, what is the problem with placing a platform above the the insulation and using it for storage?

I ask because I am planning to do just that in the attic space of an addition I'm planning for my house.

Jul 25, 2011 1:45 PM ET

Reply to Ray Sten
by Joe Lstiburek

I argue against storage in the attic because the hatch rarely gets
sealed tight - especially if you actually use the attic for storage.
And even more rare is limiting the "stuff" being stored in the attic to the attic platform. Every time I have seen this done it sucks (actually it blows - but you get the idea). In theory it works -
build the platform above the insulation and have a tight attic hatch.
But communism works in theory too.

Jul 25, 2011 1:46 PM ET

Reply to Gerald
by Joe Lstiburek

Listen to the video clip and read the text - again. I think I was pretty clear - the ceiling needs to be airtight so that air is not pulled out of the house. Then all those other variables you mention don't matter. And guess what? The passive approach works just fine - everywhere. No fan. Amazing, eh?

Jul 25, 2011 2:59 PM ET

Insulating top plate
by David Argilla

So what is the reason for making sure that the top plate is better insulated than the wall? Are there moisture issues for the top plate if it is insulated less than the wall? What is suggested for renovations were there is very little room for insulation above the top plate. Believe it or not my house has 2x4 rafters sitting on the plate (in Seatlle, so low snow load), and there is very little room for insulation above the top plate. If you add exterior insulation to the wall, there is no way to get higher R value above the plate, even if you use closed cell spray foam for the attic perimeter.

Jul 25, 2011 5:23 PM ET

Reply to David Argilla.
by Joe Lstiburek

Yes, the issue is mold on interior surfaces where the ceiling meets the exterior wall due to the cold spot created at the top plate. And yes, renovations are a bear, and you do the best you can. If the moisture levels in the house are low during the coldest month of the
year you can avoid the mold spot. What happens in some cases after
the renovation airtightness goes up, air change goes down and interior moisture levels go up and the cold spot can be an issue. If you add a controlled ventilation system and keep the RH below 30 during the coldest period of the winter you can pretty much relax - unless you are in Barrow. In Seattle, the coldest part of the winter means you put on a long sleeve shirt and you get your latte double hot. So I wouldn't worry much.

Jul 25, 2011 11:33 PM ET

by David Argilla

No insulation and no central heat = In the winter routinely waking up to a 45 degree F house. Maybe you can tell my wife to put the long sleeve shirt on, I do it one more time and my behind is single again ...

Jul 26, 2011 8:21 PM ET

Edited Jul 27, 2011 6:46 AM ET.

Vented Attic in a Humid Climate
by John Brooks

Dr Joe,
I will ease up on trying to sell the Perfect Ceiling ... getting back to a good ole vented attic with airtight drywall and Ample Cellulose above. You mentioned that a vented Attic "works" in Hot climates...
What about Hot Humid Climates? Any worries or other considerations for a Humid climate?


Jul 27, 2011 7:53 AM ET

Response to John Brooks
by Joe Lstiburek

In hot humid climates during the day when the sun is shining the attic air temperatures run to 130 degrees F and the sheathing temperatures run to 160 degrees F. The attic becomes a solar kiln. Let us assume that the outside air is at 90 degrees F and 90 percent relative humidity. When this air enters the attic space through the roof vents and is heated to 130 degrees F its relative humidity drops like a rock. The sun heating the roof sheathing drives the moisture out of it into the air in the attic. So even though attics in humid climates are ventilated with humid air, incident solar radiation leads to a huge drying potential.

Now at night, the situation changes. Venting at night brings moisture into the attic and the wood materials pick up moisture. But this wetting effect is significantly less than the drying effect during the day.

So, on balance, even in hot and humid climates venting attics make sense. Even if it is cloudy during the day for weeks. But only if the attic ceiling is airtight and there are no ducts in the attic.

Jul 27, 2011 10:47 PM ET

Ridge vents in snow?
by David McNeely

Joe, I'm a fan. I've bought your "Builder's Guide" for my climate and it helps me to sleep at night.

But here's my question: when there's 3' of snow on the roof, how much performance can the ridge vent provide?

And since I'm asking, how does one keep the vents going through the roof from melting the snow and creating ice dams downhill? How do you convince the snow to not notice the hot air from the furnace vent, the water heater vent, the range hood, the bathroom fans...

And finally, shouldn't they be called "ice: damn!-s"?

Jul 28, 2011 2:19 AM ET

Using the attic as storage.
by Vincent Alvarez

Possible on new construction or an extensive remodel this would all make sense. But when you live in a small house that storage area is important. My house is also 100 years old. I bet you anything that my energy bills are smaller than the overly large houses that are being built today. If I take reasonable measures to stop air leakage and to upgrade the limited insulation that my home has now, I can still use the attic for storage, make the hatch as tight as needed and keep my coldest month energy bill near 100 dollars. This makes more sense then to put the attic off limits and build a storage unit or pay for offsite storage.

Jul 28, 2011 7:16 AM ET

If I may brevify my understanding:
by Ed Voytovich

V1: Do not manage unconditioned attics by bringing the outside into the attic; manage them by putting the attic outside.
V2: Complete the building enclosure either at the ceiling plane or at the roof roof plane, but for god's sake complete it.

Jul 28, 2011 9:03 AM ET

Edited Jul 28, 2011 9:07 AM ET.

Durable,Affordable,Buildable & Best use of Material
by John Brooks

Which High Performance Roof Assembly has the highest Rating?....
The Vented Attic

title.JPG vented roof.JPG

Jul 28, 2011 11:03 AM ET

How do I vent around solid blocking at roof?
by Dan Burgoyne

Is seismic zones, we usually install solid blocking between the top plates and the roof sheathing to transfer shear loads between the roof and walls. What are some effective methods to vent rafter bays and still provide this shear transfer?

Jul 28, 2011 12:05 PM ET

Venting at solid roof blocking
by Tiffanie Turner


I usually plan way ahead of time for this and coordinate with the structural engineer to have oversized blocking, so that I can design venting notches along the top of the blocking, say 1.5" deep by 4" long for example, really depends on my calculations. It gets complex, in that the notches can't land on a truss or joist connection, so they have to be thoughtfully spaced.

If the blocking is oversized, then the 1.5" +/- depth taken out of the top of the blocking doesn't compromise the actual depth needed for structural integrity. As long as they get the nailing they require, which also gets complicated, the engineers are okay with it.

Jul 28, 2011 12:50 PM ET

Response to David McNeely
by Joe Lstiburek

Check out:

Do not vent your vents up through the roof - that is why God invented walls. Yes, I know, I know plumbers are Gods unto themselves and there will be plumbing vent stacks penetrating the roof. Not much heat with that. B-vents up through the roof and chimneys in general not a big deal for reasons that will not make sense unless you read the referenced article above. The melt water up above in the field of a roof does not lead to an ice dam most of the time due to the capillary uptake of the snow melt into the snow pack. Ski Patrollers who deal with avalanche control know about this stuff - speaking as an old and crotchity Ski Patroller - who only wished he skied in a place with enough snow to worry about avalanches.

In terms of ridge vents covered with snow - snow, like dense pack cellulose is not an air barrier. A couple of inches of snow covering a ridge vent is not a big deal if the ceiling is airtight and the roof is well insulated. Again, see referenced article above. In high snow load areas - like in ski resorts or in Syracuse - ridge vents are a bad idea and you need gable vents or ridge vents that vent into monitors or cupolas or vent at their ends as mini gable vents.

Jul 28, 2011 12:53 PM ET

Response to Vincent Alvarez
by Joe Lstiburek

I don't have a problem with your logic and approach. But in defense of my views, most folks are not as fastidious as you or as savvy.

Jul 28, 2011 12:55 PM ET

Response to Ed Voytovich
by Joe Lstiburek

Yes. It takes people with funny last names to make English as clear as you just made it. Joseph Conrad must have been a relative.....

Jul 28, 2011 3:07 PM ET

Habitable Attic
by Charles Shade

If I understand my building official correctly and you are required to build under the 2009 IRC if an attic is big enough to be considered a room; i.e. minimum 7' dimension minimum 70 sqft and the ceiling for 35 of those square feet is over 7' high, you will have to now design this as a habitable space AND provide a code compliant stair or access to the space. Therefore the whole idea behind the insulation being free to live as insulation wants to do would be useless.
Trusses would be more accommodating to this idea.
If indeed this is the interpretation of the building code.

Jul 28, 2011 7:36 PM ET

Edited Jul 28, 2011 7:38 PM ET.

Great article and video
by Eric Novotny

Good piece. It should be required watching for roofers. I can't tell you how many homes we go into with improperly vented (often time over ventilated) attics. My personal favorite is the homeowner that wants to keep her attic cool so she insisted on the installation or a gratuitously large power ventilator. There is a reason that your attic is now within 10 degrees of conditioned temperatures and it isn't a good one.

Most recently done roofs we look at have the standard retrofit ridge vent because roofers are afraid of the warranty exclusions from shingle manufacturers. All they have done in most cases is increased the stack effect and pulled more conditioned air from the living space below. Trying to tell a roofer that attic temperatures aren't really the issue usually nets the blank stare.

Good read as always.

Jul 28, 2011 11:15 PM ET

by Pam Kueber

Just installed a powered attic fan, like, yesterday. I also have those stupid tiny soffit vents every three or four bays. And stuff in the attic. Drats -- and that's not what I wanted to say.

Great video / presentation!

Jul 28, 2011 11:32 PM ET

by Pam Kueber

... Oh and yes, we got sold ridge vents when we added a new roof about 5 years ago. And, when I added insulation last year, I also put in platforms for storage, of course (So far so good in terms of keeping all the Xmas decorations etc etc on deck.) Gosh, we are a case study for doing almost everything wrong!

Jul 29, 2011 8:39 AM ET

Edited Jul 29, 2011 3:26 PM ET.

How do you vent valleys?
by David McNeely

Every valley results in an area of unvented roof that is the square of the distance to the ridge (i.e. a 15' distance to the ridge results in 225 sq.ft. of unvented roof). Plus, snow likes to collect in valleys and takes longer to melt there also. Amazingly, ice dams are often densest in valleys!

And yet, I have yet to read an article on ice dams that mentions this problem, let alone a solution. Joe, in your "Builder's Guide" book you suggest doubling the valley rafters and reducing their dimension from the jack rafters, so there is a space above them (resulting in about a 2"x3" channel). But this doesn't obey your command to have more vents at the bottom than at the top. Wouldn't you be reduced to the soffit vent in the first adjacent bay to feed all the subsequent bays created by the jack rafters (and the area available to that vent is further reduced by the diagonal...)? In my example of 15' to the ridge, with rafters @16"o.c., that means 11 bays on each side!

And for all the homes already built, is there retrofit that would solve this problem?

This is not an academic challenge. My MN in-laws live in an expensive house built by a successful builder for himself (not for profit), and there are still major ice dams every winter.

Jul 29, 2011 10:03 AM ET

Unvented attic/roof assmbly
by Chad Fuller

With the above on a stick built or w/ perlins roof (tung n grove ceiling on top of roof rafters, sheathing then 6" phyliso, paper then slate or 3 tab asphalt ) what is a good way to keep the "as much insulation on top of the plate as in the walls"?
The building has the craftsman wall brackets external to this area. The attic is habited space in that it's part of the lower room (i.e. no attic). We call it cathedral ceiling. I am imagining a shamwiched top plate with more phyliso board, may be extra external phyliso board at the sheathing and for the 2 ft sofits to be ladder design added after the external insulation. Can't see how to get that top plate area up to the R-30 walls.

Jul 29, 2011 4:39 PM ET

Response to David McNeely
by Joe Lstiburek

The valley issue can be handled by dropping the valley rafters. See figures from my Cold Climate Builders guide. Hips are handled the same way. This also extends to framing for dormers and skylights.
See images below. Notice the horizontal framing on the large dormer below.

Figure_11 51.JPG Figure_11.50.jpg AF61.jpg AF62.jpg AF63.JPG

Jul 29, 2011 5:29 PM ET

Unvented attic/roof assmbly for Chad Fuller
by Eric Novotny


I would first apply a peal and stick membrane (Ice/Water) to air seal the T&G boards. You can then apply your 6" of Poly Iso on top of that sealed T&G and then put down your decking or purlins/battens for the roof system.

You can run your venting over deck and you should not have any ice damning issues after that.

6" of Poly Iso is over R-30.

Jul 29, 2011 10:02 PM ET

Edited Jul 29, 2011 10:03 PM ET.

But what about the valleys?
by David McNeely

I had already seen these schematics (as I mentioned in an earlier post, I have bought your book and I am a fan). I'm disappointed that you did not respond to my concerns. One soffit vent feeding 3 rafter bays as in your drawing might work, but what happens when you have a real valley that has 11 or 12 or 15 rafter bays all being fed by one little vent? This combined with all the snow melt feeding from a large roof area, like streams becoming a river...

Valleys seem to collect ice. When you combine this with the very large hurdle of making drywall airtight despite everyone wanting cans everywhere, plus relying on all future homeowners to be cognizant of the need to maintain this barrier, and never store their stuff up there; well, you've helped me make up my mind. Seems to me the airtight barrier should be around the perimeter of the whole house, as in your "perfect wall" description, and the insulation should be up at the roof level. All problems solved.

I've always preferred the idea of a sealed crawlspace—why would anyone want to invite hot humid air into the environment they are "housing?" I think the same reasoning applies to attics: why would you want to superheat the air immediately adjacent to the space you are trying to cool? And in winter, why do you want to worry about how perfectly you've controlled that same environment exposed to the outside?

And rewiring a fixture when all the wiring is below 30" of very dusty cellulose is like trying to re-plumb a bathroom on a slab. Except on a slab at least you know where you can walk.

Jul 30, 2011 10:58 AM ET

Insulated roof decks
by Eric Novotny

Are certainly and option, just more expensive. If you valleys and ventilation limitations dictate moving your envelope and insulation layer to the roof deck, you can certainly do that.

Spray foam is just a bit more expensive than cellulose and foam combo for sealing and insulating the attic floor.

Jul 31, 2011 9:54 AM ET

Edited Jul 31, 2011 12:24 PM ET.

Questions for David McNeely
by John Brooks

McNeely:"This is not an academic challenge. My MN in-laws live in an expensive house built by a successful builder for himself (not for profit), and there are still major ice dams every winter."
David, does your in-law's house have a simple vented attic?

What does the price of the house ...or the fact that it was the personal home of a "successful builder" have to do with the quality of the Enclosure?

What was the Air Control Strategy for the Ceiling? ADA,spray-foam?
Does the house have Can Lights? How were they Air-sealed?
Are can lights really necessary?
Was the house Blower door tested?... what was the ACH50?

Have you read the Straube/Grin Report #1006 for High-R Roofs?
What's not to like about a vented attic?

attic storage.jpg

Aug 1, 2011 5:53 AM ET

Missing photos
by Martin Holladay

Three missing photos have just been added to Joe Lstiburek's last response. GBA apologizes for the delay in attaching these photos.

Aug 1, 2011 6:47 AM ET

Dropping Hip & Valley Rafters
by John Brooks

Martin or Joe,
Is the detail from post #46 mainly intended for vented attics in cold climates?
What about Climate Zones 2 & 3?..
Most of the roof designs in Texas are Not-So-Simple
Lots of Hips and Valleys and most roof rafters are only 2x6

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