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HOT-ROOF QUESTION
I have been studying ventilation therory for vaulted ceiling roofs. There are two design approaches available, which are known as the cold roof design and the hot roof design. I am thoroughly familiar with the cold roof principles, but the hot roof leaves me a bit confused.
So what I would like here is for someone to present a complete definition of a hot roof in as few words as possible. This would be a definition for vaulted ceiling roofs only.
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Hot Roof: A completely (post #205802, reply #1 of 27)
Hot Roof: A completely insulated and sealed roof system. Roofing surface is directly on top of insulation.
Cold roof: A Hot Roof with an additional layer that provides ventilation over the insulated and sealed area, with roofing surface applied over the ventalation channel.
I like the idea behind a cold roof better than a hot roof for a vaulted ceiling, as the cold roof can provide one additional layer of water penetration protection if detailed correctly.
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I too prefer a cold roof (post #205802, reply #2 of 27)
I too prefer a cold roof design with a film vapor barrier, and I would also ventilate as a backup for the vapor barrier performance.
Here is my definition for the hot roof:
An unvented roof system for vaulted ceilings that relies on a vapor barrier to prevent vapor condensation.
That has the same meaning as your definition, but in what I have read, this whole hot roof theory seems to me to be way off in the weeds with tangled details and terminology. That is why I was looking for a definition.
In an article by Joseph Lstiburek called A Crash Course in Roof Venting, he discusses the design of unvented roofs. He does not use the term, cold roof, but I assume that is what he means by unvented roof.
He says, “The goal is to keep the roof deck—the principle condensing surface in roof assemblies—sufficiently warm through the year to prevent condensation from occurring.”
He shows this being accomplished by spraying foam onto the underside of the sheathing or insulating with foam board on top of the sheathing, under the roofing. The objective, as stated, is not to seal moisture out of the insulation cavity, but to prevent it from condensing by not presenting any surface cold enough to condense it. So when spraying foam onto the underside of the roof deck, it must be thick enough to prevent the surface from getting below the condensation temperature of water vapor. Since the vapor cannot penetrate the foam, it cannot reach colder part of the insulation where condensation might be possible.
However, when you think about it, this amounts to an objective of keeping moisture out of the insulation cavity. The insulation cavity begins with the layer of foam, and the foam surface is the vapor barrier. If you don’t want moisture to condense, you must stop its movement toward the cold.
Mr. Lstiburek says that fiberglass insulation is okay for an unvented roof as long as you have a layer of foam between it and the roof deck. If you need foam behind the fiberglass, that must mean that the system has no film vapor barrier. Why not use a film vapor barrier? If you did provide a sufficiently performing vapor barrier, why couldn’t you insulate with fiberglass all the way to the roof deck and forego any ventilation?
It seems to me that this hot roof concept is only based on the relationship between venting and the vapor barrier. The type of insulation should not matter. But in most explanations of the hot roof, I find the suggestion that closed cell foam must be used, and no mention of a film vapor barrier.
The problem with Film (post #205802, reply #8 of 27)
The problem with Film Barriers... like 6 mil plastic for intance... as that the first thing you do with them is COMPROMISE the film when you put it up. Then, you compromise it again when you put up the drywall.
The foam, applied correctly, has no penetrations that compromise the envelope. Any penetrations there are get extra detailed attention (vs. the thousands of nails penetrating through the drywall into the sheet of plastic).
YAY! I love WYSISYG editing! And Spellcheck!
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Film Vapor Barriers (post #205802, reply #9 of 27)
I agree that film vapor barriers get compromised more often than not. I believe in using them, but only with exceptional diligence and quality control. I would use an extra heavy material in the 10-15 mil range, and it would be reinforced besides.
For a new design I am working on, the vapor barrier is secured to the studs with a 2 X 2 batten screwed to the studs. The screws pierce the vapor barrier, but I think the material usually seals quite well around nail or screw penetrations. I would also use a bead of polyurethane adhesive under the batten if a lap seam is made there. The adhesive would fill any irregularities between the stud and the batten in order to provide continuous squeeze on the film. I would also tape the film lap seam.
I would leave the space made by the 2 X 2 uninsulated and run the electrical in that space, so it would not need to penetrate the vapor barrier.
A lot of hot roof theory involves the use of foam board, and that must be taped at the seams. This poses issues with the possibility of tape separation or with vapor passing through the joint and condensing on the tape having been placed on the cold side. Spray foam also presents issues with possible separation from rafters and permitting vapor to pass through the crack and condense on the cold roof deck.
To me "hot roof" means any (post #205802, reply #3 of 27)
To me "hot roof" means any setup where the roofing surface is directly adjacent to the major insulation layer, with no intervening ventilation area. Doesn't matter whether it's a cathedral, conventional pitched roof, flat roof, etc.
There are no doubt gray areas, such as flat roofs where there is a substantial amount of insulation under a mopped roof, but then there's further insulation inside the structure.
The most "conventional" "hot roof" in home construction is probably the standard shingle roof with sprayed foam on the underside of the roof sheathing. Somewhat less conventional is one of the several designs that places foam on top of the roof sheathing, and the roofing (somehow) on top of the foam.
We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison
That is my (post #205802, reply #4 of 27)
That is my understanding.
So if you had fiberglass batts say a foot thick between rafters, and they ran right up to the contact with the bottom of roof deck, and there was no ventilation, but there was a film vapor barrier on the warm side of the insulation-- then that would be a hot roof, right?
I think it would, but Joe Lstiburek seems to be saying that it would not be a hot roof.
I don't see how the placement (post #205802, reply #5 of 27)
I don't see how the placement of the vapor barrier makes any difference. The same essential issues apply to a vapor barrier regardless of hot or cold roof.
We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison
Well I am just thinking that (post #205802, reply #6 of 27)
Well I am just thinking that the lack of ventilation requires a vapor barrier. Joe Lstiburek seems to require a vapor barrier with a hot roof, but he requires it in the form of using closed cell foam, so vapor cannot permeate it. I don't see why he stipulates foam to qualify for a hot roof. I think you could use fiberglass or any other air / vapor permeable insulation as long as you sealed it with a film vapor barrier.
As far as I can tell, a hot roof can be used in a cold climate like Minnesota. If such a roof had no ventilation, wouldn't the code require a warm side vapor barrier?
I've no idea what the details (post #205802, reply #7 of 27)
I've no idea what the details of the code require here, but I doubt that they're nimble enough to cover all the variables. The need of a vapor barrier depends on the dynamics of insulated cross-section -- what the moisture permeability is of the various components. Having the sheathing, building paper, and shingles of a typical roof above the insulation increases the need for a vapor barrier on the warm side, but not appreciably more than, say, a floored attic with the insulation beneath.
We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison
Hot Roof, Cold Roof, & Hot-Cold Roof (post #205802, reply #10 of 27)
I have been trying to crack this hot roof idea and understand the point of it. Here is one of the most informative and detailed explanations of roof design including the unvented roof:
http://www.buildingscience.com/documents/reports/rr-0404-roof-design
For unvented (hot) roofs, there are two sub-categories. They are systems where condensing surface temperatures are controlled, and systems where condensing surface temperatures are not controlled. So, you either keep the condensing surface warm enough to be above the dew point of contacting air, or you keep that air from contacting the condensing surface. In the first case, the air must not move beyond the condensing surface, and in the second case, air must be stopped before reaching the condensing surface. So air movement must be controlled in either case, or essentially in all variations of the hot roof.
I find it incredibly difficult to assimilate all of the variations within the unvented roof general classification, and also difficult to fully see the point of them. I would also conclude that if a person did thoroughly understand all of these variations, it would be impossible to fully communicate that information to someone else who was unfamiliar with the information. The above link is quite well done, despite the complexity. But there are many other Internet references that seem thoroughly confused about the topic.
My general conclusion about the purpose of the hot roof is that it is intended to produce an insulated roof without any venting in applications where the roof configuration and design would make venting relatively difficult or impossible. Therefore, I see the hot roof as a compromise that is prone to performance failures. So, I would not use the hot roof concept for any house that I intended to build. I would design a roof that was ideal for ventilation, and I would use the cold roof design with an airtight, warm-side vapor barrier, fiberglass or cellulose insulation, and full-length eave-to-ridge ventilation.
One main compromise of the hot roof is two things it does not do well. The two things are prevention of ice dams and reduction of summertime heat gain. But like so many things in construction details, you can compensate for the fundamental inability to cool the roof with ventilation by providing so much insulation that the heat loss is so minimal that the roof deck stays relatively cool. The extreme insulation would also work to prevent heat intrusion in the summer, but it would not protect the roofing from overheating.
However, another option is to add a ventilating space between the roofing and the roof deck. So then you have a hybrid system; a hot-cold roof.
Condensation (post #205802, reply #11 of 27)
For unvented (hot) roofs, there are two sub-categories. They are systems where condensing surface temperatures are controlled, and systems where condensing surface temperatures are not controlled. So, you either keep the condensing surface warm enough to be above the dew point of contacting air, or you keep that air from contacting the condensing surface. In the first case, the air must not move beyond the condensing surface, and in the second case, air must be stopped before reaching the condensing surface. So air movement must be controlled in either case, or essentially in all variations of the hot roof.
I don't understand how the issue is unique to "hot" roofs. To avoid condensation it's necessary to keep the dewpoint below the temperature at all points in the assembly. This can be done by either blocking humidity from the "wet/warm" side or adding insulation on the "cold/dry" side. Any combo of humidity barrier and insulation that achieves tthe goal is fine.
We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison
I don't see why you say that (post #205802, reply #12 of 27)
I don't see why you say that a "hot roof" would be bad for ice dams. Ice dams are caused by poor insulation (or, more precisely, warm "conditioned" air bypassing the insulation). In most cases a "hot roof" design is less likely to suffer from air infiltration problems that lead to ice dams.
We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison
Hot Roof vs. Cold Roof For Ice dams (post #205802, reply #13 of 27)
I always see mention of warm conditioned air bypassing the vapor barrier and working its way through the insulation to the roof deck; and the heating of the deck then causing ice dams. I agree that that can happen, but it requires either a faulty or lacking vapor barrier in conjunction with air-permeable insulation. Whether it is a hot roof or cold roof, this condition should not exist if the work is properly executed. But a lot of what I read seems to indicate that heat loss through the ceiling is more likely caused by air exfiltration than by thermal conduction. I would think the opposite would be the case. Perhaps these references to warm air escaping the conditioned space as the cause of ice dams is the correct explanation for most existing cases of ice dams in traditional construction where not enough attention has been paid to vapor barriers, ventilation, etc.
But, setting that aside, consider these examples to compare a hot roof to a cold roof for their relationship to the formation of ice dams.
Suppose you have an airtight vapor barrier (just above the ceiling), and then fiberglass insulation on top of it, and then no ventilation or air space. So the insulation runs right up to the roof deck. That would be a hot roof. So any degree of heat loss will be by conduction through the ceiling material and insulation. There will probably also be some convection cycling from the bottom of the insulation up to the bottom of the roof deck as well. But there will not be any warm air coming from the conditioned space and passing into the insulation cavity.
Then, with this setup, the more insulation you have, the less heat loss will occur, and the cooler the roof will be in wintertime as heat loss is conducted through it to the outdoors. So with enough insulation, you could prevent ice dams. However, that might require more insulation than a person would want just for heating economy, so you would be adding extra insulation only for the purpose of preventing ice dams. For instance, 2 feet of fiberglass might be sufficient for heating economy, but it might require 4 feet to prevent all ice dam formation.
So an alternative would be to add a ventilation space above the 2 ft. of insulation, which would turn the roof into a cold roof system. And then draw outside air through the air space to cool the underside of the roof deck to equal the outdoor temperature.
So with the first instance of the hot roof, the cooling of the roof only occurs as the outdoor temperature cools outside of the roof deck by removing heat that is being conducted through it from the inside. And with the second instance of the cold roof, the cooling of the roof occurs as the outdoor temperature cools both the top and the bottom of the deck, so no heat even reaches it, let alone conducts through it.
Even with the cold roof, however, a certain minimum amount of insulation is necessary so the heat loss does not simply overwhelm the ventilation and heat the roof deck to the point where ice dams can develop.
I can assure you that the (post #205802, reply #14 of 27)
I can assure you that the vast majority of ice dams occur on older homes with lots of air bypasses. And on our house the only place where we get significant snow melting on the roof is where I missed a bypass over the bathroom. (Gotta crawl up and take care of that one of these days, but it's not an appealing task.)
We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison
Yes, I know what you mean (post #205802, reply #15 of 27)
Yes, I know what you mean about warm air passing though the ceiling and causing ice dams in existing houses, especially older ones. But what I find perplexing is how information about the newest construction methods often states that (with these new methods in place) more heat will be lost through the roof by air passing though leaks, as opposed to heat loss from conduction, convection, and radiation heat transfer mechanisms.
And yet, at the same time, these information sources will state that a leak-free vapor barrier must be provided.
Well, actually the air (post #205802, reply #17 of 27)
Well, actually the air passing through leaks IS convection, and that's probably part of it. A standard attic setup lacks the top membrane of a hot roof and the ventilation creates a draft that draws air from the house. Air sealing a conventional ceiling is quite difficult.
The "leak free" vapor barrier is of course a theoretical goal, rarely achieved in practice.
We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison
Yes, I understand that air (post #205802, reply #18 of 27)
Yes, I understand that air leaking through the ceiling is convection. I was referring to convection within the insulation layer in comparing heat loss through an insulation deficiency, as opposed to heat loss by conditioned air leaking through the ceiling. The main point I was making is that I see a lot of contradictory explanation, advice, and theory about the "hot roof" concept. There is an abundance of confusion in the basic principles of the cold roof, ventilation, and vapor barriers as it is. But the hot roof concept is way over the top when it comes to confusion.
In your comment, you say that the ventilation of a standard attic setup creates a draft that draws air from the house; and that air sealing is quite difficult. I agree that air sealing is difficult. However, just to be clear, an air sealed vapor barrier is required for a cold roof. And the ventilation is intended to draw air from an intake outside of the house, not from the conditioned space inside of the house.
I do agree that many houses have been built in which the difficulty of air sealing was not overcome, and therefore, their attic ventilation draws air from the conditioned space. But that is a defect in the proper execution of a cold roof.
When I refer to a leak-free vapor barrier, I use the term as an aspiration to aim for. I think there is a certain proximity that is close enough to be acceptable for a successful execution of a cold roof. But it does require some extra effort to get beyond what is commonly achieved in vapor barrier integrity.
Yyyyeep! You've got it... (post #205802, reply #16 of 27)
Yyyyeep! You've got it... well, most of it at least. What you are missing is the "In the field" aspect - you've got the theoretical down pat.
"In the field" is where the compromises get made, and things can get ugly.
Here is a compromise: Rafter thickness. Rafters are sized based on engineering load, not on insulation performance. This is why you have people trying to hot roof a 2x6 rafter construction in the great white north. Good luck with that! I'm redoing my roof with much thicker members precicely because of insulation values... and every step of the way when I talk to trades people I have to remind them of the extra thickness of the rafters beyond what would be called out for in the span.
Here is another compromise: Tradespeople. Your invincible vapor barrier meticulously detailed and sealed is no match for Grandpa Bob and his jambsaw, hacking hole up there so he can put in a new pendant light he can read by... don't worry, he sealed up the penetrations in your vapor barrier with some10 y.o. duct tape he found in the garage. Who's fault would the future damage done by the vapor penetrations there? You (as the original builder of the system) as you didn't forsee how stupid everyone else who will interact with your structure will be for the next 100 years. This is the reason wire is set back in the stud bay, or at least armored with conduit... my Grandpa Bob.
One more thing: Temprature Delta. What makes perfect sense in one location doesn't in another. Insulation choices like fiberglass make sense in warm climates... not so much in -0 climates. Doesn't stop them from being sold in those climates, of course.
As an aside - you coments about foam, tape, and shrinkage/pulling away from rafters. This months FHB provides a close up look at how some rigid board foams do shrink, and why it's important to do offset layers like you would a subfloor. In addition to offsetting seams, I think filling seams with foam AND installing a tape with a depresssion in the middle of the seam to allow a little shrink would be prudent (Adhear tape to one side of seam, press finger into seam to indent tape in, then adhear tape to other side of seam). As to the foam pulling away issue, I agree that is a concern - but an easily managed one. Just like getting flashing details correct. You don't have foam applied when it's too cold, you don't apply it in heavy lifts, and you make sure the mix is correct. You miss any of these three and the foam will shrink.
YAY! I love WYSISYG editing! And Spellcheck!
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Paul, I agree with (post #205802, reply #19 of 27)
Paul,
I agree with your observations about the main reason for a hot roof being the limitations in the framing of a vaulted ceiling. Specifically, those limitations would most likely be insufficient space for anything but insulation with the highest R-value per inch and insufficient space for ventilation.
I understand what you are saying about the “in-the-field” problem and the peril of getting other people to install an infallible vapor barrier. Vapor barriers should be in a class of their own for being the most hated installation detail in all of construction. In fact they are held in so much contempt that they have become capable of being rationalized away as being unnecessary. The air infiltration barrier, on the other hand, seems to be the darling of membranes. Perhaps this is because its purpose is so immediately comprehendible. It seems to me that a lot of what is commonly a problem of insufficient insulation is mistakenly perceived as being an air leakage problem.
I am interested in house design for the coldest U.S. climates. But I am designing as though I will be the builder. I may indeed build one, but if others build them, they will have to be as careful as I would be. I have ruled out spray foam for several reasons as follows:
1) The cost is relatively high.
2) I do not need foam’s ability to achieve the highest R-value in a conventional frame.
3) I do not feel that it would be possible to control the quality of the foam.
4) I do not need foam’s ability to seal without a separate vapor barrier because I would use a separate vapor barrier even with foam.
5) I do not need foam’s attribute of making a structure stronger.
I am only curious about hot roofs so I can understand them. But generally, they seem like a compromise with poor structural design of a proper roof. And what I have also noticed is that hot roofs are often explained in contradictory terms. So I would just set the hot roof principle aside for anything I would design or build.
(I replied to your post above, not a post of mine.)
Denial can be a blissful (if temporary) thing (post #205802, reply #20 of 27)
Fact: Someone, sooner or later, will do something to compromise your carefully detailed vapor barrier. The very best design takes that into account. Simply saying that others must be careful, isn't good enough.
What Do You Mean? (post #205802, reply #21 of 27)
Sapwood,
When you say the very best design takes into accout the fact that somone will eventually compromise the vapor barrier, what kind of design are you talking about as being the very best?
The best design is a simple (post #205802, reply #22 of 27)
The best design is a simple system that takes into account the fact that people will not behave in ways that can be entirely forseen. Sooner or later somebody will compromise your carefully detailed vapor barrier. Good design practice will take this into account.
So what is the (post #205802, reply #23 of 27)
So what is the "good practice" of vapor barriers that takes into account the possibility of somebody compromising the vapor barrier in the future? How much should I worry about what somebody else might do in the future? They might burn the house down. Are you saying that it does not pay to do it right because somebody will screw it up in the future?
Aside from what somebody does to the vapor barrier in the future, there is a major challenge to get it right in the first place. That's all I am concerned with. If somebody in the future knocks a wall out for an addition, they are on their own.
OK, here are the two (post #205802, reply #24 of 27)
OK, here are the two practices we are talking about:
You are advocating #2, but there are clear reasons why this practice is not recomended by people like Listerbruk anymore. One, it's hard to get the details right. Two, when a detail fails it can fail for the whole structure not just an isolated point.
The first method is alot more forgiving of all the details that happen below it, the other is not. The first method actually has a pathway back out for drying vapor, the second doesn't.
Plus, the first method is easier to get all the details right - and those confirmed details don't change when they get covered by insulation and drywall.
YAY! I love WYSISYG editing! And Spellcheck!
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The two practices (post #205802, reply #25 of 27)
OK, here are the two practices we are talking about:
You are advocating #2, but there are clear reasons why this practice is not recomended by people like Listerbruk anymore. One, it's hard to get the details right. Two, when a detail fails it can fail for the whole structure not just an isolated point.
The first method is alot more forgiving of all the details that happen below it, the other is not. The first method actually has a pathway back out for drying vapor, the second doesn't.
Plus, the first method is easier to get all the details right - and those confirmed details don't change when they get covered by insulation and drywall.
When you list the two practices we are talking about, I am not sure what you mean by drawing that distinction. We are talking about hot roof versus cold roof, and either one of those practices can incorporate any of the following:
1) Spray foam full depth.
2) Spray foam only deep enough to keep its surface above the dew point.
3) Any type of insulation besides spray foam including fiberglass.
4) A film vapor barrier on the warm side.
The one and only distinction I see between a hot roof and a cold roof is that a hot roof does not have cold-side ventilation and a cold roof does. Therefore my definitions for a hot roof and a cold roof are as follows:
HOT ROOF: An unvented roof system using any type of insulation with a vapor barrier to prevent vapor condensation.
COLD ROOF: A vented roof system using any type of insulation with a vapor barrier to prevent vapor condensation.
I do not see that Lstiburek is advising against the use of a vapor barrier for either the hot roof or the cold roof. The foaming to create a surface that remains above the condensation temperature is one method used in a hot roof. It could also be used in a cold roof if ventilation were provided above it. In that case, the foam would simply be a vapor barrier like the film vapor barrier in a cold roof.
I disagree when you say that your listed method #2 does not provide a path back out for drying vapor that happens to enter the insulation cavity. The path out is through the ventilation to the outside. Furthermore, while your method #1 provides a pathway back out for drying vapor, it has a limitation that method #2 does not have. With method #1, the escape path for any moisture that happens to condense in the insulation cavity is the same path it took to get in. Therefore during the times when conditions are such that vapor is coming in, no vapor can go out.
On the other hand, a cold roof provides full time ventilation whether vapor is entering the space or not.
Hey - neat how you got the (post #205802, reply #26 of 27)
Hey - neat how you got the text background a different color!
As far as what we are talking about:
"We are talking about hot roof versus cold roof."
We were - but now we are talking about what the best way to do a hot or cold roof. You specificly talked about building a best in class film vapor barrier system, and everyone else said that film vapor barriers are so easily compromised they aren't worth depending on in a hot/cold roof system.
1) Spray foam full depth. Yep, this works but is pricey
2) Spray foam only deep enough to keep its surface above the dew point. This works and is cheaper
3) Any type of insulation besides spray foam including fiberglass. This is a crapshoot, you are trying to dry as fast as moisture permiates, but you could end up with an insulaticicle in your rafters
4) A film vapor barrier on the warm side. This works, as long as you and everyone else in the home doesn't make a mistake in application and maitenence. After that, you are back to insulaticicle in the winter + mold/rot/bug colony in the summer.
YAY! I love WYSISYG editing! And Spellcheck!
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Four Methods of Roof Construction (post #205802, reply #27 of 27)
...now we are talking about what the best way to do a hot or cold roof. You specificly talked about building a best in class film vapor barrier system, and everyone else said that film vapor barriers are so easily compromised they aren't worth depending on in a hot/cold roof system.
1) Spray foam full depth. Yep, this works but is pricey
2) Spray foam only deep enough to keep its surface above the dew point. This works and is cheaper
3) Any type of insulation besides spray foam including fiberglass. This is a crapshoot, you are trying to dry as fast as moisture permiates, but you could end up with an insulaticicle in your rafters
4) A film vapor barrier on the warm side. This works, as long as you and everyone else in the home doesn't make a mistake in application and maitenence. After that, you are back to insulaticicle in the winter + mold/rot/bug colony in the summer.
Paul,
Considering your four points for “best way to do a hot or cold roof,” I believe that only #4 is a cold roof; depending on further details in its execution.
Items #1 and #2 are exclusively hot roofs.
I am not sure about your item #3. If you are saying that approach simply has no film vapor barrier, and it uses vapor permeable insulation such as fiberglass; then I agree that it will produce disastrous results in a cold climate. But I do not see how this could even be an option. Would the code allow this type of roof construction? I assume that it would not.
Item #4 could be a cold roof if cold-side ventilation is provided with a space between the top of the insulation and the bottom of the roof deck.
Item #4 could also be a hot roof if there is no cold-side ventilation.
I understand your reservations about the film vapor barrier, and I agree that a film vapor barrier has to be properly executed. However, I have reservations about your items #1 and #2. With either approach, there is no guarantee that the foam will not break its bond to the framing members, and thus allow vapor to enter the gap and reach a cold enough region to condense. This would leave water filling the thin gap by capillary action, and such wetting would be slow to evaporate. Not only would water fill the gap, but it would also saturate the wood surface in the gap. Some of the water in the gap might even freeze. So, not only would this lead to wetting in the insulation cavity, but also, the wetting would be explicitly concentrated on the wood framing.
I also have reservations about the actual R-value of the spray foam. There is no way to know what the R-value would actually be because it can vary depending on mixing and application. For the flash coat in particular, controlling the R-value is critical because it is supposed to only be the minimum required to do the job of presenting a warm-side surface that remains above the dew point. So it is threading the needle to provide just enough R-value in the foam for preventing condensation, but no more.
Additionally, the calculation for the R-value required is seeking the minimum requirement base on average weather conditions. So that too is threading a needle.
If I were to install a film vapor barrier, I would be left knowing that the job was done right. But with a flash coat of spray foam, I would have no way of knowing whether or not it would perform properly.
In general, approach #4, with or without ventilation, could use any type of insulation including spray foam. If it used a vapor impermeable insulation such as spray foam, the film vapor barrier would be redundant if the spray foam were properly performing as a vapor barrier. If I were using spray foam for its attribute of relatively high R-value, I would still use the film barrier as backup insurance against vapor penetration.
My overall preference would use your item #4 in the cold-roof variation, that is, the version of #4 with an air circulation ventilation space above the insulation. So, overall, I would use a film vapor barrier, fiberglass batt insulation, and ventilation system above the insulation.
By the way, I am wondering why you believe that there is a consensus in the industry that film vapor barriers have been ruled out because they are unreliable. I find many references to using film vapor barriers as well as development of new film products, including smart vapor barriers.