I own a old house that at some time had the attic sheetrocked. I have my office up there because it’s the only room where I can smoke cigars. It’s insulated with foil faced batts with aluminum foil face. The sheetrock looks old. The roof at some time used to be wood shakes. The roof was replaced and it seems the wood scraps have piled up on the insulation and therefore reduced the r value. I’m thinking of tearing everything out then: 1. have foam insulation spayed in between rafters, between studs on kneewalls, and between joists behind kneewalls 2. trim it flush 3. nail up 3/4″ polyisocycanurate 4. nail up 1/2″ sheetrock. 5. install vents in gable ends so air moves through knee wall cavity.
Is this a good plan?
Replies
Greetings, WEG, and welcome to BT. If you'll fill in your profile, the information will help up answer your questions as many times the answers depend on the climate in which your home is located.
I don't like 'hot' roofs in cold climates because the potential for ice-damming and condensation is too great. If you live in a snowless--or essentially snowless--environment, you could make a case for it.
Have you considered leaving what you have in place and insulating the outside of the roof, then laying a second, cold roof on top of that? There are some very efficient foam panels available now which make this a more economical alternative to full demolition and spray foaming the inside.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
I'm in Connecticut. The roof is only 5 years old so I don't want to mess with it. The interior is pretty shabby and needs to be updated.
Given the climate in Connecticut, I would recommend against a hot roof.
After you rip out the gyprock and existing insulation, nail 2x2 blocking along both sides of each rafter where it meets the roof deck, from the building wall up to a foot or so above the attice-space ceiling. When you get to the ridge area, use more 2x to frame an air channel below the rafter bottoms running from gable to gable.
Nail Black Joe or Ten-test over that blocking, thus creating a clear ventway that ties into the ridge area. Sheathe the air channel at the ridge too.
Make sure you channel intake air through the eaves soffit vents into the ventway you've just created, of course, and install gable-wall vents to allow outflow at the ridge.
Spray your urethane foam over the Black Joe. Adding ¾" of polyiso board over that won't hurt, but why don't you go to 1½"? You're going to need strapping over the foam panels in either case. Trying to screw your gyprock right on top of the polyiso isn't a good idea.
Also insulate any plumbing vent pipes that penetrate the roof as completely as you can; they are always a source of heat leaks to the roof deck. A good trick is to create an insulated pipe section at the roof penetration by slipping a two-foot-long piece of 6" ABS DWV pipe over the vent pipe and squirting canned foam into the space between the two pipes. Make a centering template out of ½" plywood to keep the outer pipe centered on the vent pipe while the foam cures. Use low-expansion foam.Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Q1; What is black joe or ten test?
Q2; Why not put sheetrock over iso directly?
1. Black Joe is ½"-thick compressed paperboard which has been treated with bituminous tar for weather resistance. It's commonly used as exterior sheathing on low-budget construction for walls where racking shear is not an issue. It also has a small but calculable R-value as an insulator.
Ten-test is the essentially the same thing without the tar. Commonly used as an insulation containment board, as in the example I gave you.
There is very little price difference between the two. In that you will be installing this directly under a roof deck, you might want to spring for the extra few pennies to use black joe which will stand up to any roof leaks that occur over time.
2. It's a bad idea to lay gyprock directly on the polyiso because you will increase substantially the chances of screw pops/pull throughs. Polyiso is a so-called 'rigid' foam board, but it's still somewhat compressible which will lead to uneven screw tension and potential waves in your gyprock surface. We generally use it to insuslate roofs on the exterior of the roof deck; in that application we trap it in a crib of 2x on edge which stands at least ½" proud of the foam-board surface.
In addition, any condensation which does occur on the 'warm' side of the insulation will soak the paper backing of the gyprock. Theoretically, you should not have condensation at that point...but 1x3 is cheap and an air space between the two materials won't do any harm.Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Black Joe is ½"-thick compressed paperboard which has been treated with bituminous tar for weather resistance. It's commonly used as exterior sheathing on low-budget construction for walls where racking shear is not an issue. It also has a small but calculable R-value as an insulator.
Which, in CT, might be better known as Homasote board.Occupational hazard of my occupation not being around (sorry Bubba)
Asphalt impregnated homasote.
Canadians talk funny, don't they?Oh, Hi, Dino!;)
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Canadians talk funny, don't they?
Oh, Hi, Dino!
Dunno, Ryan is Canadian, but wouldn't Dino count as ex-pat?
'Course, my guess is that Homasote does not translate into Francais, thus the Jaques Noir nomme de commerce.
<G>Occupational hazard of my occupation not being around (sorry Bubba)
Thanks for the 'translation', LOL.
And I'm gonna leave Piffin's comment about 'impregnated' Homo-sote for somebody like Gunner to play with. I wouldn't touch that one with a 3.048 metre pole....
Yeah, 'Black Joe' and 'Ten Test' are two old brand names up here; I've actually never even seen those brands myself...but the names have passed into the common parlance. As I learned an awful lot of what I know about construction after I moved to Canada, I do tend to use the local terms (except for those things my old man taught me when I was a kid. Gem boxes, for instance. And I still think of drywall compound as Spackle, and remember the blue and orange carton it came in...).
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
And I still think of drywall compound as Spackle, and remember the blue and orange carton it came in...)
And the whippersnappers just roll their eyes when a person talks about the red and yellow box of Duc-Seal; or the ugly tubes of Pookie; and having to remember they only know "clean 6mil polyethylene" not "Viskween" . . . Occupational hazard of my occupation not being around (sorry Bubba)
I always thought that was spelled 'Visqueen'....
I still prefer to use the foil-backed kraft paper VB anyway. Besides the fact it lets these old houses breathe better ('cause they're not gonna install an air-exchanger in a 40-y-o 'winterised' summer cabin), I'm told the old-timers used to get it almost free around here as it was a 'waste product' from a local company that manufactured laminated panels.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
I always thought that was spelled 'Visqueen'
Could be, been so long since I saw any under a name-branding--it's all " 'Pro Grade' 6mil clear sheeting," now . . . Occupational hazard of my occupation not being around (sorry Bubba)
We used to call that stuff "Celotex", but I'm not sure it's available under that name anymore. "Homesote" used to be used instead of plaster or dry wall in cheap buildings.
I'll disagree with Dino - I think hot roofs can work well in CT. Which, btw, is where I live as well. I've seen ice dams on barns, for Pete's sake, so I question the ability of venting to prevent ice dams. Sometimes the orientation of the house allows solar melting that freezes later in the day, creating ice dams. Nothing you can do short of moving the house or changing the Earth's orbit will fix that. Insulation is your best bet. The only worry I have about hot roofs is that when the roof leaks, which all roofs eventually do, how will you know?
Also, the CT Supplement to the 2006 IRC allows hot roofs only with closed cell foam. That said, I've seen numerous occasions where the AHJ either ignored that provision or didn't understand it, and allowed open cell foam.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Well, if I'm gonna get disagreed with, I'd just as soon it be done by you....
You are right--that solar-heating ice-dam is damn hard to beat. (I get them on my south-exposure deck roof, which is very much an unheated roof, LOL.) Which is why I recommend the membrane, too.
But ventilation has other benefits besides reducing ice dams due to heat leakage, of course. In a climate like here (or CT, to a lesser degree) where the extremes of the temperature range can be 70ºC apart (-40º to +30º) or more, having a roof which can suck in fresh air to cool itself off will not hurt the longevity of your shingles, for instance. And it should reduce your cooling bill by a noticeable amount.Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
So they say.
If I lived where you do, I might put more faith in roof venting than I do. I think there are more effective ways to cool an attic, beginning with radiant barrier roof sheathing. Doubt they've got that up north, it's mainly a hot climate product. It's kind of magical in the same way that low-e windows are. That is, it reflects inside heat back in, while only radiating 3 to 5% of outside heat in. Keeps you warm in the winter and cool in the summer. How does it know? I'd guess it will be as common as low-e windows in a decade.
I'm reading Water in Buildings, by Wm. Rose. Bill's a research architect at the U. of Illinois. I've been his editor at FHB in the past. One of the fascinating things about this particular book is that Bill delves into the history of roof venting, going back to the first reasearch done in the 1930s. The focus is, of course, water, but he covers heat a bit, too.
It's apparent that the early reasearch was incomplete - even the scientists who did it said so twenty years later. Trouble was, it got written into the first national codes immediately post war pretty much verbatim. The prescriptive solutions - venting ratios of 1-150, 1-300, numbers we all know - were swags in the 30s. But an industry grew up around them, and we take them as gospel now. We shouldn't. Sometimes they're helpful, sometimes not. But they keep us from thinking.
BTW, Bill's own reasearch specialty? Roof venting. He's told me that shingle color has a far greater effect on roof temperature than venting. Great book, but about $70 US. You'd enjoy it. Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
He's told me that shingle color has a far greater effect on roof temperature than venting.
Roof colour has a major effect on attic/roof temperature. The diff between a black roof and a silver or white roof can be 20 or 30 of those little, undersized F degrees you use down there on a good hot summer day.
In the early days of sailboarding, guys used to 'scoop' their boards to keep the bow up out of the water more easily. The way they did it was to turn the board upside down and set a milk crate under it about where the mast step was. Then they'd tape a black plastic garbage bag to the hull from there up to the bow. Then they'd pile a couple of concrete blocks on the bow and go take a quick swim.
'Quick' was the operative word; within 10 minutes (or less if the sun was really hot that day), the solar heat collected by that garbage bag would have softened the plastic hull...and the nose, weighed down by the concrete blocks, would have bent almost down to the ground. At this point, the garbage bag was removed and buckets of water were thrown on the hull to cool it. Once it was cool, the weights were removed and most of the new curvature would stay in the hull.Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
The actual temperature differences are even greater than that.
white 135 degrees F
brown gray or green 175 degrees f
black 195 degrees f
While we're on the subject, more than 90% of the heat that builds up in the shingle is radiated back out to space. Only a small fraction is removed through the bottom and into the attic. That's why radiant barrier sheathings and unvented attics have only a small effect on shingle life. Color and type of material are much more important to roof temperatures.
When two objects face each other they radiate their heat to each other. The sun radiates lots of heat onto your roof. Your roof radiates its heat towards the sky. The sky is made up of the atmosphere, clouds, and deep space. Deep space is very cold. The atmosphere is warmer and clouds are warmer still. The sun is only a tiny part of what your roof sees. It mostly sees space. If I point my IR temp sensor at the sun on a summer day it will read around 5-30 degrees F. This is because it reads a big area of the sky that includes the sun. On a hazy summer day it will read around 30-60 degrees F. This is because the haze holds more heat than clear air.
The top of your roof sees a much colder object(sky) than the bottom of your roof(attic). Most of its heat energy will be radiated from the side that ses the cooler object(topside). Once you start looking at the importance of radiant energy in roof temperatures, it's easier to see why attic ventilation is anemic at removing heat from the roof system. Convection is a very weak mechanism for heat transfer compared to radiation. That's why your car still gets hot when you park in the sun with the windows wide open. No amount of ventilation can overcome the radiant energy of the sun.
Excellent post!Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
brown gray or green 175 degrees f
Which explains why I needed to wear kneepads over double-knee Carhartts, and insulated winter work gloves just to be able to get onto that green roof I had to repair during the height of the hot spell last summer....
You are correct; there is no doubt the roof transfers the majority of the calories it soaks up back to the outside atmosphere. Once the air-temp in the attic equalises with the temp of the roof assembly, no further heat transfer will take place between them until one of them heats up some more (or cools down).
And that doesn't take long on a really hot day: There's not a lot of mass in the small amount of stagnant air contained in an attic. Whereas the outside air is a heat sink of virtually unlimited size as far as that roof is concerned. And that air is generally moving: Even if there is no wind, the air near the roof is constantly moving past it, absorbing heat as it does so, due to convection currents of the heated air moving up away from the hot roof.
But in an unvented 'hot roof' structure, once the attic temperature reaches the same level as the roof structure, it's going to stay there for a good while because that heat has to get back out through the insulation to escape into the outside air...and the insulation is designed to slow that process down--in both directions, of course. To speed that up, you need to physically change-out the air mass trapped below that roof, and introduce cooler air capable of absorbing more calories.
No amount of ventilation can overcome the radiant energy of the sun.
I'm sure you would rephrase that statement if you'd ever stood in a 20-knot breeze on a bright, clear, sunny day...when the temperature is about -35º. <G>
Maybe what you mean to say is that no practical amount of unpowered ventilation that can be built into a roof can eliminate all the heat the sun is capable of putting into that roof, and in that case I agree. But I never said ventilation was the answer to cooling a roof; it is just one element that helps do so. It also happens to be a practical one that's easy to install, and it does not require any outside power source. It's one of Mother Nature's freebies.
But here we are agreeing that cooling off your roof is a Good Thing, both winter and summer.
And we agree that passive ventilation can't do the whole job by itself.
So my question is, why would anyone want to make things worse by removing the ventilation and whatever cooling capacity it does have? What do we gain from that?Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
So my question is, why would anyone want to make things worse by removing the ventilation and whatever cooling capacity it does have? What do we gain from that?
I'll take a stab. Because venting in most cases is not neccessary to keep shingles cooler than the temperatures they were designed to withstand. It's a solution searching for a problem.
It does little to achieve its original intended purpose, removing attic moisture. Attic moisture is nearly always a symptom of a damp foundation and a leaky house. Fixing those problems at their root not only dries the attic, but saves energy and improves the liveability of the house.
Venting creates potential leaks in the roof, and costs time and materials for little or no purpose. Finally, venting a roof can create negative pressure in the attic, which sucks conditioned air out of the house.
Have I ever mentioned that I don't think venting roofs is a neccessary thing?Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Have I ever mentioned that I don't think venting roofs is a neccessary thing?
Maybe once or twice....
I think maybe we forgot about one aspect of the OP's situation, which is that his attic space is used for an office. Which means he's got what is effectively living space with a cathedral ceiling. All heat/moisture removal going on up there has to happen in the very restricted, inaccessible space between his gyprock ceiling and the underside of the roof deck. He doesn't have a nice, big, empty, unheated attic to act like a thermal buffer and reservoir up above his conditioned space.
It's also an older house, IIRC, which means he probably doesn't have forced-air HVAC or an air-exchanger. So tightening up the house by foaming the underside of the roof, etc., is likely to do a fair amount of harm by trapping moisture- and spore-laden air for which there is no other way out.Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Here again I'll disagree. Air sealing the lid of an older house can make a huge difference in the amount of moisture moving through the structure. Moisture, at least in mixed climates like CT's, mainly moves on air currents, not through vapor diffusion. The classic example is a leaky attic and a damp, vented crawlspace.
As warm air rises, it creates a zone of higher pressure at the top of the house, and a zone of lower pressure at the bottom levels - the stack effect, like a chimney. This higher pressure is generally greater than the outside atmospheric pressure, so the inside air leaks out.
The air on the lowest level of the house being at a lower pressure, outside air is drawn into the crawlspace. Here, it gets loaded with moisture, and sucked upward to replace the air exiting at the top. This is the predominate way in which moisture gets transported through houses, although this is an extreme example. Capping the lid short circuits the stack effect in the area of highest pressure. If air can't leave, you'll still get convective currents, but you won't draw outside air into the foundation.
Without air currents to transport it, moisture can tend to stay in foundations. They're usually cold enough that there's little drive to make the molecules leave their cozy hydrogen bonds with the concrete. Absent air currents, a foundation has to be a swamp for much to happen in the upper floors. Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
We may be accused of belonging to the mutual admiration society. You do know your stuff.
Straube and Burnett's book is available at the building science press. Cost is 80$. It's somewhere around 800 pages? I'm almost through it but will need to read it more than once. I wish I had taken physics in college.
Yes, sitting around with those guys is quite an educational and entertaining experience. The liquid of choice seems to have changed to fine wine.
Thanks.
I heard rumours of a very good bottle of some French red varietal making its way from NY through CT into MA a couple of years ago, violating the laws of the land, if not those of physics.
Yeah, I wish I'd taken physics as well. I'm grateful though, to Mr. "Buggy" McHale, who was an outsanding high school chemistry teacher. Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Excellent post but I am more confused than ever!
I am planning a similar project in my 100 year old foursquare house's unfinished attic space (pyrimid shape). Thinking of converting the space to a master suite with bathroom, kneewalls, and high cathedral ceiling (10+ ft).
I live in New Jersey. Is the vent/no vent decision easier to make in my location due to the more mild climate? Any recos here?
George
In a way, your roof makes for an easier decision. Unless you've got a vented cupula up there, there's no way to effectively vent that roof. So what's stopping you, then?Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
RayCan we get a review of Straube's book from you? $80 is a lot to drop on a book for someone with just an interest in the field and no professional justification for buying it.
I don't know - What's it cost to take your wife to dinner?Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
She's a cheap date. I chose wisely.Maybe I'd better re-ask the question with "$80 is a lot to drop on a book that is unclear or just a repackaging of what's already out there." I imagine its pretty good given Straube's reputation, but I've seen more than a fair share of poorly written textbooks by very intelligent people.Plus $80 goes a long way to the next power tool.
I bow to your wisdom. <G>
Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
If you only have a passing interest in building science for building enclosures and money is an issue, save your money. It takes a geek to swoon over this book.
On the other hand, if you want a definitive reference book on the physics related to building performance and building science, complete with all the formulas and a thorough explanation of all the relevant theories, I don't think there is another comparable source for the information.
It is one of those books that you can easily put down. My head starts to hurt after awhile. When that happens, I skim ahead, find out where it's headed and then back up and fill in the details. Later it becomes a reference to go back to when you want to find say the heat of fusion or some other helpful bit of info. Keep it next to your ASHRAE guide and you've got the whole shebang at your fingertips.
Builders' guides are fine but with the knowledge in this text you will graduate past a reliance on prescriptive methods and gain a new sense of understanding of the issues that once made you wonder if one method was better than another or was safe in a certain climate zone.
Life will take on new meaning and women will find you unexplainably attractive. Your teenager will respect you. Food will taste better. Your vision will improve. OK maybe the teenager part crossed the line into fantasy.
Buy the book. Cross over to the dark side.
>>>>>>>>>>>>>>We may be accused of belonging to the mutual admiration society. You do know your stuff.Get a room.
"Let's go to Memphis in the meantime, baby" - John Hiatt.
GRANTT LOGANN - THE LEXINGTONVILLE COPPERWRIGHT
http://grantlogan.net/
Here again I'll disagree. Air sealing the lid....
I'm not sure we're actually disagreeing, because what I'm trying to propose won't do any of those horrible things you talked about.
The venting system I use for cathedral ceiling roofs like this poster's is completely isolated from the interior space of the house. Totally. Absolutely. Sealed off from it. Might as well be on another planet....
The system I use creates an isolated cavity between the insulation layer and the underside of the roof deck. There are only two access 'ports' to that cavity: One is the soffit vents, and the other is generally gable vents, roof-mounted charley-noble turbines, or as in the case of the hip roof, in a cupola.
There is no communication between air inside the house--whether at the foundation level or anywhere else--and the air moving through these ventilation cavities in the rafter bays. There is no pressurising/depressurising effect felt inside the house due to anything going on in there. It is impossible for air movement inside this roof ventilation system to draw in air from the foundation; it can only draw air from the outside atmosphere.
The only thing the air in these vent channels does is absorb heat from the roof above them and the insulation below them and take it outside.
That's it.Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
OK, but I tell you, convective cooling does very little in the face of radiant heat. I'd venture the savings aren't worth the effort. OK, maybe in your climate, a cold roof like you describe makes sense. Where I am, I've got doubts that it's worth the effort.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Just for fun, we ought to compare climatic data. Here are the figures for my area:
Elevation: 230m
January design temperatures: 2.5%= -27ºC; 1%=-30ºC
July design temperatures: Dry=27ºC; Wet=22ºC
Degree-days below 18ºC: 5300
15-minute rain: 25mm
One-day rain: 90mm
Annual total precipitation: 1025mm
Ground snow load: Ss=2.6kPa; Sr=0.4
Hourly wind pressures: 1/10=0.25kPa; 1/30=0.30kPa; 1/100=0.36kPa
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Where'd you find all of that in one place?Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Appendix C, National Building Code of Canada. "Design Data for Selected Locations in Canada."
My edition is a few years old, so some of that may have changed slightly, what with global warming and all that, LOL....Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Appendix C, National Building Code of Canada.
Oh. Could you be a stud muffin and look up Roxbury, Connecticut in it?
<G>Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Just take the cosine of your latitude, apply the corrections for refraction and dip, and enter the tables with the natural log of pi.
<G>
Actually, the closest I could come without US code books here was the NOAA climate summaries for Bridgeport:
CT BRIDGEPORT BDR 0 -4 0 0 -6 -1 -999 -999
But I ran a Google search on Climatic design data for the U.S., and, a few levels down, I found this:
The builder's guide to cold climates: details for design and construction / Joseph Lstiburek. New town, CT : Taunton Press, 2000. EnvDesign TH153 .L78 2000
Man, that guy is everywhere....Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Andy,I just finished reading Water in Buildings about a month ago and it its really exceptional, in my opinion. I highly recommend it to anyone here with an interest in building science. Its also nice to hear from someone besides Lstiburek, who is maybe a little overly quoted here - though not necessarily underservedly so. The book really comes at it from a science point of view whereas buildingscience.com is more of an applied science point of view. I really like Rose and I wish we heard more from him - he writes well with lucid explanations and good examples. I'm hoping to read Straube's book next but its not so readily available in libraries.From what I've read of attic venting codes origins, the fact that we are still using them, as is, is kind of embarassing.
I didn't know John Straube had a book out. I'll have to look for it.
One of the most fortunate experiences of my life was to be able to spend an evening drinking beer with Bill Rose, John Straube, Joe Lstiburek, and a couple of other gents of that ilk. Talk about eye opening.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Hello Dino. How does a "hot" roof increase the risk of ice damming? In the winter I would call this a "cold" roof for lack of a better term. Possibly with the way WEP wants to insulate it with the knee wall and the roof above it it would leave the bottom conventionally insulated to be most accurate. I would recommend insulating with the foam all the way from the soffit up to the ridge. This makes the entire roof cold during the winter, eliminating the melting up top and freezing down where the soffit and overhang is.
check link for pic.http://www.foam-tech.com/problems/ice_dams.htmCondensation is a major concern within insulation, I agree there. Foam, however, is solid and allows no air movement. Traditional fiber type insulation does nothing to stop bulk air movement. No air through the foam equals no condensation in the insulation, thus no problem.
see this link for an extreme case:
http://www.foam-tech.com/case_studies/winterplace.htmThe warm side of the insulation is actually warm with foam and the temp is well above the dew point when applied even less than code R values.Stu
How does a "hot" roof increase the risk of ice damming?
We start from the premise that no insulation (outside of an absolute vaccuum) is perfect. There will always be some heat leaking through even the best insulation job, and if there's no provision to remove that heat before it is absorbed by the roof deck, underlayment, and shingles, they will absorb it. 'Heat always moves from the hotter to the colder,' to quote Flanders and Swann.
We then have to remember that snow is one of Mama Nature's better insulators...which is why igloos work so well at temperatures that would freak out the average Connecticutian. Even a piddling foot of snow on a roof will insulate it from the outside air quite well. Were the air temp to drop to -20ºC (about 0ºF), the temperature of the roof shingles underneath that warm blanket of snow would still be stable at close to 0ºC.
So it doesn't take a lot of heat leaking up through our admittedly imperfect insulation to cause the temp on the surface of the roof to rise above freezing...which causes the snow to melt...at which point it starts to dribble downslope...until it hits the eaves.
The eaves are beyond the building wall, outside the conditioned building envelope. The roof surface of the eaves is always cold, and, if the ambient air temp is below freezing, it will also be below freezing. So when that liquid water trickling downslope under the snow hits the cold eaves roof, it freezes and creates a very small bump of ice.
The next drop of water hits that little bump, and freezes on top of it.
And so on and so on, until you have a dam of ice high enough to retain pooled, liquid water uphill of it, over the warm part of the roof. And since roofs aren't designed to hold water but to shed it, you get leaks. That standing water will be wicked uphill under the shingle tabs by capillary action, and will then weep down along nails or staples through the roof, run down the underside of the roof decking until it hits a piece of framing, and follow that down until it makes a nice ugly wet spot on your gyprock--usually a good fifteen to twenty feet from where the water actually penetrated the roof, LOL.
All this will happen unless you provide some way of evacuating leaked heat from the roof structure before it warms the roof deck and shingles up above freezing. The way we do this is to create a cavity between the insulation and the roof deck and ensure that this cavity has unobstructed flow-through ventilation of cold, outside air. Hence soffit inlet vents and gable-wall outlet vents near the ridge.
It should be noted that direct sun can also cause some ice damming, Use of a self-sealing membrane (or full-pitch double-coverage felt job) will help prevent leaks due to these.Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....