Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
VENTILATION in BUILDINGS
AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK SEALING PROCEDURE
AIR POLLUTANTS, COMMON INDOOR
AIR SEALING STRATEGIES
ROOF ICE DAM LEAKS
BASEMENT CEILING VAPOR BARRIER
BASEMENT HEAT LOSS
BASEMENT LEAKS, INSPECT FOR
BLOWER DOORS & AIR INFILTRATION
BRICK WALL DRAINAGE WEEP HOLES
CATHEDRAL CEILING VENTILATION
CEILINGS, DROP or SUSPENDED PANEL
COMBUSTION AIR for TIGHT buildings
COOLING LOAD REDUCTION by ROOF VENTS
CONDENSATION on WINDOWS & SKYLIGHTS
DEW POINT CALCULATION for WALLS
FIREPLACES & HEARTHS
FLAT ROOF MOISTURE & CONDENSATION
GREEN BUILDING CONSTRUCTION
HEAT LOSS in BUILDINGS
HEAT LOSS DETECTION TOOLS
HEAT RECOVERY VENTILATORS
HOT ROOF DESIGNS: Un-Vented Roof Solutions
HOUSEWRAP AIR & VAPOR BARRIERS
HOUSE DOCTOR, how-to be
HUMIDITY LEVEL TARGET
ROOF ICE DAM LEAKS
INDOOR AIR HAZARDS TABLE
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INDOOR AIR QUALITY IMPROVEMENT GUIDE
Insulation Air & Heat Leaks
INSULATION INSPECTION & IMPROVEMENT
INSULATION R-Values & Properties
LOG HOME GUIDE
MOISTURE CONTROL in BUILDINGS
ODORS GASES SMELLS, DIAGNOSIS & CURE
ROOF VENTILATION SPECIFICATIONS
ROOF ICE DAM LEAKS
SHEATHING, FOIL FACED - VENTS
STAIN DIAGNOSIS on BUILDING INTERIORS
STUCCO WALL METHODS & INSTALLATION
SWEATING (CONDENSATION) on PIPES, TANKS
THERMAL MASS in buildings
THERMAL TRACKING Indicates Heat Loss
VAPOR BARRIERS & AIR SEALING at BAND JOISTS
VAPOR BARRIERS & HOUSEWRAP
VAPOR CONDENSATION & BUILDING SHEATHING
VENTILATION in BUILDINGS
WIND WASHING INSULATION At EAVES
WINDOWS & DOORS
Hot roof designs, aka "dense-packed" insulated sloped roofs: this article describes various solutions for un-vented cathedral ceilings and similar under-roof spaces, offering advice on how to avoid condensation, leaks, attic mold, & structural damage when roof venting is not possible. This article series about roof and ceiling ventilation describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
Un-Vented Roof Solutions: how to avoid condensation, leaks, damage when roof venting is not possible
Here and at Correcting Roof Ventilation we argue that while some experts like the "hot roof" design that omits attic or under-roof ventilation entirely, and while there is good science behind that approach, risks of mistakes in construction, variations in building occupancy use and moisture levels, and wear and tear can lead to very costly surprise rot, mold, or insect damage on buildings where leaks and moisture are trapped in building cavities and remain un-noticed.
Some buildings, by their shape or design, simply don't make it easy to install continuous intake venting at the eaves or lower roof edge, or continuous outlet venting along a ridge.
For example, a house which has no roof overhang at all makes intake venting at the eaves difficult.
A house with a pyramid roof shape or complex roof shapes makes outlet venting at a ridge difficult.
Sketch (above left) is courtesy of Carson Dunlop Associates shows the two basic strategies for insulating cathedral ceilings and flat roofs.
Problems with Partial Roof Venting
On these roofs, partial venting can be worse than no venting. For example, adding a ridge vent, or several roof "spot vents" or roof turbine vents on a few roof slopes, typically mid-slope or in the upper third of the slope on roof surfaces not visible from the front of the building, may please the installer, but they are worse than ineffective.
Placing an outlet vent on a roof without adequate inlet venting (see Roof Vent Area Ratios) works against the interests of the building and its occupants. As convection currents and heat loss into the roof space or attic vent out through these vents, the intake air needed to satisfy the exhausted air leaving the building will be drawn from the building interior - increasing building heating costs and possibly increasing particle movement from basements or crawl spaces (if there is a mold concern in the building).
If you can't provide enough intake venting it is probably better to not vent at all in these conditions.
Solutions for un-vented roofs: Avoid cold-climate ice dam leaks & reduce cooling costs in hot climates
Ice Dams on Un-Vented Roofs in Cold Climates
Grange and Hendricks (1976) recommended a combination of eaves and ridge venting to avoid ice dams on buildings. Other authors found that ice dams seldom occur at temperatures above 22 degF. or when attic temperatures are below freezing. Our photo (below left) shows an ice-dam prone roof on a tall building with a slate roof.
Design Suggestions for Unvented “Hot” Roof Designs Where Venting is Difficult
-- Adapted with permission from Best Practices Guide to Residential Construction.
Worries about the "hot roof" un-vented Cathedral Ceiling & Hot Roof Designs
As explained in our "hot roof" discussion at ROOF VENTILATION SPECIFICATIONS and at HOT ROOF DESIGNS: Un-Vented Roof Solutions, we don't have confidence in the long term durability of "hot roof designs" because any future roof leak into this cavity produces trapped moisture and rot. We call this a "hot roof" design because failing to vent the roof from below not only misses a chance to avoid ice dam leaks and condensation damage in cold climates. In hot climates the roof temperature will be much higher on an un-vented roof, resulting in much shorter shingle life. This is less of a concern for slate and similar product roofs.
Also see INSULATION LOCATION for CATHEDRAL CEILINGS for more information.
In buildings where there is no roof venting anyway, an un-vented, well insulated "hot roof" is a second-best alternative to preventing ice dam related leaks in cold climates. Be sure to inspect the roof surface from outside for leaks and damage every year and to fix any damage quickly.
FAQs about un-vented cathedral ceiling under-roof spaces
Question: Some older buildings with no roof venting seem to be ok anyway - is there a concern?
Our house was built in 1920 it has never been vented. The second story is finished with the insulation directly on the roof sheeting. This year, after 30 years, we had the house reshingled the roof deck was in perfect condition no problems anywhere.
If we had the the choice I would lean toward venting but because of the way the home was built it's impossible but their has been no negative affect being unvented. - Jim
Reply: Some un-vented homes seem not to suffer, but not all: explanation and some warnings about un-vented cathedral ceilings on older homes
I agree that many older homes were often drafty-enough that combined with the good luck of no unusual indoor moisture source (like a recurrent wet basement or crawl space) that they fared pretty well without more aggressive attic venting.
Building ventilation and moisture entry patterns change over the life of a structure
But one needs to be careful in drawing conclusions from those examples. The way buildings are used, heated, ventilated, insulated, and sealed changes over time. I have inspected homes that were more than 100 years old that had been in good shape as far as moisture problems were concerned, until energy costs led new owners to change the way the house worked.
Reader comment: Replacing a wood shingle roof with asphalt over plywood changes how the building works
Lot of older homes vented naturally thru their wood shingle roof ...... When you tear off and replace with plywood and an asphalt shingle you are changing the design and could be asking for problems if not properly insulated and ventilated . Ansel 9/4/2011
Good point, Ansel, and we agree. Your comment is an example of how changes to a structure over it's life also change how a building works and thus can cause new problems to arise, in this case higher moisture. We might add that a replacement wood shingle roof installed over solid plywood decking won't last as long as wood shingles over ventilated spaced nailers.
Question: We Can't Afford Spray Foam - Can we Just Cut Rigid Foam to Fit?
We have TJI rafters in a very weird roof area. no venting at all. I need to insulate and cannot afford the $750 minimum in our area for spray in foam. since the TJI has a flange top and bottom, what would be the best combo of rigid foam sizes? the flanges are 1.25" high and 1" deep and the remaining rafter depth between them is 7". the distance between the flanges within the bay is 21.5". the rafters are 24" OC . Each bay is 5' long for a distance across the roof is 21' please help me calculate. Claire - 9/16/12
in the interim, I figured out how I will do it. I will fill the space between the flanges with 7" strips of 1" foam. then I will cut and place 2" foam boards and 1" foam boards to fill the bays. tape and vapor barrier to block air
Our photo (above and below left) illustrate before and after spraying foam between the TJI rafters in our own project in 2011. The "R" value of the roof with the cavity filled will exceed what you can conveniently jam between the TJI's in your own roof and more, it's an air-leak-proof ceiling in the "hot roof" design we discuss below. To reduce our anxiety about future leaks into this roof structure (the foam and ceiling would hold water from a future roof leak until the roof rotted or the ceiling collapsed) we also opted for a durable standing-seam metal roof above. Photos courtesy Galow Homes.
There is no technical reason why you couldn't cut solid Hi-R foam into slabs and custom fit them between your TJI roof truss bottom chords. And it's true that a spray foam job may cost twice that of installing fiberglass batt insulation in the same space.
But having done some cut-and-fit foam board insulation jobs myself, I have some reservations about the DIY approach using cut sections of foam board insulation in the case you describe.
Not only is the cut-and-paste effort labor intensive, but I worry about a leaky design that makes the insulation job ineffective. An advantage of sprayed foam under-roof insulation is that it seals perfectly against air leaks. Depending on the foam type used you'll still need a vapor barrier.
Insulating with cut sections of solid foam board
Our photo (left) illustrates a project using 2-inch high density styrofoam insulating board under the floor over a (dry) crawl space.
We used cleats to support the foam, not having the convenience of the I-joists that you have in your ceiling. You can see that even working with some care, it was difficult to keep the foam slabs cut for a perfect fit (green arrow).
A second problem with retrofit foam insulation using cut-foam boards is shown by our photograph of a feeble attempt to insulate around the waste pipe penetrating the floor. (Next photo, below left).
A combination of cutting solid foam board insluating slabs followed by judicious use of a few cans of spray insulating foam can improve the performance of a do-it-yourself cut and paste insulation job.
The R-values of sprayed foam depend on the foam type and weight-rating. Two common versions are Icynene® half-pound open celled foam and CertainTeed CertaSpray® two pound closed cell foam. Open-celled Icynene LD-C-50 type foam has an R-value of about 3.6/inch.
Properties of Spray Foams
In comparison, any properly-installed spray foam insulating job fits building cavity and mechanical system variations and penetrations perfectly.
Open celled foams are a bit less costly (per inch of finished thickness) than closed cell foams, are sprayed and trimmed, and are lower in R-value. Closed cell foams (two pound foam) are more dense and heavy, are usually sprayed with little trimming (there may be a concern about the care with which ceiling or wall or floor cavities are filled to an adequate depth) and this foam job may cost 20 to 30% more, gains in both air and moisture barrier properties, adds structural rigidity to the buidling roof or wall where it is installed, and has an R-value of about R 6.5/inch - almost twice as much. More details are at INSULATION R-Values & Properties and at INSULATION CHOICES.
You'll want to compare the R-values per inch of the foam board you planned to use and figure how much thickness you'd need to fit into place to get even close to the R-values of the spray approach. 1-inch Dow Tuff-R board has an R-value of 6.5/iinch for one-inch thick boards. That's not making any allowance for heat losses because of looseness of fit or cutting errors.
Once you've installed a makeshift insulation job, you'll be tempted to install the finish ceiling. And once the ceiling is in place you're not going to want to disturb that work for a long time, meaning you may be living with a poor or leaky insulation job and high energy costs.
Take a look at the "hot roof" or "packed" roof insulation deiscussion above and at the end of this article before making up your mind.
Question: dense packing sloped roofs, residential - Is there Data Supporting Dense-Packed Un-Vented Sloped Roofs?
Is there any data out there on condensation issues when dense packing a sloped roof. Cape cods and other dormer type construction have ceilings that traditionally have ventilation to the upper attic area and out the ridge vent. Sometimes these sloped roof areas have battens of fiberglass insulation.
We are running a residential program in Wisconsin and the contractors have been dense packing the sloped roof/ceiling in these areas.
Their anecdotal evidence is that they have had no problem with condensation or moisture. Are there any resources out there on data for zones 4 and 5. By the way, I want to thank you again for joining us on the radio program, Constructive Solutions, in Wilmington NC and answering listeners questions. - J.A., Madison WI, 5/15/2012
Our sketch (above left) shows the traditional 1940's - 1960's design for insulating and venting a cape cod roof. Current design, among those who are venting the roof at all, provides continuous soffit or eaves intake, continuous ridge outlet venting, and we eliminate the gable end louvered vents. - Ed.
Reply: there is science supporting un-vented roofs provided construction is perfect and no roof leaks occur
Also see the notes in our hot roof article (above) and also at ROOF VENTILATION NEEDED?.
Roof venting for cathedral ceilings remains a contested issue, though as we elaborate below, there has been some expert work and plenty written about the topic. Roof venting for buildings with attics is also discussed below.
Our photo at left is not a Cape Cod style home, but this New York house illustrates differential snow melting indicating where heat is being lost into the roof structure.
Even in climates not subject to freezing temperatures and heavy snowfall, the snow-melt patterns on these Northern homes can inform us about how heat and air can move from the occupied space into the under-roof space of a building.
Special Venting (or "packed" hot roof) Problems for Cape Cod Roofs
Cape cod roofs present a special problem in that in the usual cape design there is a lower attic knee-wall enclosing what amounts to a "low attic" followed by the equivalent of a cathedral ceiling over the middle section of the roof, often penetrated by dormers when the second floor of the Cape Cod is designed to be occupied, followed again by an attic design for the upper third of the roof.
When I examine Cape Cods built in the Northeastern U.S. in winter, homes built in the 1930's through 1960's especially show ice dams on the lower roof, snow on the center roof, and all snow melted off of the upper third of the roof.
Those conditions mean leaks into the building walls, unnecessary heat losses (higher heating costs), and too often roof sheathing and insulation contaminated with several genera of mold.
And where we have removed insulation from between the rafters of such roofs we often find moisture has been trapped against the roof sheathing. Sometimes that has led to mold, delaminating plywood sheathing, or even rot.
In new construction we have an opportunity for better building design, implementation, and energy performance.
The bottom line (in my opinion) is that roofers and builders who like the "hot roof" approach trust their implementations of the hot roof approach and assert that it works fine but water or moisture leaks from either outside or from within the building into an un[-vented building cavity (like a cathedral ceiling roof) cause more damage than leaks into a vented roof.
Hot Roofs vs Vented Roofs in Hot Humid Southern Climates - Building Heat Gain
Our photo (left) shows an extensive Penicillium sp. mold contamination in an attic where moisture condensation had gone unnoticed. [The identification of tihs mold as Penicillium happened in the lab, of course, not based on just the visual inspectinon.]
TenWolde and Rose pointed out in 1999 that
Our photo (left) shows stains on the upper building walls under an inadeqruately-vented soffit. This New York Home suffered recurrent ice dam leaks sending moisture through the building walls, with the expected problems that result.
Joe Lstiburek's view is that particularly in the hot humid South (of the U.S. - and not the local ofthe buildings we are illustrating here) there is not much gained in venting a roof (2-3% reduction in heat transfer in a vented attic) and quite properly he continues to point out that once A/C ducts are placed into the vented attic heat transfer to the occupied space goes up to 5-7% (for tight insulated ducts) compared to routing those ducts through conditioned space of the home, and a much worse 25% heat gain to the home if the HVAC ducts are leaky.
And his argument that air movement through the attic under the roof will not flush heat being radiated towards the ceiling below is both interesting and compelling as well.
Joe also argues that the moisture from warm humid outside air run through an attic to ventilate it can move through the attic insulation where it can condense on the cool building ceilings below. 
Those data are compelling, though I'm not entirely convinced that that nasty warm southern air my respected acquaintance describes actually moves "down" to the cool ceiling in the attic. In roofs I've worked-on and examined with air tests and smoke guns, warm air rising in the hot attic went zooming out at the ridge vent, drawing cooler (nasty moist Southern) air in at the eaves. The incoming air followed the underside of the roof upwards on the air currents exiting at the ridge. without however, recapping construction costs, or considerations for an existing home rather than new construction.
Joe pegs the impact of venting on roof temperatures at 5%. I measured the temperature drop in a hot, un-vented attic in New York from over 145 degrees to 95 degrees after we cut in soffit and ridge venting. I was measuring air temperatures in the attic, not roof surface temperatures, and not radiated heat effects. That was a 35% drop in air temperature in the attic.
Hot Roof vs Vented Roofs in Cooler Northern Climates
The arguments above were aimed at homes in the hot humid South, not your area, in much cooler Wisconsin, and perhaps not in the hot but arid climates of the Southwest. [We need to ask Joe his opinion on that climate.]
Our image of a New York 1940's Cape Cod home (left) illustrates a snow-melt pattern telling us where the roof was insulated, where heat is flowing from the occupied space below, and where to expect ice dams.
TenWolde and Rose, ASHRAE experts from the Forest Products Laboratory and the Building Research Council respectively addressed these issues articulately in 1999, citing research on attic ventilation in the U.S. dating back to 1939 (Rowley et als) who recommended controlling indoor moisture and venting the attic.
The first explicit quantitative attic venting recommendations came from FHA in 1942 when I was just (-1) years old. Rowley et al. (1939) documented that attic ventilation could reduce condensation on roof sheathing during cold weather, though Tenwold/Rose point out shortcomings of their study and cite further research by experts in the 1940's suggesting that condensation occurred only in homes with high indoor humidity.
TenWolde and Rose also point out that
I concur also with the view of those authors that a cathedral ceiling (or for that matter any roof) with ridge vents but without sufficient soffit intake venting acts as a chimney, not only admitting harmful humid indoor air into the cavity but also increasing heating costs. They conclude that,
I agree with this view as well, based on the science, but I do not entirely agree about the long-term durability of un-vented roof designs subject to the vagaries of leaks and time, as I have elaborated here, and even those authors pointed out that
Roof Temperatures of Roofs over Attics Compared with Roofs over Cathedral Ceilings
Nevertheless it seems obvious that being hotter longer is almost certainly going to mean wearing faster for asphalt shingles. We lack quantitative specifics, not concept.
Roof Temperatures: Roof Ventilation vs Roof Color & Roof Shingle Life (and Shingle Warranty)
A less exciting and less convincing argument but one that may matter to some is the observation that a ventilated roof is cooler in hot climates, potentially giving both lower building cooling costs and a longer life for asphalt shingles than the identical shingles installed on the same slope at the same exposure in the same climate, but on a hot un-vented roof design. Dr. Joe argues that shingle life depends less on roof temperature than shingle color. He's correct in what I think he meant to say, but the statement needs clarification.
The chief effect of changing roof shingle color from, say black to white, is in fact a reduction in the surface temperature of the shingles when exposed to hot sun. It's not like wind and rain know what color the shingles are, nor that we use different asphalt binders for white mineral granules than we do for black ones. He may be right but I'm unclear on the argument. Most likely this is what was meant by the earlier statement.
I do agree that roof venting alone may not do much to reduce the temperature of shingles on the roof surface, as others have explained with some care.
Watch out: I and others also point out that many roof shingle manufacturers do not warrant their shingles if installed on an un-vented roof because higher shingle temperatures accelerate aging.
Roof Leak Risks Need to Be Addressed When Comparing Vented vs Un-Vented Roof Designs
There are two principal moisture pathways into a "packed" sloped roof
Watch out: when enthusiasts recount differences between the performance of a "packed" or "hot roof" design (presumably very high-R and "tight") and a ventilated roof design (more trouble, more cost, sometimes poorer insulation and higher energy cost) they sometimes may not have realized, or may fail to report that when a leak into the packed roof occurs, water stays there a long time - inviting rot, carpenter ants, mold. The result is a very large repair expense.
At the Journal of Light Construction building conference in Boston way back in 1985 we heard from the hot roof camp, we heard research reporting that most moisture leaking into (and thus potentially wetting) building cavities was through penetrations, and following those talks, we saw slides of horror stories by a roofer, Henri De Marne's, whose business was going to New England's hot-roofed cathedral-ceiling homes where a leak had gone unnoticed long enough for the repair to require more than just fixing a leak.
The repairs for the cases he documented required a complete roof tear-off, roof sheathing removal, wet insulation removal, moldy drywall removal, replacing rotted rafters, and then finally, reconstruction.
It would appear that
Ending Remarks About Hot vs Vented Roofs
Joe has much better education and credentials than I do (Joseph Lstiburek, Ph.D., P.Eng., ASHRAE Fellow), compared to About Us and Daniel Friedman's Resume, and he's an exciting speaker, and truth be told, he's better looking too. We agree on much of the theory but I maintain some practical objections elaborated below. First and more significant: TenWolde and Rose have written a seminal work summarizing the issues and their conclusions on the topic of attic and cathedral ceiling ventilation.
Consistent with those authors I propose that increased consideration should be given to the effects on the building, roof, heating and cooling costs, and maintenance costs associated with
Watch out for real world snafus, damage, leaks in roofs. In sum, I'm left unsure about the gap between new construction designs and a perfect world where roofs never leak and the roofs I and more importantly, repair and renovation roof contractors have found when we inspected, tore apart, and repaired leaky roofs of both insulated cathedral ceiling homes, and homes with vented roof cavities.
Our expert Steven Bliss commented to offer a final word on this topic:
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
No FAQs have been posted for this page. Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Check the FAQs just above, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
HTML Comment Box is loading comments...
Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.