Question: how can I add venting if the roof and interior cathedral ceiling are already finished?
How do I properly vent my roof if the attic has been renovated into a living space and there is no access to the roof sheathing which is covered by finished ceilings, etc. Thermal imaging [image at above-left] indicates that there is a lot of heat loss from the ceiling. A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone. If we knew the actual date we could look up the outdoor temperature for that time and date in your city. Even then, the image does not show the house roof - which is the part of the structure about which you inquire.
The thermal image does raise an interesting question: the upper half of the upper floor windows look notably cooler than all of the other windows on the one side of the home we can see.
I also note a few very bright spots both inside and outside on this image that might be light fixtures. In sum, if all you got for your money was the one page you sent to us - a thermal image of one side of a house, I hope you didn't pay much for this report which to me looks like little more than a pretty picture tossed over the wall with no helpful information. Let's start by seeing how we would ventilate a cathedral ceiling if we could - say during construction .
If both the roof exterior and the building ceiling interior are "finished" clearly the options are more limited.
Shed dormers are often built as a "cathedral ceiling" though with just one sloping surface; and often because they were a retrofit or add-on project the builder packed the roof with insulation and didn't allow for venting. Shed dormer roofs, being a lower slope than the main roof, may also be more inclined to leak, have faster roof shingle wear, and if the slope is quite modest, venting may be infeasible; in that case I'd focus on using a very reliable roof material that works well on low slopes, perhaps EPDM (EPDM ROOF), and if necessary I'd add insulation (solid foam then covered by drywall) on the ceiling interior to get the R-value up where needed.
IF an investigation from the roof surface (a test cut at the ridge say where a ridge vent would be installed) and from the house eaves (soffit covering is removed to give view into the roof structure) show that the ceiling-to-roof space was packed with insulation - with no air space between the insulation top and the roof under-side, you cannot add ventilation without reconstruction of the roof or ceiling.
One experienced engineer said that I shouldn't have a problem with rot or mold as stands because the fiberglass is air permeable and so the main problem is losing heat through the attic. And then she said I should consider installing a "diffusion" type material (with properties similar to Tyvek homewrap) underneath the ridge vent so that water vapor would diffuse out (minimizing problems from rotting), no moisture would come in, but it would serve to "airseal" the roof better so I don't lose so much heat. I will take a look at the TI report but just shooting from the hip and thus admitting I don't have all the requisite data, the description in your email does not sound right to me. A "diffusion material" like Tyvek is specifically intended to STOP air movement while permitting moisture to move through the barrier. We give up and go to a hot roof design - which is most reasonable if the roof covering outdoors is metal or perhaps another very very reliable material.
Next, I'm a little confused anyhow - if we have a hot roof design, the theory, per Joe Listburek is that there should be NO air movement through the roof structure. If you go that way, and your design leaves a STILL AIR SPACE inside the roof structure, you'd probably get some added R-value with that design. Joe is a better speaker than I am, but frankly I don't buy all of his views on the wonder of hot roofs. The owners don't notice trouble until damage is severe because even though rot or carpenter ants or mold or all 3 are chewing away at the roof structure and sheathing, water and stains are slow to fully penetrate the structure to show up in the finished space. I recently built a hot roof design (I don't disagree with Joe completely) over my lab - but we installed a standing seam coated steel metal roof that won't be punctured easily even by a falling tree limb. In sum, if I had an inaccessible cathedral ceiling roof design on an older home, and if there were no provision for effective ventilation of the cavity under the roof sheathing, soffits to ridge, for the whole roof, I would figure I had an un-vented roof and I'd think about adding solid foam insulation inside, increasing the R-value of the structure, eliminating penetrations through the roof deck (pot lights for example), and until I was ready to re-roof with a metal roof I would inspect the roof directly, annually, for signs of leaks or damage. Reader Question: how can I reduce the heat coming through a metal-shingled roof - cathedral ceiling structure in Athens?
I am constructing the roof of my house in Athens-Greece- using metallic tiles (DECRA), which, after the end of the work, should look similar to Figure 1 at left.
Now, I want I„I? solve the problem of high temperatures induced by the sun during the summer, despite the fact that there are ventilation tiles fitted (Figure 2 shown next below). Note that the max temperature of the surface reaches 77 C, the bitumine membrane 55 C and the wood rafter 50 C. I would also like to ask you if you think the air vents and openings at eaves and the ridges will be capable of reducing the temperature considerably so that the above components are not subject to heavy thermal stress. Are you sure that there is unobstructed air path from the lower roof edges to the ridge vent and that the ridge vent itself is open and venting out?
Reader follow-up: Yes, an obstructed air path will be constructed exactly as shown in the plan, by placing a ridge vent directing the hot air towards the acme of the roof. What you want is an UN-obstructed air path of aderquatge dimensions - from eaves to ridge, to permit good air flow under the roof: that is, an air path that is not blocked by anything. Other factors that prevent the air venting system under the roof, or between the roof under-surface and the upper surface of the cathedral ceiling insulation is if the air space is too small to get enough air movement from soffits up to and out at the ridge.
Watch out: keep in mind that you will have to reduce the effective cross section of intake or exit vent area by the effect of screening or perforated covers.
We want good intake vent at the house eaves or soffits, unobstructed air flow - at least 2" distance and full width of the space between rafters - between roof underside and top of insulation (3" is better), and un-blocked air exit through the ridge vent. Typically the total amount of soffit intake vent opening is 2 x the exit vent at the ridge, and both soffit and ridge vents run the full length of the building roof. For example if a builder installs a common nail-down, shingle-covered "mesh" type ridge vent, that system moves much less air and will not vent as well.


While it requires some further thought, I'd also look into using a reflective barrier in the roof structure to keep un-wanted heat out of the interior. Illustration adapted from BEST CONSTRUCTION PRACTICES GUIDE, used with permission - Steven Bliss & J Wiley Publishing. Does use of a reflective barrier increase roof surface temperatures and thus shorten roof life? Can placement of the reflective barrier cause a moisture trap problem in the roof structure? On flat roofs some builders install turbine vents hoping that the actively rotating vent will pull venting air through the roof structure to improve under-roof ventilation.
In the construction of our lab and offices we chose a metal roof covering for a fairly low-slope roof; with the reliability of a metal roof we decided to insulate the roof cavity with spray foam - buildingA  a "hot roof" design with no ventilation. While generally I am not a fan of hot roof designs, with a very reliable roof covering and where low slope roofs are hard to vent, my view is that they are an reasonable alternative worth considering.
Reader follow-up: I would like to ask how tall to be the ridge, so that it improove ventin,.
Reader: The final construction are shown in attachement, always the reflective barrier abone the insulation- that is between the rafters- will incur farther by reflection the bitumine membrane. I ask because the sketch does not show how the additional fiberglass insulation is supported in place, and because if solid foam insulation is available, you could obtain higher R-value in less total thickness, and also because using solid foam insulation would make construction much easier and less costly.
I built a roof much like this one, insulating first between the rafters (leaving an air space), but I finished the interior by installing a continuous layer of 2-inch High-R solid foam insulating sheets across the rafter faces on the interior side of the structure.
Do you mean to increace the insulation gap by elimination of polysterine plates and install under the rafters?
Here, there are a few kind of insulations, and however i have already bought these components.
Do you estimate that if the gap risen I„I? 5,51 inch is enough to reduse the thermal amount, you know i thought again your thoughts and could install the polysterine plates under the rafters.
The rise of ridge vent at 8,26 inch from the top of the roof could destroy the Bernouilli effect around the ridge? Reply: Generally you want twice as much exit venting at the ridge as intake venting at the soffits or eaves. Yes whatever we have for total input vent area opening at the ridge we want AT LEAST two times that amount at the eaves (that's the sum of front and rear or for a single shed dormer roof it's all at the single lower edge).
Could i to chanel the counterbatten space into the ridge opening, then the existing venting tiles (pict 2)-wich lay before ridge- will create short-sircit of flow? Today searching the internet i found the essey about : Natural Convection Heat Transfer in Roofs with Above-Seathing Ventilation.
Openings may be obstructed by plastic mesh, vent design, or even failure to cut an adequate gap in the roof sheathing on either side of the ridge board. Prevailing winds vary enormously seasonally and by geography; prevailing wind direction effects will completely swamp natural convection effects up through a roof vent opening system, and will cause venting effects to be completely different on different roof slopes. Ridge vent gap height has no significant effect on improving total ventilation effects under a roof surface until the height is tall enough (typically 24" or more) to create a strong-enough natural updraft effect to "draw" air up through the roof vent system. I describe this problem at ROOF VENTING for DEEP SNOW where you will see a very tall cupola type vent that was constructed to create a venting effect when a normal ridge vent would be snow covered. Since few building owners like to put one or more three or four foot cupolas along the ridge of their home, I have not found many takers for this design.
Instead we rely on providing plenty of space for more typical passive under-roof ventilation systems at the building eaves and ridge. The metal shingle roof in your photographs appears to have a scattering of individual spot vents at intervals along the roof.
But even a continuous ridge vent will not vent a cathedral ceiling design roof structure if there is not sufficient air space between the top of the insulation and the under-side of the roof sheathing.
And even that design will not vent a cathedral ceiling roof structure if there is not adequate air intake at the roof eaves. Finally, because we do not want an un-balanced roof ventilation system to increase building energy costs by drawing conditioned air out of the occupied space, and because restrictions at intake or within the passage will slow air flow inside the roof cavity, most writers want to see at least twice as much intake venting at the building eaves as at the ridge.
Tests at FSEC also indicate that simply switching from dark to white asphalt shingles in a cooling climate can reduce peak cooling loads by 17% and seasonal loads by 4%. Continue reading at HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS or select a topic from the More Reading links or topic ARTICLE INDEX shown below.
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The attic had a shed dormer installed and was renovated and there is very little access to install vent baffles now. If we then understood if the heat were even on in the home we could make more sense of the image. If you knew, for example what was covering those upper window halves you could form a wild guess about the relative differences in heat loss rates between windows, covered windows, and the building walls.
If you knew, for example which home had what sort of insulation in its walls that might be instructive.


Thermography is a wonderful and very useful tool, including its use looking for areas of building heat loss. But before tearing the home apart or adding a layer of inside insulation, giving up ceiling height, take another look at your thermal scan of the building.
Thus, she suggested I do a compromise and head in the direction of a "hot roof": to seal the soffits. Any other suggestions that doesn't require tearing up the finished attic ceiling or the roof? If our object were to provide for under-roof ventilation, that by definition means there is air movement under the roof and out at a ridge vent. The idea put forth about the Tyvek wrap is a HOT ROOF effort which involves sealing the soffit vents (not a VENTED roof). I have seen too many homes that were severely rotted and required very costly repairs when there were undiscovered leaks into a closed-off, inaccessible roof cavity.
In case the openings are not enough, is it necessary to create additional convection by natural or artificial way?
I would know if these spaces are enough to reduse the hight temperatures considering the factos slope and length.
The larger you can make the opening space between insulation top and roof underside and the larger you can make the entry and exit vent openings the better the roof will ventilate.
An advantage is that you might be able to get a higher R-value with a foam-insulated roof.
I didn't go into that detail because the height you would have to add is so much (typically about a meter) that it would be so ugly that almost no one takes that approach. That gave an very high-R-value roof overall with the added advantage that the inside insulation was continuous - there were no thermal break points at the rafters.
Nevertheless, Is there a software that calculate the drop temperature along the venting ducts so that t determine the exact gap? Could i to chanel the counterbatten space into the ridge opening, then the existing venting tiles (pict 2)-wich lay before ridge- will create short-sircit of flow?This is the hotest place of the roof.
Or i should bloking this comunicate so that the counterbatten space to ventilate throught ventilation tiles. In figure 4 described temperatures just my roof but couldn't found height of gap, maybe because of i am not engineer. While the research is important, in my opinion it is nonsense to think that you can reliably translate theory to an individual building without extreme attention to a great many details about the individual building.
And on the opposite side (we call it the lee side) of a building against which wind is blowing, ventilation effects may be completely the opposite: a vacuum may be created on the down-wind or lee side of the structure. In my OPINION the older and less attractive aluminum ridge vent that stood up about 3-4 inches above the ridge surface and that had wide open louvers on either side, combined with a ridge vent opening of about 1.5" or more on either side of the ridge board below the vent, provided far more effective exit venting for a building roof than the more attractive shingled-over mesh type ridge vents currently in wide use in many countries. There is in my experience no way that these can possibly adequately vent the entire roof structure. The text is intended as a reference guide to help building owners operate and maintain their home effectively. No one thinking clearly and wanting such a vent system to work would then stop air movement by installing Tyvek to close off the ridge. I would not place a reflective barrier on top of the insulation; rather I'd prefer it right under the roof sheathing - something that is easy to do only at the time of original construction.
To cover the foam insulation with drywall we simply held each drywall sheet in place and used very long drywall screws that penetrated through the 2-inch foam insulation into the faces of the rafters above.
We are not going to actually see a doubling of air flow rates through the roof, especially on a low slope shed roof like yours. A cathedral ceiling, by affixing a covering to the under-side of the roof rafters, will not have any cross-movement of air from one rafter-to-rafter space across to the next one.
I would take another look at the suggestion to coat the roof with a white or very light color to reduce the heating load. If the building walls or ceiling are leaky at penetrations for lighting or wiring or plumbing, for example, finding and fixing those leaks is very worthwhile. You could, however use rubber - EPDM which, IF properly installed, I've found quite reliable. So you would need a spot vent between every rafter pair on both sides of the ridge - a silly and unattractive design. Some of our references find that this improvement can be significant in reducing building cooling costs. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home.
Carson Dunlop Associates' provides extensive home inspection education and report writing material. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order.



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