POST a QUESTION or READ FAQs about attic moisture, condensation & ventilation: what are the effects of blocked soffit intake venting at roofs? This article explains the effects on buildings caused by locked soffit intake vents and we explain how blocked soffit venting causes or contributes to attic condensation, moisture, and potential mold contamination problems in buildings. The photograph at page top shows an attic whose intake venting is blocked by fiberglass insulation. The photograph at left shows severe mold on the underside of roof sheathing in a 1920's cape cod attic where there was no under roof ventilation.
The page top photo suggests (by the absence of visible mold) that luckily we may not find a mold problem in every poorly-vented attic or under-roof space. When removing problem mold from an attic we must also correct the moisture problem by both removing the moisture source and by correcting any attic venting defects. See the Mold Information Center for guides to inspecting, testing, and removing mold in buildings. Even if continuous soffit venting was installed (seen from outside) it may be blocked in the attic (shown here). In this case the builder and insulator were fortunate because even though roof venting was blocked, there was not an attic moisture problem. And we were pleased to note the absence of significant ice dam leak stains on the roof sheathing or rafters.
Insulation stuffed into building eaves: Some homeowners and even some insulation installers love to stuff insulation into the soffit to reduce drafts, in order to try to warm the attic and thus slow heat loss from the building. Perforated soffit vents over solid wood: many older homes have had exterior siding and soffit coverings installed, often of aluminum or vinyl. If they contact a solid surface they were installed over solid plywood or wood boards at the building eaves.
Deliberate Blocking at Eaves: in some older buildings, in an attempt to "warm up" a cold attic someone may have installed wood blocking between the rafters at the building eaves.
Heat and warm air flows into and is lost from the building roof cavity or attic - warm air rising creates upwards convection currents in the building. As air flows readily out of the exit venting high on the roof (ridge vent or gable end vents) it creates a negative pressure with respect to the air pressure in the building occupied space. But if there is not adequate intake venting of outside air, that same negative pressure tends to draw still more conditioned air (or heated air) out of the building space.
Gable end vents combined with a ridge vent tend to become intake vents feeding air flow currents created by air exiting at the ridge, thus failing to draw air up along the roof underside, failing to cool and dry that area, even if soffit intake venting is present.
If your building has adequate intake venting at the soffits or eaves, and good outlet venting at the ridge, you may still find problems with attic condensation, attic mold, or roof ice dams (in freezing climates) if the attic insulation blocks the venting system.
An inexpensive solution is the addition of styrofoam roof vent baffles which are placed between every rafter pair.
The attic roof vent baffles baffles hold the insulation away from the roof deck to permit air to enter the attic or under-roof space. We use this same design under roofs that cover cathedral ceilings, but on occasion find that more air space and air flow up under the roof sheathing are needed in those structures.
See UREA FORMALDEHYDE FOAM INSULATION, UFFI for a detailed discussion about formaldehyde and indoor air quality issues concerning this UFFI urea formaldehyde foam insulation. The best place for locating or placing attic insulation, from the view of avoiding attic condensation and ice dams, is in the attic floor or up the sides of attic knee walls. This leaves a cold, drafty attic, but it means longer shingle life and no attic condensation problems. Avoid placing insulation between the rafters unless special venting measures are also taken. You can't cut holes into the soffit from inside most attics - it's just too difficult to crawl down into the roof edge and almost impossible to reach into the soffit with a saw. The work to provide air intake venting at a building's eaves or soffits is almost always performed from outside. Our photo of our friend Jess Aronstein's attic (Poughkeepsie NY) shows light at the house eaves - so we expect that there was pretty good intake venting at that location. From outside, if the soffit undersides are safely and readily accessible (don't work from a ladder if you're not fit and experienced, don't work alone, don't put a ladder on an icy surface, etc) then often we can push upwards on those vinyl soffit panels. The best procedure would be to remove the vinyl panels that your gutter guy installed, cut in a continuous soffit vent strip along the entire soffit - continuous, not intermittent. We like the openings and vents to be just behind the fascia board - it's easier to install (one saw cut instead of two), it lets us leave the rest of the soffit plywood covering in place, and by its location we reduce the chances of wind-wash (movement of loose fill attic insulation away from the wall tops in the attic) and we reduce the chances of wind-blown rain entering the attic space. But if the soffit plywood was in poor shape, stained, rotted, needs painting, a better alternative is to simply remove the soffit wood entirely and replace all of it with perforated vinyl panels.
Question: do I need to replace soffit intake venting that has been blocked by a building addition? We extended out roof line out twenty four feet and built a garage covering 26 feet of soffit. Next photos show the inside of the garage which shows the rafters meeting the logs on one end, and that 26 feet of soffit is now "in the garage". The house is only 2 years old and main portion is 1100 sq feet without old garage converted.
A third said we had to drill holes in the end of the soffit and have some sort of channel down the new garage rafters with holes to outside -- all below the insulation.


At above left we see the new roof extending off of the edge of the3 original roof eaves, where the perforated white soffit covering remains in place on the original roof overhang.
In general, if a garage addition onto an older home caused a portion of the original roof to become "indoors" by building a new, higher or over-roof over the garage that itself would be vented as well. But the case you describe is not a roof-over but a roof-extension - extending an existing roof line out from an existing eave line. On your photo above I marked in blue the original roof venting system in theory - we don't know if it is actually working.
This option is almost right, except that we don't like to put under-roof on the warm side of the building envelope - doing so subverts the insulation and increases building heating or cooling costs, and it also fails to achieve the original purpose of under-roof ventilation - that of preventing condensation and moisture on the under-side of the roof sheathing or in building insulation in an un-vented roof cavity, or of keeping the roof cold enough to prevent ice dam formation. If you are not insulating the garage interior, and if the garage is to remain an un-heated space, depending on the air-tightness of the garage there will be some air inflow into the roof cavity of the original roof over the heated space, entering from the garage. If on the other hand you plan to insulate the garage and make it a heated space, then it would make good sense to provide a free flowing air path from roof eaves all the way to roof ridge under that roof slope.
Provide at least 2" of free air space from the new lower roof eaves all the way to the original roof ridge.
If I had built this addition I'd have removed the original roof fascia and sistered the upper ends of the new addition garage rafters to the last 24" of the original roof rafters.
Watch out: if the original under-roof venting design did not leave adequate airflow space between the top of the ceiling or under-roof insulation in the original house, or if the ridge vent does not provide adequate outflow opening size along the entire ridge, then this entire question is moot because the original house under-roof ventilation system is not working anyway. In sum, I would not do a thing before first investigating the original roof venting design to be sure that there is one, and I might, if the garage is to be un-heated, wait to insulate its roof and wait to see the effectiveness of air intake into the old roof eave soffits entering from the garage space.
Reply: Cracking a window makes sense for a temporary measure - assuring that there is plenty of air available to feed the natural in-draft at the soffits that is caused by warmer air exiting at the ridge. When the contractor originally replaced the soffit vents I expressed concern that just looking at the vinyl soffit vents would provide much less air into the attic. I estimated that the 1" aluminum continuous soffit vent originally on the house was providing 545 sq-in of net free air(NFA). I have a concern that the water on the boxes was an indication that the soffit venting was insufficient. Are the any rules of thumb that contractors should be using when replacing aluminum vents with vinyl ones? Rule of thumb for venting a standard gable roof, soffit to ridge ratio: Basically we want the soffit intake to be 2x the outlet vent opening. Some hip roofs add a spot vent on the smaller triangle hip roof ends at the triangular apex near the end of the ridge to add exit opening. Continue reading at SOFFIT INTAKE VENT INSPECTION or select a topic from the More Reading links or topic ARTICLE INDEX shown below. My roofer wants to cut into decking a small area here and there along gutter line because my soffits are covered in attic with insulation.
The best solution is to bring insulation out over the wall top plate and at the same time keep it off of the underside of the roof deck and give an air path by inserting baffles (carboard or styrofoam) sold for that purpose, between every rafter pair - work done from in the attic. Then either go to a perforated soffit cover or cut a vent opening continuous along the soffit to accept a perforated vent strip. We do agree with roof deck removal as an intake port along the entire lower roof edge just for the case of roofs built with no roof overhang at all. Watch out: even adding a starter vent and cutting off roof decking for a couple of inches won't work if the space inside is filled with insulation. Opening additional soffit vents that are blocked should not at all reduce the benefit of the ones that are already open.
For soffits that we can't reach from within an attic another approach is to work from a ladder or scaffolding from outside, removing the soffit covering, making necessary clearance openings, then replacing the covering. Do you think I should have a contractor take down the panels covering the soffits, cut a continuous vent, pull the insulation back and have vents put in?
Adding insulation won't make matters worse but will make installing baffles to permiypt venting much more trouble later. Winnipeg, MB, Canada: MY house has drastically changed from low humidity to high humidity after fiberglass loose attic insulation was added from R20 to R50 on Oct 22,2014.
Adding fiberghlass loose fill itself should not change humidity in the space below, though you might have slowed air leaks up into the attic and thus changed the house air movement pattern. Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. We also explain that attic or roof exit venting without adequate soffit intake venting increases building heating cost. But after we confirmed in our lab that the mold was Penicillium sp., in our opinion the attic at left needed to be cleaned. Still, opening the soffits for venting can result in a cooler roof surface and longer shingle life, even if there are no moisture problems in the attic. If we make the mistake of providing exit venting from a roof cavity or attic, such as a nice open ridge vent or gable end vents, we also need about twice as much (by square inches) of intake venting at the building eaves. If you can get into the attic when it's dark (or bring a flashlight but then turn off the attic lights), see if you see light when looking into the eaves - if so there were openings cut.
We need roughly twice as much air intake at the house eaves as we have air outlet along the ridge.


At above right is the new roof eaves end seen from the garage interior - showing that any intake ventilation has been blocked at the wall top.
And if a contractor built a second roof atop an original roof, leaving the original shingles in place he may have violated local building and fire codes. On that photo I marked in red the rear roof slope vent path that is now blocked by the new roof extension, and I marked in dashed green a possible new air path that may be needed to reactivate the red section. The person who made this remark probably thinks that air entering from other soffit intake vent sections provides enough under-roof ventilation. This will require opening the space between the original roof eave rafter ends and the rafters in the garage roof extension.
If it was just tacked onto the fascia board of the original eaves I'm not sure this is a structurally adequate connection.
The reason I prefer continuous soffit intake venting is that rafter pairs form a bay that conducts airflow along the roof underside. The contractor assured me it would be enough [ventilation in the attic] but now I've noticed mold in the attic.
Indeed the smaller perforations in some soffit vent coverings can have the effect you describe. I'd consider that improvement if we also need more hip roof exit venting for a hip roof design.
IN this case a Hicks Starter vent or equivalent is added to support the shingles while providing a (modest) intake vent opening.
So you're back to inserting baffles, which means there was no reason to cut roof deck in the first place. The soffit looks to be unvented as I see no light coming through ( after moving back the insulation) and there are no ventilation panels covering the soffits, only solid panels. Or do you think I'm ok to just have additional insulation put in the attic ( which is what started me down this path in the first place). I'd install baffles, soffit ending, and then close of the gable end vents to stop that air short circuit. The text is intended as a reference guide to help building owners operate and maintain their home effectively. The risk of problem mold bothering building occupants was increased when the attic was in a knee-wall area adjoining a bedroom. What does remain is the under-roof ventilation question you raised and that it took me some time to understand. This is not the case because soffit intake venting at other roof areas will not effectively cross horizontally to keep the now blocked roof section cool and dry. I would have built a similar roof overhang (looks like you have 24" or more) at the new garage roof lower end or eaves and vented it. We have air seal the attic added an R60, which is great but the company did not install or install baffles correctly. Before there was no insulation at the edges of the attic (over the soffits), now they all covered up with fiberglass insulation. In effect the garage extension has converted one 26-foot length of one roof slope to an un-vented roof. But if you are you want an air path between every rafter pair and an air intake for each of those bays. I do have triangular openings at each end of the house at the top of the peak, a ridge vent and a solar fan.
Would that enough to change the environment of the house so much from low humidity to high humidity? Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Other than that, he said smart vents are dumb vents except in the south bc they can cause ice damming in winter in gutters and back up that ice into shingled areas. No indication of any mildew or other problems other than one ice dam years ago after a blizzard. A In 1993, the Council became part of the School of Architecture, and since then has been known as the School of Architecture-Building Research Council. 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. The Council's researchers answered many critical questions that would affect the quality of the nation's housing stock. 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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