Attic venting blocked by fiberglassEffects of Blocked Soffit Intake Venting
a Cause of Attic Condensation, Ice Dam Leaks & Attic Mold in buildings

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Blocked or inadequate roof ventilation air intake openings at soffits or eaves:

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. We also explain that attic or roof exit venting without adequate soffit intake venting increases building heating cost.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

This article series describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings. It describes proper roof ventilation placement, amounts, and other details.

Blocked Soffit Vents Invites Attic or Roof Cavity Moisture, Condensation, & Mold

Photograph of attic mold due to bad venting

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.

But after we confirmed in our lab that the mold was Penicillium sp., in our opinion the attic at left needed to be cleaned. The risk of problem mold bothering building occupants was increased when the attic was in a knee-wall area adjoining a bedroom.

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 Correcting Roof Ventilation for more details about correcting under-roof ventilation.

See the Mold Information Center for guides to inspecting, testing, and removing mold in buildings.

Why is Blocked Soffit Intake Venting a Factor in Attic Condensation Problems?

soffit blocked by mineral wool - note ice dam leak stains

Even if continuous soffit venting was installed (seen from outside) it may be blocked in the attic (shown here).

The photograph shows building eaves blocked by mineral wool insulation. 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. 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.

Here are some examples of blocked intake venting in an attic:

Why Does Blocked Soffit Intake Venting Increase Building Heating Cost?

It's simple. 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. Otherwise here is what happens:

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.

The rate of movement or "strength" of the up-flowing warm air current from the building occupied space increases as it enters the attic and finds a ready exit vent at the ridge or gable ends. (We prefer continuous ridge vent to assure even ventilation across the roof deck underside).

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. Essentially we are increasing the heat loss from the building.

Conversely, if there are open soffit vents to allow free flow of air into the attic (or cathedral ceiling roof space), the negative pressure or "vacuum" created by the exiting attic air is more easily satisfied by inflowing (cooler, more dry) outdoor air than it is by leaking air from the occupied space. That slows building heat loss during the heating season.

Why aren't gable end vents the best idea for attic venting?

Gable end vents alone do not uniformly cool and dry the whole roof underside.

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.

How do I Unblock Obstructed Roof Eaves by Installing Roof Vent or Soffit Baffles in the Attic

Attic roof vent baffle installed

Photo of roof vent baffles installed in an attic

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.

Photo of UFFI foam insulation in a building atticWe 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.

Eaves blocked by foam insulation: some older homes were super insulated during the energy crisis in the U.S. in the 1970's by pumping urea formaldehyde foam insulation (UFFI) into building cavities.

Newer open celled (Icynene®) closed cell foams are in use today.

We may spot this foam oozing into an attic even if it's not visible elsewhere in the building (though you can find it by strategic probing and inspecting at building cavities). If excess UFFI pumped into building walls has blocked attic insulation, it may need to be removed.

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.

Questions and Answers on Un-Blocking Soffit Vents or Eaves Vents on buildings

Question: Faux Soffit Vents - Can I Vent My Soffits by Working From Inside the Attic?

Soffit vent strip intake (C) Daniel FriedmanHello- I just finished reading your piece on attic ventilation and unfortunately believe that work done on my house was incorrect. I am a 71 year old female who hired a gutter man to put new gutters on my house. The soffits needed painting (there had never been any vents) and I gave him the job of replacing the soffits.

I now believe that he just covered the old soffit with vinyl soffits that had perforations every few feet without cutting any holes. This past winter, I had a problem with icicles forming along the edge of the new gutters.

I called him about the problem, but he claims that it was a bad winter.

He has agreed to return next week, as the gutters are leaking in a few places. I don't think that the attic is venting at all except for the gable vents.
My question is this : Can holes be cut from inside the attic and what would be the best way to attempt this? Would I be better off calling an insulation company to check out the attic?

Technically, there are 2 soffits - one the original plywood and the vinyl one on top. The first order of business is to get into the attic and see if he cut any openings. If not, it sounds as though the entire job needs to be removed.

If a continuous channel is cut, would I not be able to reuse the vinyl soffit? I'll attempt to send a photo that I took during the winter that shows icicles along the gutter and you can also note the type of vinyl soffit that was used. Again, thanks for your input. - P.

Reply: Usually access from in the attic is too limited. Check for venting, and add from outside if it's absent.

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.

Check for soffit vents from the attic (C) Daniel FriedmanBesides, drilling a few holes just won't provide adequate air inflow.

The work to provide air intake venting at a building's eaves or soffits is almost always performed from outside.

First, let's find out if the gutter guy did what we fear: installing perforated "soffit vent panels" over solid plywood soffit coverings - what we call "faux soffit vents" because you see the vent panels but they aren't doing a thing.

There are two easy ways to check this. 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. If not there were either no openings or they are blocked.

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. If they are over solid wood they'll feel solid - nothing moves.

If the soffit vent panels were installed only after the original solid soffit wood or plywood was removed, the vinyl panels will flex upwards easily for 1/2 inch or even more.

Soffit vent strip intake (C) Daniel Friedman

If the wood soffit covering was "vented" just by cutting a few intermittent holes before the vinyl was installed, chances are it's going to be inadequate. We need roughly twice as much air intake at the house eaves as we have air outlet along the ridge.

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.

See our photo (left) of continuous soffit vent strips.

[Click to enlarge any image]

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.

See our photo below.

Be sure you've also got a working exit vent along the ridge, and check that the vent openings into the attic space are not blocked by insulation

Soffit vent strip intake (C) Daniel Friedman

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. A contractor told us because this 26 feet of soffit was now in the garage, we would have trouble. What do we do?

Roof extension blocks original soffit intake vents (C) InspectAPedia EZ

Contractors contradict each other and one says do nothing. Here are some pictures. On the first you can see how we extended the roof line. Follow the roof line right to left and see the white soffit end and wood addition begin.
The Amish built us a log garage attachment (making a straight roof line because of the ground slope) to former garage we turned into a living room. Given property considerations, this was the only way we could add space.

[Click to enlarge any image]

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. Egg cartons between all rafters.

  1. One person said that 26 feet inside the garage was not enough to make a difference,
  2. Another said we have to have a fan,
  3. 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.

Any advise you could give is greatly appreciated. We live in Wisconsin. - Eric Zingler 12/24/2013

Roof extension blocks soffit (C) InspectApedia EZ Roof extension blocks soffit (C) InspectApedia EZ

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. 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.


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. 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.

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. So the fire code issue does not pertain. What does remain is the under-roof ventilation question you raised and that it took me some time to understand.

On your photo above I marked in blue the original roof venting system in theory - we don't know if it is actually working. 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.

My comments begin with remarks on the three options you were given

  1. One person said that 26 feet inside the garage was not enough to make a difference

    I do not agree with this view. The person who made this remark probably thinks that air entering from other soffit intake vent sections provides enough under-roof ventilation.

    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. In effect the garage extension has converted one 26-foot length of one roof slope to an un-vented roof.
  2. Another said we have to have a fan,

    This option is difficult to understand - a fan blowing where? moving air from where to where? In any event, even assuming we put a fan blowing into the attic that approach has several downsides including

    - uneven under-roof ventilation leaving problem areas of condensation

    - the requirement to have an electrically-driven motor running 24/7 all year long, an energy waster that looks even worse since it is not fully effective
  3. 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.

    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.

From the photos and your notes I see the following options

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. That would have left an adequate airflow opening along the original roof eaves. 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.

Watch out: I'm not clear on how the new roof was attached to the original one. If it was just tacked onto the fascia board of the original eaves I'm not sure this is a structurally adequate connection.

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.

If your original roof is not adequately vented you are faced with a decision to convert the building to a "hot roof" design.

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. BUT if you plan to heat the garage I'd review again the thoughts I outlined above.

Reader follow-up: I welcome all opinions and will check [here from time to time to see if anyone else has a different suggestion].  It looks like I have two options:  open window or have the two inch channel from old roof to end of new.  Bottomline, it looks like cracking a window can handle the matter until I can afford the latter. 

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.

The reason I prefer continuous soffit intake venting is that rafter pairs form a bay that conducts airflow along the roof underside. If you are not insulating the garage ceiling then it's not an issue. But if you are you want an air path between every rafter pair and an air intake for each of those bays.

Question: replacing aluminum soffit vents with vinyl reduces the actual vent opening area - seeing attic mold

7/8/14 Richard said:

Three years ago, I had my 34x24 hip roofed house have its roof and soffit vents replaced. The original ridge was continuous aluminum vent and so was the original soffit vents. The new soffit vents are vinyl and continuous but are only 3.5 to 4 inches wide.

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. The contractor assured me it would be enough [ventilation in the attic] but now I've noticed mold in the attic. And some limited amount of water had puddled on some boxes under the ridge.

So then I started looking at calculations.

  1. I estimated that the 1" aluminum continuous soffit vent originally on the house was providing 545 sq-in of net free air(NFA).
  2. I estimated the original 1" aluminum at the ridge was providing about 150 sq-in NFA.
  3. The new ridge vent is providing about the same as the old vent,
  4. But I think the vinyl soffit vents are now only providing 232 sq-in NFA.

I live in NH- Do I need adjustments of both ridge and soffit?

I have a concern that the water on the boxes was an indication that the soffit venting was insufficient.
The original seemed to be close to the 1/150 rule. But the new design does not even meet the 1/300 rule because the ridge is only about 3/4 that needed for a 34x24 attic.

Are the any rules of thumb that contractors should be using when replacing aluminum vents with vinyl ones?



Thanks for the interesting and important analysis. Indeed some contractors are not into reading instructions, building journals etc. We joke that the instructions are just used to kneel upon while doing the actual work.

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. Indeed the smaller perforations in some soffit vent coverings can have the effect you describe.

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. I'd consider that improvement if we also need more hip roof exit venting for a hip roof design.


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