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VENTILATION in BUILDINGS

AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK MINIMIZATION
AIR LEAK SEALING PROCEDURE
AIR POLLUTANTS, COMMON INDOOR
AIR SEALING STRATEGIES
AIR TEST FOR MOLD: ACCURACY
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ANIMAL ODORS IN buildings
ATTIC LEAKS, CONDENSATION & MOLD

BASEMENT CEILING VAPOR BARRIER
BASEMENT HEAT LOSS
BASEMENT LEAKS, INSPECT FOR
BATHROOM VENTILATION
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BRICK WALL DRAINAGE WEEP HOLES
BUCKLED FOUNDATIONS due to INSULATION?
BUILDING NOISE DIAGNOSIS & CURE

CATHEDRAL CEILING VENTILATION
CEILINGS, DROP or SUSPENDED PANEL
CHIMNEY INSPECTION DIAGNOSIS & REPAIR
COMBUSTION AIR for TIGHT buildings
COOLING LOAD REDUCTION by ROOF VENTS
CONDENSATION or SWEATING PIPES, TANKS
CONDENSATION on WINDOWS & SKYLIGHTS
CRAWL SPACES

DECK & PORCH CONSTRUCTION
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DRYER VENTING

FIBERGLASS INSULATION
FIREPLACES & HEARTHS
FLAT ROOF MOISTURE & CONDENSATION
FORMALDEHYDE HAZARDS
FRAMING DETAILS for BETTER INSULATION
FRAMING DETAILS for DOUBLE WALL HOUSES
FREEZE-PROOF A BUILDING
FROST HEAVES, FOUNDATION, SLAB

GREEN BUILDING CONSTRUCTION CODES GUIDES
GREENHOUSE DESIGN for SOLAR HEATING
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HEAT LOSS in BUILDINGS
HEAT LOSS DETECTION TOOLS
HEAT LOSS INDICATORS
HEAT LOSS PREVENTION PRIORITIES
HEAT LOSS R U & K VALUE CALCULATION
HOT ROOF DESIGNS: Un-Vented Roof Solutions
HOUSEWRAP AIR & VAPOR BARRIERS
HOUSE DOCTOR, how-to be
HUMIDITY LEVEL TARGET

ICE DAM PREVENTION
INDOOR AIR HAZARDS TABLE
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INDOOR AIR QUALITY IMPROVEMENT GUIDE
INSULATION CHOICES
Insulation Air & Heat Leaks
INSULATION INSPECTION & IMPROVEMENT
INSULATION R-Values & Properties

LOG HOME GUIDE

MOISTURE CONTROL in BUILDINGS
MOLD: A COMPLETE GUIDE TO MOLD

Nanomaterials Hazards
NOISE / SOUND DIAGNOSIS & CURE

ODORS & SMELLS DIAGNOSIS & CURE

PAINT FALURE, DIAGNOSIS, CURE, PREVENTION

RADIANT BARRIERS
RADIANT HEAT
ROOF VENTILATION SPECIFICATIONS

SHEATHING, FOIL FACED - VENTS
SOFFIT VENTILATION
SOUND CONTROL in buildings
STAIN DIAGNOSIS on BUILDING EXTERIORS
STAIN DIAGNOSIS on BUILDING INTERIORS
STUCCO WAll FAILURES DUE TO WEATHER
STUCCO WALL METHODS & INSTALLATION
STUCCO OVER FOAM INSULATION
STUCCO PAINT FAILURES
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
WINTERIZE A BUILDING
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY

ZONE DAMPERS
ZONE VALVES


More Information

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

  • Blocked Soffit Intake Venting as a Factor in Attic Condensation Problems and Attic Mold
    • What causes attic condensation, mold, or dark stains on roof plywood?
    • How to inspect for working attic venting at soffits
    • How to check for true soffit intake vents versus faux soffit venting
    • How to detect roof venting deficiencies, attic insulation defects, and attic condensation problems
  • Questions & Answers about attic moisture, condensation & ventilation: what are the effects of blocked soffit intake venting at roofs?
  • References

Click to Show or Hide Related Topics

  • ROOF VENTILATION SPECIFICATIONS - home
  • AIR & HEAT LEAKS
  • AIR CHANGE RATE ACH HEAT SAVINGS
  • AIR LEAK DETECTION TOOLS - home
  • ATTIC LEAKS, CONDENSATION & MOLD
  • ATTIC MOISTURE or MOLD
  • ATTIC VENTILATION
  • ATTIC CONDENSATION CAUSE & CURE
  • BASEMENT MOISTURE to ATTIC
  • BLOCKED SOFFIT INTAKE VENTS
  • BLOWER DOORS & AIR INFILTRATION
  • CATHEDRAL CEILING VENTILATION
  • COOLING LOAD REDUCTION by ROOF VENTS
  • FLAT ROOF MOISTURE & CONDENSATION
  • HEAT TAPES & CABLES on Roofs for Ice Dams
  • HEAT LOSS in BUILDINGS - home
  • HEAT RECOVERY VENTILATORS
  • HOT ROOF DESIGNS: Un-Vented Roof Solutions
  • HUMIDITY LEVEL TARGET
  • ICE DAM PREVENTION
  • ICE DAM CURE: Comparing Two Houses
  • INDOOR AIR QUALITY & HOUSE TIGHTNESS
  • RIDGE VENT, ATTIC INSPECTION
  • ROOF VENTILATION SPECIFICATIONS
  • Inspect Building Exterior - Roof Venting
  • ROOF VENTIILATION INTAKE-OUTLET RATIO
  • ROOF VENT LOCATIONS
  • ROOF VENT SOFFIT & RIDGE NEED
  • ROOF VENT if NO SOFFIT
  • ROOF VENT SOFFIT, CONTINUOUS
  • ROOF VENTING ENERGY SAVING DETAILS
  • ROOF VENTING NEEDED?
  • ROOF VENTING ENERGY SAVING DETAILS
  • ROOF VENTING NEEDED?
  • SKYLIGHT VENTILATION DETAILS
  • SOFFIT INTAKE BLOCKED
  • SOFFIT VENTILATION SPECS
  • UN-VENTED ROOF SOLUTIONS
  • VENTILATION in BUILDINGS - home
  • VENTILATION DESIGN PROBLEMS & SOLUTIONS
  • VENTILATION, WHOLE HOUSE STRATEGIES
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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.

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. Also see CATHEDRAL CEILING INSULATION.

Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.

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:

  • 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. In some instances they even block soffit vents.

  • Perforated soffit vents over solid wood: many older homes have had exterior siding and soffit coverings installed, often of aluminum or vinyl. But very often we find that perforated soffit vent panels were installed directly over the older solid wood soffit covering. So even though an outside inspection shows that soffit or eaves ventilation appears to have been installed, in fact this is not the case. You can detect this condition easily: from outside simply push up on the vinyl or aluminum perforated soffit panels. If they contact a solid surface they were installed over solid plywood or wood boards at the building eaves. From inside the attic, if there is no insulation blocking the eaves, and if you do not see daylight, there is no working intake venting at the eaves or soffits.

  • 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. In this case even if soffit vents are installed there will not be adequate air inflow. Remove this blocking when designing good attic venting. Be sure you're removing eaves blocking by noting that you can, after its removal, see into the soffit overhang at the building eaves. For safety, do not remove fire blocking found between wall studs which may show at an attic floor.)

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 installedPhoto 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.

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.


Photo of UFFI foam insulation in a building attic

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.

Besides, 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.

Check for soffit vents from the attic (C) Daniel Friedman

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.

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.


Soffit vent strip intake (C) Daniel FriedmanBut 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 at left.

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

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

  • Building Research Council, BRC, nee Small Homes Council, SHC, School of Architecture, University of Illinois at Urbana-Champaign, brc.arch.uiuc.edu. "The Small Homes Council (our original name) was organized in 1944 during the war at the request of the President of the University of Illinois to consider the role of the university in meeting the demand for housing in the United States. Soldiers would be coming home after the war and would be needing good low-cost housing. ...  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. ... The Council's researchers answered many critical questions that would affect the quality of the nation's housing stock.
    • How could homes be designed and built more efficiently?
    • What kinds of construction and production techniques worked well and which did not?
    • How did people use different kinds of spaces in their homes?
    • What roles did community planning, zoning, and interior design play in how neighborhoods worked
  • "Energy Savers: Whole-House Supply Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Supply_Vent.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11880?print
  • "Energy Savers: Whole-House Exhaust Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Exhaust.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11870
  • "Energy Savers: Ventilation [copy on file as /interiors/Energy_Savers_Ventilation.pdf ] - ", U.S. Department of Energy
  • "Energy Savers: Natural Ventilation [copy on file as /interiors/Energy_Savers_Natural_Ventilation.pdf ] - ", U.S. Department of Energy
  • "Energy Savers: Energy Recovery Ventilation Systems [copy on file as /interiors/Energy_Savers_Energy_Recovery_Venting.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11900
  • "Energy Savers: Detecting Air Leaks [copy on file as /interiors/Energy_Savers_Detect_Air_Leaks.pdf ] - ", U.S. Department of Energy
  • "Energy Savers: Air Sealing [copy on file as /interiors/Energy_Savers_Air_Sealing_1.pdf ] - ", U.S. Department of Energy
  • Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
  • "Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
  • ...

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  • Home Reference Book - Carson DunlopThe Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

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