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Effects of Blocked Soffit Intake Venting - a Cause of Attic Condensation, Ice Dam Leaks & Attic Mold in Buildings InteriorAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• 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 detect roof venting deficiencies, attic insulation defects, and attic condensation problems

This is chapter 5, "Blocked Soffit Intake Venting as a Factor in Attic Condensation Problems", part of our discussion of "Attic Condensation". This article 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. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

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 first photo suggests (by the absence of visible mold) that luckily we may not find a mold problem in that space. But after we confirmed in our lab that the mold was Penicillium sp., the attic at left needed to be cleaned.

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?

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.)

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

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.

Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.

• Alan Carson Carson Dunlop Associates, Toronto, Ontario. Mr. Carson is a home inspection professional, educator, researcher, writer, and a principal of Carson Dunlop Associates, a Toronto home inspection and education firm. Mr. Carson is a past president of ASHI, the American Society of Home Inspectors Some great illustrations of the proper under-roof ventilation pathways are offered by Carson Dunlop.Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. (727) 595-4211 mark@BestTampaInspector.com 11/06Daniel Friedman - principal author Daniel Friedman, editing, expanding, adding to comments from John Annunziata, P.E. - NY Metro ASHI informal chapter discussions.
• Technical reviewers are invited to comment or ask questions - contact us

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website. If you just "scroll down" you'll miss some important articles. See links at page left.

MOISTURE, MOLD, ICE DAM LEAKS in ATTICS & ROOFS
Attic Condensation
Inspect Building Exterior
Comparing Two Houses
Inspect Basements for Moisture or Mold
Inspect Attics for Moisture or Mold
Inspect the Ridge Vent System from the Attic
Inspect the Soffit Vent System from the Attic
Blocked Soffit Intake Vents
Correcting Roof Ventilation
Ridge and Eaves Venting Both Needed
Continuous Soffit Intake Venting Needed
Proper Roof Vent Location
Roof Vent Area Ratios
Un-Vented Roof Solutions
Air Bypass Leaks, Thermal Tracking

More Information on Effects of Building Moisture & Building Diagnostic Inspections and Repairs

More Reading about Dealing with Attic Mold, Identifying, Removing, and Preventing Mold in Attics

Be sure to review HOW TO FIND MOLD: How to Inspect Homes and Other Buildings for Mold - the Basics of How to Find Problem Mold Indoors in our Mold Action Guide. Here are other articles that will be helpful in evaluating attic mold presence, causes, and cures:

• What Mold and Allergens Look Like: mold identification photos to help identify mold - choosing what to sample in buildings
• How to Clean Moldy Wood Framing & Sheathing How to clean/seal mold from/on exposed lumber or plywood subfloor or roof sheathing indoors - some suggestions based on our field and laboratory research
• Lighting, proper use of: proper aiming of a good flashlight can disclose hard to see but toxic light or white mold colonies on walls.
• Mold spores in the Home - a Photo ID Library for detection and identification of mold allergens.
• How to Find and Test For Mold in Buildings A "how to" photo and text primer on finding and choosing the right spots to test for mold in buildings
• Stuff that is not mold but is often mistaken for it - things you may not want to test. Also, not all "black mold" is toxic - here are examples of harmless black mold.
• Simple Adhesive Tape Sampling of Moldy Surfaces - how to send a mold sample to our lab
• Mold Sampling Methods in the Indoor Environment - In-depth article: detailed critique of popular mold testing methods - Is your mold test kit worth the bother?
• Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
• Mold-Resistant Building Practices, advice from an expert on how to prevent mold after a building flood and how to prevent mold growth in buildings by selection of building materials and by anti-mold construction details.

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Use this simple, economical mold test kit by following our instructions on how to collect and mail mold samples to our lab

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