Blocked Proper Vents Causing Ice Dams

By Todd Fratzel on Insulation, Roofing

With winter quickly approaching I thought it was time to discuss one of the problems that causes ice dams. Ice dams cause significant amounts of damage each year and most of them can be avoided if you understand the root cause. The following information should help you understand some of the causes of ice dams.

Proper Vents

proper vent

Proper vents are used to ensure proper ventilation from house ventilated eaves up to the ridge vent. Proper vents are typically made of styrofoam and come in many different shapes and sizes. The one shown in the picture is fairly representative of the ones sold today. The proper vent creates adequate air flow from the eaves up past the insulation material.

Blocked Proper Vents

Yesterday I went and looked at a house that’s having trouble with ice dams. The house is only a year old and the owner is quite frustrated at the poor performance. He asked that I come take a look at the house and see if I could identify any deficiencies that might be causing the ice dams.

Sure enough when I visited the house I was surprised to see a rather significant ice dam along the eave on the north side of the house. So I ended up going into the attic to inspect the insulation and ventilation. It only took me a minute to diagnose the problem that’s causing the ice dams.

The proper vent that was used during construction appears to be a very flimsy / cheap version that actually collapsed when the insulation was installed. Apparently the insulation crew stuffed the insulation so tight against the proper vent that the air flow was cut off. Basically this caused a very limited amount of air flow and thus the ice dams.

This is a perfect example of cheap materials and poor construction techniques creating a very serious problem. The house is just barely a year old and they have ice dams causing water damage. So if you’re having trouble with ice dams on a well insulated house it’s likely to be a lack of proper ventilation that’s causing the problem.

About the author

Todd Fratzel

I'm full time builder for a large construction company in New Hampshire. I run their design-build division that specializes in custom homes, commercial design-build projects and sub-divisions. I'm also a licensed civil and structural engineer with extensive experience in civil and structural design and home construction. My hope is that I can share my experience in the home construction, home improvement and home renovation profession with other builders and home owners. I'm also the Editor-in-Chief and Founder of Tool Box Buzz. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

All posts by Todd »

Not what you're looking for?

Search for more articles here. Enter keywords like, 'insulation' or 'kitchens' etc to find your topic.


  1. mikes says:

    I had a similar problem. I hired someone to add blown fiberglass insulation into my attic and they sprayed it all over, including into my proper vents! Big problem. Luckily I caught it in time and had it fixed before it became catastrophic.

  2. Robin says:

    Good post! Our house was so prone to ice damming that after only 10 years the roof was completely shot and the previous owner has leaks in every room. But it wasn’t because of the venting it was the lack of attic insulation. There was only 4-6″ of blown in insulation in the attic! We fixed that right away and added two layers of R-19 batt.

  3. ArmchairDIY says:

    Very good information in this article.

    About two weeks ago I was in an attic of a home that was re insulated last year buy another contractor with blown cellulose insulation.
    They did not install any proper vents thus blocking the air flow from the soffit.

    Inadequate ventilation can lead to ice dams as well as premature shingle failure. Something so inexpensive and easy can create thousands of dollars of damage if not done properly.

  4. FredW says:

    So how many of these rafter baffles should one have? I had some ice dams and knew what the problem was without looking. I went into the attic and did discover two things. There are baffles up there, or rather a dam of cardboard keeping the blown insulation out of the soffits.

    I’ve spent the past two nights putting in 24×36″ rafter baffles in preparation for more blown insulation. I had it fairly easy. The house is a straight run of about 60 feet. I installed baffles in every rafter bay except the 3 on each end, on each side. I have gable vents in the top and wanted to try to get more air draw from the middle of the house instead of just off the ends. That is my un-scientific reasoning.

    As it turns out there is only about 4″ of insulation on average up there. I can not wait to get more in there to see if there is a real difference.

    • Todd says:

      @ FredW – You really should have them in each stud bay. Also, end gable vents don’t really work well at all. Frankly, you need a ridge vent in order for the soffit vents to work properly. The added insulation will definitely make a big difference.

  5. Jonathan Tauer says:

    Disheartening to see this mis-information about ice dams posted here. Ice dams are not caused by inadequate ventilation they are caused by heat loss. Usually they are caused by warm, moist air escaping the heated envelope and ending up in the attic (convection). Sometimes they are caused by heat moving through the sheet rock and inadequate insulation levels at the eaves (conduction). Ventilation is very important to move moisture out of an attic but we always go after the source of moisture first. Air seal first and 9 times out of 10 you have solved the ice dam.
    Venting attics was crucial to saving roofs in drafty homes in New England. Now we are moving towards understanding that a tight envelope with adequate fresh air from mechanical ventilation is a better, greener approach.

    • Todd says:

      Jonathan – I have to disagree with you. Ventilation is crucial unless you want to “spray foam” your attic as your company (I removed your link as I find it to be spam) wants to sell. I will agree that ventilation is not needed if you can completely seal the space with foam. However, if you use other insulation methods the ventilation is 100% required as proven by the millions of homes with proper ventilation.

  6. Tim says:


    I have a question for you that the manufacturer can’t/don’t want to take the time to answer. I had a contractor add aluminum soffit to my house for appearance but more for added ventilation. Every 46″ or so they drilled (3)-1″ holes in the wooded soffit and installed a 12″x6.5″ vented section of aluminum soffit that has a free air space of 13 sq inches per 1 LF. So it appears that I won’t be getting the 13 sq in due to the 6.5″ not being 12″ and the 1″ circles instead of the entire vented area being. The million dollar question is what is each vented area rated at for airflow into the attic? The area of the attic above garage and family room is 34’x21′ so gut feeling is I don’t have enough airflow coming in (only one side of house has soffit). There are 8 sections of the vents also.



    • Todd says:

      Tim – The general rule of thumb (this can change depending on your local building code requirements, which you should check) is 1 sq. ft. of ventilation per 150 sq. ft. of attic space. Your attic is roughly 650 sq. ft. which means you should have at least 4.3 sq. ft. of net openings. A 1 inch diameter hole has approx. 0.02 sq. ft. of opening. If you take 4.3 sq. ft. required and divide by 0.02 sq. ft. supplied you would need 215 1-inch diameter holes to provide sufficient eave ventilation. You would also need the same sq. ft. in the ridge vent. make sense? Sounds like you only have 8 sections times 3 holes which is only 0.50 sq. ft. of ventilation.

      If you installed a 2 inch wide strip vent along the length of the soffit you would have plenty of ventilation.

      Make sense?

  7. Handyman says:

    Todd seems to have a full grasp of the issue at hand. I’d like to add, though, that no matter how much or what type of insulation you put in the attic you WILL still have heat loss and unless the building is in the desert and unoccupied there will be moisture. That is what venting is for. One goal of ventilation is to keep the whole roof deck as near to outside ambient temperature as possible. This prevents snow melt from the bottom up, which results in melt water trickling down to the eave and refreezing. A good envelope around the heated portion of the building is important to prevent migration of moisture laden air but there is almost no way to eliminate the presence of moisture in the attic. If that moisture is not vented away it will condense in the insulation, thereby reducing the r-value and promoting mold, wood rot and damage to wall and ceiling panel. In the summer, attic ventilation keeps the roof deck relatively cool to improve shingle life and reduce heat transfer down into the living space. It also allows humidity to escape freely rather than becoming entrapped in a closed space right above your comfy air conditioned living space, where it could condense and cause the same aforementioned damages.

  8. Lynn says:

    Any advice on adding more attic insulation around a big air conditioning unit? We definitely need more insulation up there to stop the icicles and damming. How do we keep it tight up there with the air conditioning unit?

    • Todd says:

      Lynn – Well first of all ice dams are the result of two issues, one heating entering the attic and two poor ventilation. So yes adding insulation is a good idea but addressing the ventilation is just as important i.e. ridge vents and soffit vents.

      Typically there’s not much you can do under the A/C unit, however, you can create a frame around it and increase the insulation around it.

  9. john says:


    I have an attached garage that is not insulated at all except the wall into the house. Above the garage, the builder finished it with drywall.

    I don’t have any plan on making it conditioned space, just to make it a little bit warmer compare to outside. Does insulating it (I will do this – stud wall with faced insulation R-19) and 3.5 inch of blow-in insulation in the attic will help (angie’s list special for $99 for 500 square feet of blow in insulation)


    • Todd Fratzel says:

      John – It will absolutely help that space feel warmer. Just be sure that attic is ventilated well. Good luck.

  10. Scott says:

    I am an Architect/Builder building a log home in Montana. I have installed 8-1/2″ SIP panels for the roof structure. I am putting on a Coretin metal roof soon. I am very interested in building a cold roof prior. Your information parallels what I am thinking and was hoping to get some advice on my process. I notice you only use 1x strapping to create the airspace. Other sites mention that 3-1/2″ airspace is MINIMUM. That seems excessive. I plan to use 2×4 flat at 16″ o/c with 5/8″ cdx on top. My roofing manufacturer recommends cdx for fasteners rather than OSB. I will then install ice and water to complete roof prior to metal. The ridge will have a vent detail and I have a detail to provide a continuous 1-1/2″ x 1-1/2″ eave vent between sub fascia and finish fascia with spacer blocks that will minimally block air in each bay. I am concerned about the valleys because the air will get trapped. So I have decided at the valleys and a few bays adjacent to provide the strapping with 1×4 vertical and 1×4 horizontal in a cross pattern to flush out height with 2×4’s thus allowing the air to move sideways and therefore make it out of the vents. Lastly, with this 1-1/2″ of air space, I am planning on running all of my rough electrical wires within and stubbing down through sip panels. I’ll be securing the wires tight to the sip panels and detailing wire locations to ensure no nails penetrate. Also, roof fasteners are only 1″, so they shouldn’t be an issue.

    Any thoughts, comments, or suggestions on this process?

    Thanks for any help you may provide.


    • Todd Fratzel says:


      Sounds like a solid plan. Quite honestly I think as long as there is an air space for air movement these roof details work. Whether it’s 3/4″ or 3-1/2″ it allows air movement and keeps the roof from becoming too hot. Love your valley idea! I’ll keep that in mind for the future.


  11. vicki says:

    our furnace is in our attic So would we still follow the same rule when there is no furnace in attic as far as ventilation?

Leave a comment

Be nice. Keep it clean. Stay on topic. No spam.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Copyright © 2009-2023 Front Steps Media, LLC. All Rights Reserved. Home Construction & Improvement™ is a Trademark of Front Steps Media, LLC.