Attic Vapor Barrier & Insulation

By Todd Fratzel on Insulation

Attic Vapor Barrier for Insulation in Cold Climates?

Insulation details change about as often as the weather here in New England. One question that seems to come up is whether or not to use a vapor barrier with attic insulation in cold climates? The answer to this question seems to have changed in recent years and may surprise you.

For this article, I’m strictly focusing on vented attics in a cold climate. The details will be different for un-vented attics and/or warm climates.

Air & Water Vapor Movement

Installing-Cellulose-Insulation-In-AtticThe key to this discussion is understanding the movement of air and water vapor. In a perfect world none of the conditioned air (from the house) would enter the attic and none of the unconditioned air (from the attic) would enter the house. In addition, ideally all water vapor would be managed with mechanical systems. Unfortunately we do not live in a perfect world and building construction is FAR from ideal.

Water vapor wants to move from warmer to colder air. That means in the winter time water vapor inside the home wants to move to the colder attic space. In the summer the warm humid air will want to move to the interior of a cooler home.

Ideally ceilings in homes would be perfectly sealed with drywall and good quality latex which would drastically reduce the movement of air and water vapor. However, in reality most ceilings today look like swiss cheese. Ceilings are riddled with holes from lights, mechanical vents and access hatches. Each one of those “penetrations” typically allows a significant amount of air and water vapor to move back and forth from the conditioned to unconditioned spaces.

Attic Vapor Barrier Detail for Cold Climates

Even though we apply vapor barriers to exterior insulated walls in cold climates it’s no longer recommended for ceilings (floor of the attic). Because it’s nearly impossible to seal penetrations in the ceiling, the reality is water vapor will get into the attic.  Due to this inevitable fact it’s actually better to omit a vapor barrier between the insulation and drywall of the ceiling below the attic.

The best approach for a vented attic, in a cold climate, is installing a layer of drywall with a good coat of latex paint (the paint creates a semi-permeable vapor barrier). The insulation (fiberglass or cellulose) is then placed on top of the drywall with no vapor barrier above or below.

The latex paint allows the ceiling to breath a bit but still help prevent significant movement of air and water vapor. This really helps prevent trapped moisture between a traditional vapor barrier and the drywall.

Should A Vapor Barrier be Used Over Attic Insulation? The answer is no. Either install un-faced fiberglass, loose fill fiberglass or cellulose directly on top of the drywall.

 

 

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 author of Tool Box Buzz and Today's Green Construction. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

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13 Comments

  1. Joe McGeady says:

    In my attic I currently have USG Thermafiber insulation in the 5.75- 6.75″ between joists with 9″ unfaced fiberglass batts on top. Below the joists is 1×5 16″ center to center supporting lathe and plaster for ceiling below I am trying to create some walkways and storage space by placing boards on top of the joists. What is the best way to get r value in the space between the joists. I believe in most places drywall has covered the original plaster ceilings.

    • Todd Fratzel says:

      Joe – First off, be careful you don’t overload that attic. Many attics are not designed to handle anything more than occasional access, and certainly not storage. Having said that, many people do create storage space. When we create a space like that, we typically build a frame to get the floor up high enough to allows the proper depth insulation. Good luck.

  2. Hi Todd- My hubby and I own a small (15x 30) cottage in southern Vermont. We are using the attic for storage with easy access via pull-down stairs. We wanted the attic to be somewhat part of our living space i.e. there’s some carpets on the floor of the attic with no insulation between the attic floor and main floor. (It’s about 50 degrees in the winter.) As part of an energy audit program last year a contractor told us to install 8″ thick fiberglass paper-faced insulation between the rafters to keep any heat that came through the attic floor from escaping through the roof and to also install 6 ml plastic over all the insulation. There’s no soffit (not open anyway) and no ridge vent. There are 2 normal openable windows.
    Big Problem is: with warmer weather and the sun beating down on the roof , there’s a lot of condensation forming on the inside of the plastic and dripping down from light sockets. As a stop-gap measure we’ve been using a dehumidifier to keep the water and mold at bay but we want to fix the problem itself. What do you think?

    • Todd Fratzel says:

      Linda – Can you be more specific about where the insulation is, where the paper facing is, and where the poly is? Sounds like a double vapor barrier problem.

      • Linda says:

        Thanks for your time on this problem, Todd. The insulation is between the roof rafters with the paper facing inward. The plastic is stapled to the rafters, covering all the insulation. The few places that we didn’t seal well with the plastic actually drip with condensation on sunny days. (Like around light fixtures at the peak.) We’re also not sure when and if we should open the windows.
        Thanks so much! Linda

        • Todd Fratzel says:

          Linda – Not sure where to start with this so I’m going to toss out some thoughts.

          – First off, here in this part of the Country, 8″ of insulation is NEVER enough for an insulated rafter application. Doesn’t even begin to meet code. Attics today are typically insulated with R30 to R49 at a minimum. You’ve got probably about R25, well short of the required insulation value.
          – Secondly, I’m not sure why he installed plastic on top of the paper. This is trapping moisture that’s caught between the plastic and roof surface.
          – It’s odd that moisture is trapped in that space, it makes me think that either the roof has some leaking, the plastic isn’t sealed well and moisture from below is getting behind the plastic and trapped, or there was moisture in that space prior to the installation that got trapped.

          A dehumidifier won’t fix this problem. I’d inspect the roof, remove the plastic, and be sure to install the appropriate amount of insulation. Furthermore, batt fiberglass against a roof without ventilation is BAD!!! Each bay should have a vent, tied to soffit and ridge venting.

          Good luck.

          • Linda says:

            Thanks, Todd! Your comments make a lot of sense and line up with what a local roofing guy told us this week. We will be making some changes (for the better!) for sure. The dehumidifier has worked quite well as a stop-gap, symptom – not problem- fixer. It will be good to fix the problem. Thanks for your excellent input!
            Stay cool–Linda

  3. Larry says:

    I have faced fiberglass insulation batts in my attic. While the R value was to code when the house was built, I’d like to increase the R value in order to maximize the efficiency of a geothermal climate control system we recently had installed.

    My thought was to temporarily pull up the fiberglass batts in between the ceiling joists, install 2 inch blue board in the joist cavities, then place the existing fiberglass batts back on top of the blue board.

    Will this cause issues down the road?

    • Todd Fratzel says:

      Larry – I would leave the fiberglass, and blow in additional cellulose over that. You can rent a blower and do it yourself for cheap money. We still use a ton of cellulose in attics and it works quite well. good luck.

  4. Ahad says:

    I’ve bought a 40 year old semi-detached and noted that at in winter, 3 rooms form condensation at the end of the ceiling and drops of water.

    An expert company advised replacing our vents with a Max Vent, add baffles, remove all insulation and put a spray foam vapour barrier and blow cellulose insulation. This was too much to remove all existing insulation.

    Another company came, insulated my dry walls (I think retrofit), ie spray foam insulation between drywall and bricks, put the MAX Vent, 21 baffles and blow cellulose insulation. They advise not to put the vapour barrier which will be costly to remove all existing insulations, etc.

    Last winter, it was better but the condensation continues in the 3 rooms.

    Is the vapour barrier the only solution? Is there an alternative type of vapour barrier where we can shift the existing insulation part by part and put the vapour barrier? Please advise the most economical way.

    Thanks

    • Todd Fratzel says:

      I think you’re miss-understanding the issue. From what I can tell, you’ve got two problems.

      1. You have too much humidity in the house. You need to deal with lowering the relative humidity. Showers, baths, cooking, washing clothes and dishes all generate high humidity. You need better ventilation.

      2. It’s likely the insulation value is not high enough at the “pinch” point where the walls meet the roof, this results in the ceiling being cold, so when the damp warmer air hits it, it condensates. So….you likely need to increase the R value, if there is not enough space for cellulose to get a proper R value, then you need spray foam.

  5. M. David says:

    Hello, I just wanted to understand your words (in quotes further below) —
    The drywall is installed on the attic floor itself? And, below that drywall, would be open space/wiring/light outlets, ect…
    The drywall is painted, and then insulation is put on top of it. Is this correct?
    And, it would not add to much weight to attic (size is 550 sq. feet); Victorian Home… Much Thanks! M
    (RE: Attic Vapor Barrier Detail for Cold Climates) —
    “The best approach for a vented attic, in a cold climate, is installing a layer of drywall with a good coat of latex paint (the paint creates a semi-permeable vapor barrier). The insulation (fiberglass or cellulose) is then placed on top of the drywall with no vapor barrier above or below..

    • Todd Fratzel says:

      You misunderstood.

      The approach is basically ceiling joists with drywall below (living side). The drywall is painted with a good latex paint. There is NO vapor barrier other than paint. Insulation is installed above that.

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