Attic Vapor Barrier & Insulation

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• 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 1x5 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.

  • 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.

• 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?

  • 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.

    • 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

      • 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.

        • 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

• 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?

  • 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.

• 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

  • 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.

• 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..

  • 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.

• your description was confusing. it also sounded to me, like you were saying install drywall on the floor of the attic/ painted one side.

  • Mack - It's pretty straight forward...the drywall is attached to the house side of the attic floor joists or the bottom of the ceiling joists, typical location.

• Hi, I live in Montana and our climate is pretty arid. We bought a 1960's house with batt insulation which is faced. The contractor used two layers of insulation one is R13 the other is R11 which gives us R24 (no where near what is recommended for our area). First, I noticed that both layers of batting are faced (the facing is towards the drywall) and I am curious if it is a problem that the second layer of batting is face and am curious if it should be removed? I am also wondering if it is okay to blow cellulose over those layers of insulation to bring the R value above 50, or if it would be better to remove all of the batting and then blow in cellulose. If I do remove the old batting and blow in cellulose it it necessary to paint the drywall first as it is very dusty and dirty up there and I have doubts about how well the paint would stick. P.S. I am planning on air sealing the attic first.

  • Bill - It's not ideal, but I don't think it's a huge deal, if it was a wall I'd be more concerned. I've seen cellulose blown over it with success.

• Hi Bill,
  Was is the best material to be used when creating a vapor barrier in the attic? I'm installing new bathroom ventilation fans and want to make sure they are air tight, especially under the duct exhaust of the fan. I didn't want to compress the fiberglass, and was thinking of putting blown insulation around the fan box, then put unfaced fiberglass on top. Should I use the backing that comes on the "faced" insulation and tuck under or is there something else that would do a better job, like the "Frost King" Self-stick vinyl foam with aluminum foil backing?

• Hi Todd,

  We have a commercial building with a flat roof and a vented attic with cellulose insulation. We just added 4" of polyiso to the roof deck since we put on a new TPO roof. I am looking to close off the attic vents and turn it into a sealed space. Currently the floor below the attic is un-occupied, but when we renovate we will put up drywall and paint on the ceiling. Do I need a moisture barrier on the ceiling or attic? Is it OK to close off the vents now, or should I wait until we also seal up the ceiling so as not to trap moisture in the unvented attic?

  Thanks for your advice

  • I would wait until you've got that sealed from below.

• HI Bill
  I'm from new york, and I have a condensation issue in my attic. I had an energy audit and they said I was also losing a lot of heat through my attic because I only have r19 batt insulation between 2x6 joists. They recommended raising my attic floor and putting cellulose blow in insulation and capping it off with 2 inch xps foam board. So my question is will that cause problems covering the insulation with the foam board. Will the inulation get moldy from moisture or will the foam board prevent the water from condensing on the bottom side?

  • I would NOT install that foam board. I'd just install the proper depth of blown in insulation.