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Thread: Attic Insulation facing question...

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    DIY Junior Member dizzo72's Avatar
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    Default Attic Insulation facing question...

    I have a 1920's house with knob and tube, Zero insulation, and flooring nailed down in the crawl space behind the knee-wall in my half story. I would like to run with the easy solution for now and just roll fiberglass batts out over the floor, two layers perpendicular to each other. My question is about facing:

    Since I would be leaving a 6 inch air gap, between the ceiling and the attic floor, will I cause damage with a facing side down on the attic floor? Or do I need a vapor barrier?

    Basically, should I have one layer of batts with the facing down? Then another unfaced on top running perpendicular? Or is the uninsulated air gap going to cause condensation? Seattle WA so this is winter consideration only, no AC.

    Thanks

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    Retired Defense Industry Engineer jadnashua's Avatar
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    With an older house, that space is likely to have lots of air infiltration...not sure how much good it will be - fiberglass is a good air filter, but only good at insulation when it is in still air without possibilities of air currents. While it might be more of a pain later on when you do rewire, blown in cellulose under the floor might be better. Before you go off and do something...see what some of the others have to say.
    Jim DeBruycker
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    Forum Admin, Expert Plumber Terry's Avatar
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    The facing always goes on the living side. Mositure comes from inside the home, dry air is on the outside. The facing is to keep the insulation from getting damp. So below the floor, the facing would be high, and in the attic, the facing would be low.
    Always between the living space and the outside.

  4. #4
    DIY Junior Member dizzo72's Avatar
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    Quote Originally Posted by jadnashua View Post
    With an older house, that space is likely to have lots of air infiltration...not sure how much good it will be - fiberglass is a good air filter, but only good at insulation when it is in still air without possibilities of air currents. While it might be more of a pain later on when you do rewire, blown in cellulose under the floor might be better. Before you go off and do something...see what some of the others have to say.
    Some insulation, even if not perfectly airtight, must be better than none though, right? Honestly rewiring all the light fixtures (most other wiring has been upgraded from the new panel) doens't sound cost effective. It is all in good shape, and runs CFLs throughout, a lot of hassell just to be able to blow in cellulose.

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    In the trades Dana's Avatar
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    Blown insulation would be better, but is probably disallowed in WA (as in many/most areas) where it could cover a knob & tube splice. There's a theoretical fire hazard where an aging k & t splice builds up too much heat when surrounded by insulation, but SFAIK it's only occured in a laboratory- no known cases of it happening in a house, which is why that restriction is starting to be repealed. If you're allowed to, blowing cellulose under the floorboards would be a very good start- you can get ~R35 in a 2x10 joist cavity, ~ R27 in a 2x8 cavity, or ~R20 in a 2x6 cavity.

    Filling the cavity with a higher density fiber also acts as an air-retarder for potential thermal bypass air currents: A bunch of batts laid down over the flooring may end up buying you nearly-nothing if the kneewall spaces are ventilated to the outside, since the joists provide a thermal bypass allowing outdoor air to flow in one side of the house across to the other and out. A better bet would be to use 1-2" of closed-cell spray foam on the roof deck closing off all venting, followed by 3-6" of wet-sprayed cellulose "blown in bag", neither of which are good DIY projects.

    If the space is already unvented and you can verify that it's reasonably air-tight (pressurize the house with a window fan, and see if air flows freely toward the kneewall access points), batts will do SOME good, but use unfaced batts, and monitor the humidity in the now colder space with an RH meter. If it stays over 70% RH in winter you may have to reduce the insulation level to avoid condensation and mold issues. Facers on batts are still a good vapor retarder even when it's not air-tight, but since it's guaranteed to be nowhere NEAR air-tight you'll have a higher risk of getting moisture in via air-transport to condense, but then take forever to dry due to the vapor retardency of the facer.

    Also, the batts will perform better if you roll out a (vapor permeable) air barrier such as a housewrap (like Tyvek or Typar) over the top after you have the batts laid out. In cold-side-up applications the air-permeability of low-density batts causes them to DRAMATICALLY under-perform their rated value due to convection losses. The R-rating of the batt is only valid when there is a reasonable air-barrier on both sides. (The floor being one of those sides.) The thicker and higher density the fiber, the less of an issue this is, but in colder climes than yours even R38 batts will perform well under R30 over a winter unless there is either a higher density overblow of cellulose or a topside air-barrier due to convection currents in the insulation.

    Do NOT put down anything as vapor-retardent as a poly vapor barrier anywhere in this assembly unless you know to a high degree of certainty what the drying mechanism will be. Housewraps, or fan-fold XPS (sold as siding underlayment) would be about it for moderate/high perm air-barriers.

    What causes condensation is warmer more humid air coming into contact with surfaces that are below the dew point of that air. The dew point of typical 68-70F 30-35% relative humidity wintertime conditioned-space air is between 35-40F. If it's more than 35F outdoors, you basically won't have any attic condensation as long as your ventilation rates to the house are reasonable and you run the bathroom fans after showers. During cold snaps in Seattle you may have some temporary condensing conditions, but as long as there's a drying path for the water vapor either through the insulation toward the interior, or out the attic ventilation it won't usually create a problem in the kneewall-attic. In cooler climes where it stays below 35F for weeks/months on end significant moisture can build up unless you get serious about adjusting the vapor and air-retardency of the layers within the assembly. Kraft-facers on batts are only semi-impermealbe to water vapor in order to not trap moisture in the walls, but even that can't be relied upon completely in the upper midwest. Still almost ALL moisture problems are due to air-leaks, not vapor permeation through wall/ceiling assemblies, but putting a vapor-barrier in a non-air-tight assembly can be asking for trouble, since the air-leaks let the moisture in, and the vapor barrier keeps it from leaving quickly. Air barriers are thus FAR more important than vapor barriers for moisture control.

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    DIY Senior Member dlarrivee's Avatar
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    I would hold off on this project until you decide if you're going to re-wire anytime soon or not...?

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