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Thread: attic insulatiom

  1. #1
    DIY Senior Member DIY's Avatar
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    Default attic insulatiom

    Hi all.. I did not see a forum that relates specifically to insulation questions ,issues etc., so i thought i woud try this forum. What i have going is an attic space of 1,344 sq. ft. I have not gotten a lot of information on what would be best or how to get the most out of insulation except for the last 2 estimates i've got. It was told/suggested to me that R-19 would be great in the sq. ftg. i have. R-30 i was told would work wonderful,but maybe a overkill because it Is not the thickness of insulation that makes it work good or not, but the area it covers.Can anybody here who installs or is very well versed with how insulation works put their thoughts on this? I am also considering batts or blown in... Is one better than the other?

    Thanks to all

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    DIY Member RRW's Avatar
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    First, where are you? Second, if you use batts will they be installed between ceiling joists? If so you need another layer over the joists, at least. It is the thickness of the insulation, or actually the R value that makes it work.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Think of the area like a radiator...the bigger it is, the more heat it can move (both in and out). Then, think of the insulation - the more you have, the slower that heat moves. Both of the guys that gave quotes are off base. Now, depending on where you live, you may not need a lot of insulation. Check this out http://www.energystar.gov/index.cfm?...sulation_table
    Jim DeBruycker
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    DIY Senior Member DIY's Avatar
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    Thanks for the reply. I am in the north central Florida area. I mean it makes sense the higher the Rvalue and the thicker the insulation the better,but there is a point when going thicker or up in R value to where insulation gets useless i have been told by these insulation contractors..
    I am going with blown in R-30 for the 1,344 sq.ft. non insulated attic soon to be insulated. The 11.5"to 12"s of fiberglass loose fill will be fun to hand back rake over if/when service work etc. will be needed up there...

    Another question: Does insulation of any kind deaden sound if put in existing walls? (drill a hole between studs with 2-3" hole saw blow in quietness....?..Ideas from anybody)?

    Much appreciated!

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    Retired Defense Industry Engineer jadnashua's Avatar
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    In FL, you must be careful about where the vapor barrier goes. Blown in cellulose has a better R-factor per inch than fiberglass. Insulation in the walls will improve the noise suppression - cellulose better than the fiberglass since it is denser. To meet Energy Star ratings, from the map, R-30 just makes it.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    What jadnashua said. R19 may still be code-minimum in some areas of FL but more is still cost effective.

    Blown is better than batt as a rule, since it fills in all gaps & voids. In situtions with big temperature differences between attic & conditioned space cellulose retains R-value whereas with fiberglass it's insulation value falls with big temp differences due to induced convection within the fiberglass. With either, a radiant barrier at the roof deck reduces both peak & average air conditioning loads.

    If you have ducts, air handlers in the attic they too must be insulated (and radiant barrier affects that too!) If that's the case it's often easiest/best to spray 2-3" of closed cell foam at roof deck as air-barrier/vapor retarder/insulation/hurricane glue for R13-19, sealing all ventilation, and make up the difference in cellulose (up to ~R40 foam+ cellulose) on the attic floor. The interior side of the foam then never reaches the temps where it's radiating a lot of heat, the ducts are inside the pressure boundary of the structure, and the attic temp is now between the roof temp & conditioned space temp. (If you're feeling rich you can make it all closed-cell foam at the roof deck, but it's a pricey proposition.) You could also use 6" of open cell foam in place of 3" of closed cell foam for less money, similar thermal performance, but without the structural & vapor control enhancements.

    If blowing cellulose into the attic, it'll perform better if you give it at least 3" over the joist tops to form a thermal-break. R30 is only ~8", and if you have 2x6 joists or bigger the wood makes a large thermal short-circuit through the insulating layer. Wood is also very absorptive of radiated heat, and the joist tops will run hotter than the attic air temp if you don't have a radiant barrier in place. R30 + r.b. is cost effective in FL using even the most conservative assumptions about utility costs, etc., but it's more than just the price of electricity- more insulation will be just plain more comfortable. If you assume energy price inflation (always a difficult factor to prove) or you live in a high electricity price area, R38-R40 can be cost effective as well, and if it covers the joist tops where R30 didn't the performance enhancement will be better-than the simple-math suggests.

    Cellulose & fiberglass in walls both deadent sound. In a standard 2x4 framing FG gives you about an STC 38, cellulose dense-packed to 3lbs?ft^3 yields about STC 41 (noticably quieter). At typical loose-blown densities cellulose will still outperform fiberglass, but not by huge margins.

    Under no circumstances should an air-conditioned house in FL have interior side vapor retarders. But many can do just fine without any vapor retarders interior or exterior. The issue is vapor drive or air from the exterior finding it's way to a layer inside the cooler building to condense on wood that's below the dew point. If you have stucco or brick siding there is usually a ventilation gap between the siding & structural wall to handle the super-high vapor drives of sun on a rain-wetted siding. In stick-built homes with fiberglass insulation, convection within the fiberglass can transport the high-vapor drive moisture to condense on the cool interior wall-board creating mold conditions- it NEEDS an exterior side vapor retarder. But blown cellulose has only ~10% of the internal convention of fiberglass batts (if dense-packed, less than 2%.) It also wicks condensation away from structural materials and can be used without a vapor retarder in N. FL. But in a flood situation it's miserable- takes forever to dry on it's own and it's usually remove & replace, whereas fiberglass can dry fairly quickly if you open up the walls. (If you live in a flood zone, spray foam insulation would be preferred, as expensive as that is.)

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    Moderator & Master Plumber hj's Avatar
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    The "R" value of insulation is what determines its effectiveness, BUT that R value usually is based on the insulation having a certain "thickness" and thus an amount of entrained air. IF that insulation is compressed, either because of the building construction, (such as putting 6" batts in a 4" wall), or adding additional insulation on top of it to press it down, the "R" value will decrease. The only way the "area" covered would be a factor is if the insulation were NOT applyed over the entire living area. That can happen with haphazard installation if the installer does not insure that the insulation is extended into the "edges" and corners of the attic.

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    In the trades Dana's Avatar
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    Quote Originally Posted by hj View Post
    The "R" value of insulation is what determines its effectiveness, BUT that R value usually is based on the insulation having a certain "thickness" and thus an amount of entrained air. IF that insulation is compressed, either because of the building construction, (such as putting 6" batts in a 4" wall), or adding additional insulation on top of it to press it down, the "R" value will decrease. The only way the "area" covered would be a factor is if the insulation were NOT applyed over the entire living area. That can happen with haphazard installation if the installer does not insure that the insulation is extended into the "edges" and corners of the attic.
    The R value is determined by ASTM C 518 testing, which is at full specified thickness/loft, in a fixed orientation with no air movement on either side (samples are tested between plates in a closed device) and a relatively small temperature differences (delta-T). But it's effectiveness depends on a lot more than it's mere ASTM C 518 test result. Real-world performance is not constant with either orientation or delta-T. Fiberglass loses R in horizontal applications due to convection currents within the insulation. It also loses R-value with high delta-T due to the same processes, and in hot-attics even more, due to it's translucency to radiated heat. But the thicker it is, the less that temperature-dependent loss is. R30 doesn't lose as large a percentage of performance as R19 (which can lose a LOT, like 50% of it's R-value at high delta-T in very hot or cold climates, which is when you need it the most.) With R30 you may only have ~R25 when it's 100F outside and the roof deck is pushing toward 140-150F, but with R19 batts you'd be lucky to get R10 of performance out of it.

    But it's also true that in partially insulated assemblies it doesn't take a very large fraction of uninsulated area to dominate the heat gain/loss, making it difficult to get batt insuation to perform to it's full potential- the gaps & compressions undercut it's real-world performances. Blown insulation tends to work better (typically 10-15% better, from a utility bill point of view), since it fills in the gaps & voids. But "fixing" a batt installation can be as easy as blowing 3" of new stuff over the top.

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