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  1. #1
    Join Date
    Aug 2004
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    WV, Cold Climate w/ wind, HDD 7136, 56" avg. precip.
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    Default Cellulose on top of fiberglass batts

    I'm trying to increase ceiling R-value in a house with attic.

    The existing insulation is r-38 faced insulation and I'm thinking about blowing in 5-10 inches of loose cellulose on top of the existing batts. I have never used cellulose but have read that it settles...will this be an issue with compressing the fiberglass beneath it and reducing the r-value?

    Is there any issue with moisture movement/condensation with blowing 5-10 inches of cellulose on top of faced insulation(drywall ceiling)?

    thanks, Tucker

  2. #2
    Join Date
    May 2006
    Location
    Moultonborough NH
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    Default Re: Cellulose on top of fiberglass batts

    Three hours and no reply?? Anyway, your plan to add cellulose on top of the FG batt is fine. The little extra weight of the cellulose layer will compress the FG layer slightly, which would tend to reduce the R of that layer. But that would be partly offset by the slight increase in R/inch of the now more dense FG batt. Far more than offsetting that slight decrease is the addition of the cellulose, at perhaps 3.4 to 3.6 per inch. Adding that 5-10 inches of cellulose will give you a total on the order of 50-90% more than you have now.

    Then there is the often discussed matter of the ORNL measurements of thermal performance of attic assemblies as outside temperature decreases. Their measurements showed that the R-19 attic that had chopped loose FG insulation dropped to about R11-12 by the time the temperature above it got below zero. It can be argued that your R38 FG layer is tighter and that the effect of cold, more dense air above the insulation would be at least less pronounced than in the ORNL experiments. Still, one recommended solution to the problem was the addition of a layer of cellulose insulation of several inches on top of the FG layer.

    As an aside, the ORNL experiment showed that the same attic assembly, using loose cellulose insulation instead of FG, did not suffer the drop in R value as the temperature above dropped. It actually went up.

    As far as water vapor issues go, your lowest perm layer would be the combination of drywall ceiling and the FG facing. Both the FG and cellulose are very porous, and any water vapor diffusing up slowly from inside the room will quickly diffuse to the air in the attic space and out the ridge or other vents. My only recommendation for your area would be to avoid running summer A/C so hard that your inside temperature is below the outside air dew point. But I think that would apply anywhere, eh?

  3. #3
    Join Date
    Aug 2004
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    WV, Cold Climate w/ wind, HDD 7136, 56" avg. precip.
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    Default Re: Cellulose on top of fiberglass batts

    Thanks for info! I also just read that applying the couple inches of cellulose over the fiberglass was called "capping"...not quite as catchy as "flash n batt".
    Last edited by tucker; 05-09-2009 at 11:49 AM.

  4. #4
    Join Date
    Jun 2004
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    Portland, ME
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    Default Re: Cellulose on top of fiberglass batts

    I think you'd be much better off if you could pull the FG, blow the cellulose, and then lay the FG back over the top. Too many voids in most FG installations.

  5. #5
    Join Date
    Jul 2006
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    Portland, OR
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    2,444

    Default Re: Cellulose on top of fiberglass batts

    If you have fans, mechanical equipment, or anything else in that attic that will need accessing, pre-install runners for catwalks before blowing the insulation. 2x6, 2x10, or 2x12s, whatever matches your insulation thickness.

    AFTER you blow the insulation, put on the walking surface over the 2xs

  6. #6
    Join Date
    Jun 2004
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    Friday Harbor, San Juan Island, Washington
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    Default Re: Cellulose on top of fiberglass batts

    Quote Originally Posted by NW Architect View Post
    If you have fans, mechanical equipment, or anything else in that attic that will need accessing, pre-install runners for catwalks before blowing the insulation. 2x6, 2x10, or 2x12s, whatever matches your insulation thickness.

    AFTER you blow the insulation, put on the walking surface over the 2xs
    Words to live by. Five years ago I did a remodel and at the end we had loose fill installed in the attic. Now they want more remodeling done, so I'll be shoveling the insulation around to make a pathway for crawling.
    Bailer Hill Construction, Inc. - Friday Harbor, WA
    Website - Facebook

  7. #7
    Join Date
    Jun 2004
    Location
    St Louis, Mo for the past 25 years
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    7,489

    Default Re: Cellulose on top of fiberglass batts

    Quote Originally Posted by Dancing Dan View Post
    I think you'd be much better off if you could pull the FG, blow the cellulose, and then lay the FG back over the top. Too many voids in most FG installations.
    Dan, do you think that he could get it to fill in the voids with some careful work. Not just stand at one end of the house and start shooting. Not sure just how much that stuff will fill in in voids but I thought it was fairly "fluid" and might fill things in.

  8. #8
    Join Date
    Jan 2005
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    Kansas City, Mo
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    447

    Default Re: Cellulose on top of fiberglass batts

    There is a significant difference between the stated R-value and the effective R-value of attic insulation. Energy modeling software will provide substantially more credit with just adding 1"' of continuous insulation over the batts. If my memory serves me correctly, the insulation system will perform 38% better with a continous insulation layer.

    Therefore blown insulation, regardless of type, out performs batt insulation by 38%. Check out REMdesign software and download a demo to see what the effect would be. REMrate is the DOE Energy Star raters software and REMdesign is the non-rater version. Another words, you can what-if the rating impact of different energy improvement scenarios on your own without having to pay a rater fee.
    Rick Westmoreland, CGB, CGP

  9. #9
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    Default Re: Cellulose on top of fiberglass batts

    Rick, that is good to know. I have not been to the site yet but what I am thinking it could come in pretty handy. if you were just trying to see if the extra money to increase the insulation of an attic is worth it this just might be the thing to show that the extra insulation gives you a % increase that would make it worthwhile. I will look at it later today but that is what I think you are saying it could be used for.

  10. #10
    Join Date
    Dec 2005
    Location
    Upstate NY
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    66

    Default Re: Cellulose on top of fiberglass batts

    The largest mistake about just adding insulation is the lack of consideration to an air barrier. Fiberglass (filtergalss) insulation allows air to pass right through it and Kraft paper (is there is any) isn't air sealed. So, any warm air from the home passes right through the fiberglass and into the attic. It's the same type of material used in your furnace filter except thicker. The air paths can typically be identified by looking for the grey, brown or black dust patterns within the fiberglass. This happens whether it's FG batts or blow in stuff.

    Cellulose is better because it slows the air flow more than FG, but still allows some air flow through it unless it's dense packed within an enclosed cavity. So, adding cellulose on top iof FG is like putting a lid on your coffee cup but putting a layer of scotchbrite pad under it. The lid doesn't make a tight seal and the contents just spill around the lid. This is what can happen when just adding cellulose over FG. The heat and moisture pass through the FG an follow the path around the cellulose and still up into the attic. It some cases, it makes mold situations worse because the warm moist air is directed to the underside of the roof near the edges/ soffit vents. When the warm moist air comes in contact with the cold roof you get more condensation and an environment for mold.

    Another common issue is ventilation. Many times there is not enough room to get good ventilation from the soffits and enough insulation value over to the top of the walls. Sometimes the baffles get blocked up with cellulose and you don't get enough ventilation even if you have adequate venting in the soffit and ridge. So, make sure you have plenty of ventilation with adequate baffling and adequate insulation over the walls when adding cellulose. This is sometimes accomplished by pulling back the FG from the outside edges (wall tops) and spraying the cellulose to the top of the walls. You need to make sure you have some type of wind blocking material at the wall top to prevent wind from washing through the cellulose at the soffits and prevent it from filling the soffits with cellulose

    Another way of addressing it is by using foam. Foam can be used as the wind block at the soffits and selas the top of the walls. Then you can blow in cellulose. Or you can eliminate the need for ventilation by spraying foam to the underside of the roofdeck. This is sometimes the most economical solution when you consider the effectiveness, and costs associated with adding more ventilation to construction which it was not designed for.

    Jim Coler
    www.coler.com

  11. #11
    Join Date
    Jul 2008
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    Suburbia (Washington, DC area)
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    Default Re: Cellulose on top of fiberglass batts

    Overall, adding cellulose over top of batts is supposed to be a great idea, according to the energy auditors/weatherization guys I know.
    Jim's point, that adding fibrous insulation on top of more fibrous insulation won't stop air leaking out of top plates, lights, and other penetrations, is also correct.
    Best scenario is to do "attic air sealing"--lift up the batts, caulk or foam the drywall to the top plates and seal up wire & pipe penetrations, put airtight boxes over recessed lights (without causing a fire hazard--lots of info on the web about that), etc., to stop air leaking out of the house into the batts. Then put the batts back and cover them with cellulose. Voila, air sealed, thick, complete (gap free) insulation.

    Attic Air Sealing is now a standard package that the better insulators around here sell. (Only one is really good at it though so you do need to be careful.) You can also do it yourself, again look around on line, there is good material on what to look for and how to seal it up, also a few safety concerns.

    Good luck sounds like you're really looking out for your clients.

  12. #12
    Join Date
    Apr 2007
    Location
    Windsor, NY
    Posts
    32

    Default Re: Cellulose on top of fiberglass batts

    Airsealing the attic floor before insulating is a no brainer, but in retrofit, airleakage is not the only hazard when you are trying to improve batts. My IR shows me that even very small areas where the fiberglass batt is suspended off the sheetrock ceiling allow significant heat loss. I attribute that to convection.
    Think about this, heat will pass through sheetrock very quickly. Any air between the sheetrock and the fiberglass will be very near room temperature, and it has absolutely no air barrier between it and the nearest attic vent. In fact, it is accelerated toward the vent because it is hot. I call that 'tenting' and it is a real problem for batts because any wire, pipe, duct, nailer, or off center framing member causes tenting.
    We also find that if the ceiling joist or truss bottom cord is 2x6, and the infill batt between the joists is less than 6 inches thick, a second layer of batts cross laid over the first is going to have massive tenting, and likely will not bring any benefit to the owner.
    In my business, we usually remove the batts completely, airseal the deck, and blow in cellulose, or airseal and blow under the batt as well as over it.

  13. #13

    Default Re: Cellulose on top of fiberglass batts

    At what point does convective heat loss begin to be a concern when adding depth to attic insulation? I'm guessing this would be related to density as well.

  14. #14
    Join Date
    May 2006
    Location
    Moultonborough NH
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    117

    Default Re: Cellulose on top of fiberglass batts

    "At what point does convective heat loss begin to be a concern when adding depth to attic insulation? I'm guessing this would be related to density as well."

    I would love to see some real data showing measurements of density effect, but I doubt it has been done. Bear in mind that the ORNL measurements, around 1990 IIRC, were done with loose chopped FG insulation vs. loose cellulose. The effects of air density resulting from increasing temperature difference became measurable above a certain delta. The heavier air above the insulation relative to the warm air inside the insulation just above the ceiling became an unstable situation and eventually inversion patterns developed. Their IR shots from above showed temperature patterns familiar to them (the name is in the article).

    That the situation did not occur with cellulose is a direct indication that density and the resulting closer spacing between insulation particles impedes the free flow of air that would be required for the inversion patterns to develop. One could also infer that a thicker blanket of insulation separating the same temperature difference would be less susceptible to density-induced convective flow, since the air density gradient within the batt would be lower.

    While the ORNL article didn't explain why the cellulose case saw the overall R of the assembly go UP by 10% while the FG case went down by over 40%, I think this can be explained by density and thermal conductivity of air. The much tighter spacing of the cellulose particles would result in truly dead air trapped within the particles, in the absence of holes in the ceiling (can lights, etc). The thermal conductivity of air at 70 F is about 15% greater than at -15 F. Much of the R value is due to the R of the dead air. The average over that temperature range is about 6/inch, and the overall cellulose layer has an R of perhaps 3.4 to 3.6/inch. By decreasing the conductivity of the air, and thus increasing the R, by 15%, it's easy to understand how the cellulose-based assembly would see a 10% increase in total R over that temperature range.

  15. #15
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    Default Re: Cellulose on top of fiberglass batts

    Great info everyone. Warminup, are you seeing the cold spots even after capping? When I looked at my R-30 batts in 2x8s it was obvious where air could move through it; capping with a few inches of cellulose appeared to cover most of those spots, but who knows what's going on below!
    (I'm hoping you do.)

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