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Attic insulation….fiberglass or foam?

oUykyzbh5M | Posted in Green Products and Materials on

Dear GreenBuilding Advisor
I live in the suburb of Philadelphia and I am having one hell of a time trying to find the best way to insulate my attic. Here’s my short story, I am wondering if you could provide some guidance.
I am very interested in going green with my attic and upgrading my currently degrading fiberglass insulation. The catch…and its seems a debate of who you ask.
#1) The govt suggests (as does most building codes in the area) we get a total R value of R-38 (that’s about 12″ of compact fiberglass) or 5″ of closed cell foam (at R-7/inch) or 10″ of open cell foam (at R=3.8/inch).
#2) I have had 2 independent contractors to my house, and a 3rd a verbal quote over the phone about options. ALL of them say to use less insulation with spray, resulting in less cost for product but more importantly less R value. Each say about 5-6″ of open cell is what I would need for insulating the flooring of my attic (and removing the existing fiberglass) Overall this is a less R value = R19 or R21. The reason for my email.

Who do I believe? Do I really want to put all of this money into my attic for a sub R value ( R stands for resistance of heat flow?). Why would I do that? Any why would they try to push this one me? I have asked to see data – but the few things I have seen show “efficiency” or in conditioned attics where they spray the underside of the roof.

I understand that foam will block out every area where air flow is an issue, but my bigger issue is why would I choose a lower insulating product?
Who is right? Seems both sides have some issues to settle.

Thank you

GBA Prime

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  1. user-659915 | | #1

    Although your project probably does not involve a permit, you may want to ask your local Inspections Department for their opinion. FWIW, mine will not accept the 'R20 foam is as good as R38 anything else' line and requires a ResCheck calculation for sub-prescriptive value foam. When you run the numbers you usually find you need to significantly upgrade other elements to achieve equivalent code performance with a sub-par roof.

    Have you considered cellulose?

  2. Robert Hronek | | #2

    You need to understand what your priorities are. After removing the insualtion you should be able to identify and seal any air leak into the attic. That is number 1 and most improtant- bascially a foundation for and insualtion job.

    The air sealing can be completed using can foam and treating just the areas that require the air sealing. Your second option would be to spray foam the entire attic floor. As you have seen spray foam is expensive and in order to get your business the contractors have advised you to install a substandard R value. What you can do is put a 1in "flash coating" and install another type of insulation to achive the needed R value.

    You have mentioned fiberglass but fiberglass is too air porous to be used with an open top. You have not mentioned cellulose. Cellulose will perform better than fiberglass with an open top. You can pile it up as high as you want. R 38 would be your minimum though you should go for a higher R value.

    For cost I would go with can foam and cellulose

  3. Paul McGovern | | #3

    if ducts or hvac equipment is in the attic, consider air sealing/ insulating the rafters ... flash & batt is fine if you aren't using roof fans ... flash & cellulose better.

  4. Riversong | | #4

    V Santa,

    There is no legitimate debate on this issue. Codes are very specific about minimum R-values. R-value is established by independent laboratory testing to specified standards. And the numbers you've been given are exaggerated. Closed cell will average R-6.25/inch and open cell R-3.4/inch.

    Foam contractors are trying to sell you a product and service. They are trying to sell the most expensive insulation system on the market and cannot get market share without using misleading and deceptive advertising and promises. While it's true that spray foams do an excellent job of air sealing, any good insulation contractor will air seal before insulating and air tightness is a separate issue from code-required R-values.

    I agree with others here that removing the old fiberglass, using canned spray foam to air seal any possible leakage area (including at wall plates), and blowing cellulose will give you the greatest value and the greenest attic. No other insulation comes close in high function with extremely low ecological footprint, reasonable cost and no negative health impacts (it also kills insects, prevents mold and discourages rodents). You will need a minimum of 11.4 inches of cellulose to achieve R-38. I would recommend going as deep as you can afford. It will settle about 10% over time, but that will not change it's overall R-value as the R/inch increases with increased density.

    If the attic is vented (and ideally it will be, bottom and top), then make sure to install ventilation baffles at the eaves to isolate the cellulose from the air flow and to maintain the air channel.

  5. GBA Editor
    Martin Holladay | | #5

    V Santa,
    This is a nationwide scandal -- spray foam contractors are trying to convince homeowners, builders, and local code officials that it's acceptable to install less insulation than required by code. Don't take the bait!

    Read more here:
    It’s OK to Skimp On Insulation, Icynene Says

  6. oUykyzbh5M | | #6

    Thank you for your comments. Does anyone have concerns on cellulose as far as fire? I know they treat the cellulose but I am not clear if this is still a potential risk. I have an upgraded ridgeline, soffits for ventilation and a partially floored attic for storage (no HVAC). The floor will need to be raised to accomodate 11-12" of cellulose - I assume someone makes a dense cellulose product? The unfinished side can be blown high.

    Does this sound correct? Thank you

  7. GBA Editor
    Martin Holladay | | #7

    V Santa,
    When blown into an attic, there really isn't any way to control the density of the cellulose.

    The cellulose will settle; cellulose installers are familiar with the settling phenomenon, and will blow more cellulose to anticipate expected settling. If you're aiming for a settled thickness of 11 inches, your contractor will install more than 11 inches.

  8. Robert Hronek | | #8

    V Santa

    You didn't mention cellulose. Do you know anything about it? Do you have a prior opinion on it and how was it formed? The fiberglass manufacturers have been deriding cellulose for a long time and propagated a lot of myths.

    What you should know is that it is recycled news print. It is treated with borate for its fire retardant properties. This also makes it so the insects and rodents do not like it. It does a better job of preventing convection than fiberglass. It is fire resistant and will not produce toxic fumes in the event it is exposed to a flame. In case of fire I would rather have cellulose over fiberglass or foam. It is more dusty being installed but after that it is fine. It blocks radiant heat flow more effectively than fiberglass.

    Blown fiberglass distributes small, naked to the eye, glass fibers that are a hazard when inhaled. Since you can not see them how do you know if they are the air. Fiberglass is a possible carcinogen. I think air leaks raises the possibility of these fibers making into the air your family breathes. Cellulose dust is visible and is not harmfull besides it quickly settles out of the air.

    Robert Riversong's posts on this site go into more details on the properties and benefits of cellulose.

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