Thread: Cellulose over fiberglass

There is a company out there promoting their pest control cellulose insulation that has 15% borates for bug control. They are telling potential customers it is OK to blow in 6 or 7 inches over blown in fiberglass. I see a couple of issues with this, one, it will compress the fiberglass and two, it will filter down into the fiberglass, both of which are going to reduce the R value. What do you all think of this practice?

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Originally Posted by Benjamin Thompson

There I see a couple of issues with this, one, it will compress the fiberglass and two, it will filter down into the fiberglass, both of which are going to reduce the R value.

One of the benefits of placing cellulose insulation over fiberglass insulation is the cellulose (if installed properly) will provide an air barrier (something fiberglass insulation cannot do).

Could be. I have also been reading the BI fiberglass is pretty worthless at very low temperatures.

its used up here pretty regular. The installers will make wooden sides around the attic hatch, maybe 20 inches high, then staple paper measureing tape on the gable ends and then blow it in from a removed roof vent, blow it till it reaches the R number on the tape. They must install vent baffles at the soffits to allow air circulation. BI as you called it comes in Fiberglass Pink and Cellulose. Rent the machine Home Depot and blow in your own. It works well when installed correctly...

Originally Posted by Benjamin Thompson

Could be. I have also been reading the BI fiberglass is pretty worthless at very low temperatures.

Ben,
Please explain how the intermingling of cellulose and fiberglass results in a reduced R value. Thanks in advance.

The Borate insulation can not be purchased by a Home Owner. It can only be installed by a licensed pest control applicator.

T∙A∙P Pest Control Insulation Systems | Helps keep bugs out - and Comfort in.™

Loose terms. I read BI as: Blown Insulation.

Originally Posted by Joseph Hagarty

The Borate insulation can not be purchased by a Home Owner. It can only be installed by a licensed pest control applicator.

T∙A∙P Pest Control Insulation Systems | Helps keep bugs out - and Comfort in.™

Originally Posted by Darrel Hood

Ben,
Please explain how the intermingling of cellulose and fiberglass results in a reduced R value. Thanks in advance.

Not Ben here, but ...

Fiberglass insulation has its R-value based on its depth and consistency (blown in or batt) and when fiberglass insulation is compressed the R-value is reduced, the more the fiberglass insulation is compressed the greater the R-value loss (it is not a linear loss, it loses R-value faster the more it is compressed).

Fiberglass insulation, especially blown-in type, allows the cold are to settle down through the fibers and down through the insulation, reducing its R-value dramatically. This was found out in tests run in Wisconsin or Minnesota I believe.

Originally Posted by Darrel Hood

Ben,
Please explain how the intermingling of cellulose and fiberglass results in a reduced R value. Thanks in advance.

Part of the advantage of cellulose is the tight air barrier.
As Jerry alluded to, I think there is more "air wash" or air infiltration as the cellulose becomes mixed with the fiberglass.

Also, I'm reading that you shouldn't go higher than R-30 with cellulose because of the weight. Any thoughts on this?

Originally Posted by Bruce Breedlove

One of the benefits of placing cellulose insulation over fiberglass insulation is the cellulose (if installed properly) will provide an air barrier (something fiberglass insulation cannot do).

Originally Posted by Benjamin Thompson

Part of the advantage of cellulose is the tight air barrier.

Cellulose does not provide an "air barrier" unless it is dense-packed to 3.5 lbs/cu. ft.
To do that it must be in an enclosure such as a wall cavity. You don't get air sealing properties/benefits just blown loose on top of a ceiling or on top of other insulation.

While good stuff, the insect (and fire retardancy) benefits from the borates diminish with time.

Fiberglass can also be dense-packed (2.2 lbs/cuft.). Google "Spider Insulation" made by Johns Mansville.

Jerry,
I really need to understand this principle, so I will keep asking questions until it gets through my thick elderly skull. I can interpret the conversation thus far a few ways. Is one of the following statements correct?

1. The blended R value is less than the sum of the two R values.
2. The blended R value is less than either of the two R values.

Thanks,

In the initial post by Benjamin, I read complaint against scandalous marketing of cellulose insulation. I didn't even see the issue of compression of base loose-fill fiberglass, until that carried the thread. Borate is added for fire suppression, not first as a means of preventing mold growth. Cellulose insulation - Wikipedia, the free encyclopedia


If you soak cellulose, the cellulose might never stink, but surrounding wood will develop mold and rot, without constraint. It's the wood we will care about.

The compression issue is also part of scandalous marketing. There is attempt to scare people into accepting lowest-possible density, as excuse for using less weight of insulation. Get the job done with fewer bricks of cellulose or fiberglass to be ground and blown. The result is a fragile sea, subject to greater wind washing. Touch it and there is irrecoverable collapse.

The R-value per inch goes up as density increases. Just use more weight to get needed depth and R-value that is stable.

I never use loose-fill as top-layer insulation, covering floor joists. I use ample batt-insulation-filled decking, but still see that loose-fill might be trampled and ruined some day in an inspection or maintenance task.

The math of adding insulation is summarized in practical Examples 3a, 3b and 3c here.https://sites.google.com/site/philli...nsulation-math

The effect of adding insulation is linear, once level is above floor joists, where you measure it by stated batt R-value, not thickness. I accept an R19 batt as R19, whether it is 5 1/2" thick, or 3" thick.

Putting the cellulose over the batt is acceptable and done often with good results. I know people that do it here and we verify the results with blower door testing and thermal imaging. You can get air sealing with cellulose. There is a down side that it can absorb moisture. This can lower it's effectiveness and pack it down more so attic ventilation is important. The reason they are using cellulose and not fiberglass is that you can insulate a ton but without air sealing you loose allot of the performance. They aren't interested in doing it right and remove the insulation then air seal so they are using the cellulose in hopes it takes care of some of the air sealing. The r-value per inch of batt goes up as it gets compressed but the total R goes down. The difference between an R-11 bat and R-13 batt is the R-13 is denser. Your adding more insulation on top of it so it makes up for it's compression.The problem when you compress the batts in a wall is that you allow a gap between the vapor barrier and the drywall. This allows air currents in the wall that take the heat up to the attic before it can ever get to the insulation. Fiberglass is hardly ever installed properly and blown in allows for a better over all insulation job.

Originally Posted by Darrel Hood

Jerry,
I really need to understand this principle, so I will keep asking questions until it gets through my thick elderly skull. I can interpret the conversation thus far a few ways. Is one of the following statements correct?

1. The blended R value is less than the sum of the two R values.
2. The blended R value is less than either of the two R values.

Thanks,

Darrel,

The answer is not that easy.

If the heavier blown cellulose insulation was blown in first, then lighter fiberglass blown in on top of the cellulose, the fiberglass would not cause must compaction of the cellulose and the R-value of each is probably close to be additive, if not additive.

If lighter fiberglass is blown in first, then the heavier cellulose is blown in on top of the fiberglass, the weight of the cellulose will compact the fiberglass and thus the R-value of the fiberglass insulation is reduced, which means the R-values are not additive as reduction for compaction of the fiberglass needs to be applied before adding the R-values together.

Go here: http://www.ibsadvisorsllc.com/_libra...Insulation.pdf and scroll to the end of the document for compressed R-values. An example is 6-1/4" fiberglass batt in various wall cavities:
- 2 x12 cavity = R-19
- 2 x10 cavity = R-19
- 2 x 8 cavity = R-19
- 2 x 6 cavity = R-18
- 2 x 4 cavity = R-14

Jerry,
Thanks for the explanation and the link. They show the blended R value will not be less than at least one of the two individual R values. Therefore, by placing the cellulose on top of the fiberglass, there is an increase in R value. However, the increased R value will not be as great as the sum of the individual R values. Thanks again.

I believe cellulose is the better choice in walls or attics. If the concern is holding moisture or getting wet then fix the moisture or getting wet problem. As far as loose fill fiberglass I have never liked it or believed its worth until they started getting into the R40 area or the deeper the better. Just too much air penetration is loose fill fiberglass.

As far as I am concerned Borates, in most cases, are useless in an attic space. The easiest place, in most cases, to treat for insects with out putting borates over your head with so many areas to the home allow for air infiltration/dust. Just not a good idea.

As far as pumping cellulose over fiberglass? You are decreasing R value in the loose fill fiberglass by trying to increasing the R value with another product and adding useless borates. Pump the loose fill fiberglass up or remove the fiberglass and replace with cellulose. Add a radiant barrier, whatever.

JMO

I'm also a licensed Insulation Contractor (since 1985). Contractors that like to push Fiberglass spread a lot of rumors. Its all crap-olla..

Installing Cellulose over Blown or Batt fiberglass insulation is perfectly acceptable and in most cases will not decrease the R-Value of the Fiberglass. In cold climates it is Recommended to blow 2-3 inches of cellulose over fiberglass because Fiberglass in a cold attic becomes less effective due to convection currents when you have a large temperature differential between the inside air (abt 70) and an attic that gets down below freezing. In fact when the attic temp was -18 the R-Value drops to 1/2 the rated (R-30 becomes an R-15), adding an R-8 with cellulose over the R-30 fiberglass still measured R-38 and maintained the R-38 at the lower temperatures.. Source for my opinion is Oak Ridge Natl. Laboratory.

You also should understand that an R-38 batt is 12 inches butt blow fiberglass is 14-17 inches. Fiberglass is very Dependant on density as well as depth. Cellulose changes very little with density.

Oh yes, R-60 can be blown with cellulose into most attics with no problem with weight. about 16-17 inches.

Unless there is something wrong with the insulation that is installed in a home (like exposure to Meth) there is no need to remove the existing insulation and any type can go over any other type...but your nuts if you install anything other than Cellulose...IMO

We've spec'd cellulose topcoating over installed fiberglass with good results as well here in Chicago. It's a little off topic but no one really addresses the radiant gain through fiberglass versus cellulose which is a large factor in summer cooling (luckily we don't have too much here). I've seen WAY too many childish installations with FG to cloud my memory but every now and again, i.e. almost never, do I see it installed well or properly.

Up here it is very common to see cellulose installed over fibreglass batts, never seen an issue.

Originally Posted by Phillip Norman

The math of adding insulation is summarized in practical Examples 3a, 3b and 3c here.https://sites.google.com/site/philli...nsulation-math.

Hey, thanks for that link! That Energy Conservation How To blog is a very interesting place.

Thanks, Michael Thomas, for noticing my input. This has become an interesting seminar on insulation placement, and I will add more.

I am dismayed to see acceptance of cellulose insulation in attics subject to inspection. This blog post is one of my many examples of good service, in opposition to cellulose installers:
Energy Conservation How To: Cleaning Up A Blown-Cellulose Attic
I have never seen a cellulose blown attic, done with full honesty, or with consideration for access needs. No one should ever go into an attic to store things, where top insulation is any kind of loose-fill.

I was dismayed by the table posted by Jerry Peck:
Go here: http://www.ibsadvisorsllc.com/_libra...Insulation.pdfand scroll to the end of the document for compressed R-values. An example is 6-1/4" fiberglass batt in various wall cavities:
- 2 x12 cavity = R-19
- 2 x10 cavity = R-19
- 2 x 8 cavity = R-19
- 2 x 6 cavity = R-18
- 2 x 4 cavity = R-14
__________________

In my math presentation, I more-importantly consider thermal shorting of framing. It is that which keeps an R38 batt from service as R38 in any framing. Worse, the credit of full R value of an R19 batt in a 2x12 cavity is absurd. A batt must over-fill a cavity, for best service. An under-filled cavity might have no insulation value.

The photo at first page of the pdf link is not labeled "Bad Examples." It reminds me of this conversation:
Guardian Fiberglass Threatens Blogger With Legal Action | GreenBuildingAdvisor.com

I can, and probably will, add a lot more. Perhaps we need a new thread.

Writing now ONLY about cold climate housing where R-44 ceiling insulation is the current standard.

Adding a second layer of ANY insulation to an attic with existing insulation less than about R-38 AND unsealed bypasses has a high potential to create damaging condensation on the sheathing during cold weather. This was proven by our Canadian friends during their R-2000 program. More insulation makes the attic colder more often and places the sheathing below the dew point of the indoor air more often for longer periods.

Prior to adding ANY attic insulation to attics of homes built BEFORE sealing attic bypasses was standard practice creates this potential for condensation. If the attic insulation is blown in (any type), a good way to find and seal the bypasses is to remove the existing insulation (vacuum truck) then apply a layer of spray foam atop the ceiling, then re-insulate with the BI of your choice.

Whenever I see an attic with two dissimilar layers of insulation (even a color change in blown FG) in a house built before sealing bypasses was standard practice (mid 80's here in MN) I predict that there are unsealed bypasses and warn about the potential for condensation in the attic, even if the evidence of any prior condensation is slight. (Indoor relative humidity varies with different occupants.)

The worst attic messes I've seen are where the attic was heavily insulated without sealing the bypasses (air leaks from house to attic). Several are shown on my website.

Originally Posted by Dave Hahn

Cellulose does not provide an "air barrier" unless it is dense-packed to 3.5 lbs/cu. ft.
To do that it must be in an enclosure such as a wall cavity. You don't get air sealing properties/benefits just blown loose on top of a ceiling or on top of other insulation.

While good stuff, the insect (and fire retardancy) benefits from the borates diminish with time.

Fiberglass can also be dense-packed (2.2 lbs/cuft.). Google "Spider Insulation" made by Johns Mansville.

Dave

How is borate going to diminish? Borate is a mineral and I have not heard of it "diminishing". Can you point to a study. Excessive wetting may wash it out but the moisture is a bigger problem for other reasons.

Any loose insualtion- fiberglass, cellulose, rock wool etc is not an air barrier.

The most porous is the fiberglass. Cellulose does a better job of stopping the air but is not an air barrier. Even as dense pack it is not an air barrier but does can do a good job of stopping air.

Cellulose over fiberglass will help the fiberglass work better. It will have less air flow through it. Plus the cellulose will stop the radiant heat from reaching the fiberglass.

Borate is a natural mineral that has the toxicity of table salt. It will not hurt people. It is put in as a fire retardent. An added benefit is that insects and rodents dont like it.

Some cellulose is only treated with borate. Some have a combination of borate and I believe aluminum sulfate. The 100% borate is preferrable.

Like another poster has stated air leaks into the attic from the condition space should be addressed before adding insulation. Air leaks contribute to mold and ice dams.

You guys are making my head hurt! So, considering cold climate here in Alaska, new/re-model attic space, 2x6 truss chords, baffles in place, ridge vent, give me your recipe for effective insulation.

Originally Posted by Randy Rogers

You guys are making my head hurt! So, considering cold climate here in Alaska, new/re-model attic space, 2x6 truss chords, baffles in place, ridge vent, give me your recipe for effective insulation.

this may help
http://www.uaf.edu/files/ces/publica.../EEM-04452.pdf

Originally Posted by Randy Rogers

You guys are making my head hurt! So, considering cold climate here in Alaska, new/re-model attic space, 2x6 truss chords, baffles in place, ridge vent, give me your recipe for effective insulation.

I would (almost) never use Blown Fiberglass, especially in an extreme cold or hot climate. When you have an extreme difference in temperature between the inside of you home and the attic, Convection currents are set up in the (light) fiberglass insulation and your rated R-Values diminish drastically.

This explains it http://www.ecp1.com/science/Oak-Ridge-Report.pdf

The Link talks about cold attics with (effective) R-Value ratings of fiberglass at only 50% if inside (home) is 70 deg. (f) and the attic is -18 deg. (f).

Remember that because of *Radiant Heat Loss, the outside temperature does not have to be -18 deg.(f) for the attic to be that cold...Probably only has to be about 0 deg.(f) ambient air temp outside on a clear night. So you don't have to be in Alaska or Canada for your R-38 attic insulation to be downgraded to R-19.

This same Temperature differential (and convection currents) can be seen in very hot climates where attic temps can get up into the 150 deg. (f) range.

*Why you will see frost on your roof or windshield when outside temp is still above freezing. (Cliff Clavin Fact)>>>The Temperature in Space is approximately -455 deg (f) A lot of potential for Radiant heat loss on a home.

Let's not be compromising on the floor sealing. It must be 100%, with durable material including careful and supported fit of Densarmor drywall over large openings.

Barrier and insulation needs in Alaska are like those in Sweden, source of Tenoarm, a real, thorough vapor and air barrier at interior surfaces. Even in Oregon, I apply 100-year vapor barrier, Tenoarm when I tear down drywall to add insulation in walls, or insulate any new construction. You will find superior construction gaskets too, in the Swedish technology.

I believe in hard-covering of attic floor insulation, with plywood. It is not only for access, although accessibility for service and inspection should be mandatory. Hard-covering all is rarely practical, and I want to throw down washable covering by good-quality (cotton?) sheets, where the floor is not decked. Thorough air-barrier-covering does more to protect loose-fill R-value, than covering by any insulation. No customer in Oregon has yet demanded the covering throws, but it will happen. I rarely throw away any insulation, but an happy to pack down the darned low-density loose-fill, to get space for real, compact insulation. Everything pie-in-the-sky for Oregon, should be common in Alaska.

So, take my father's place in Woodburn that has 6" truss chords, a mixture of batt and loose fill. You would cover the majority feasible with say 5/8" ply and cover the rest with sheets? Your suggestion for tougher barriers is good and I see a lot of new construction that merely feigns an attempt at a good barrier. Still looking for "recipes".

So

Why are we going to these extremes. Why not just six inch sips all around and put an end to it.

No recessed lights. incorporate the attic space as an HVAC area. Plenty of storage if you wanted an attic at all in that case. You can do anything you want to a home when building and it will not mach a timber frame and full SIPs.

Every home should be built as such. The idea that this is a horrible system and plagued with problems is silly. They have no more concerns than and well built structure and I put to you far far less concerns. Add high energy efficient windows and doors in. A candle or two to heat it. Leave the freezer open and a fan to cool it (obviously kidding on those). You are not getting any more efficient or strong a structure anywhere. Built properly and wired for future and away you go. You may spend more in materials but the labor is slight. You do not even have to go timber frame and still have a hell of a structure.

Just saying

If the current conversation is over treatment of attics in Alaska, Ted might best have the last word. SIP's are good advice. I've been engaged though in making a better photo album on treatment of truss attics, for Randy, and that truss attic, in Woodburn, Oregon.

Here is the album:
https://picasaweb.google.com/1085337...TrussAttic2012

I participate in informational moves away from the topic of home inspection, and think I do inspectors a good service in promoting accessible attics. Note: good lighting even, and power. I think access is consciously impaired sometimes, to resist inspection. I think a minimum of safe access must be in improved building codes, and must be part of any insulation job. Filling an attic with any insulation, especially loose-fill, above floor joists, must be accompanied by construction of some walk ways. If we accept that all insulation must be covered for effectiveness, even upon an attic floor, we will learn to deliver that covering.

Nice looking attic space, alright! Looks just like Dad's in the before pics. It seems like the age old issue of folks not wanting to spend the money to get it done right, and, perhaps, builders and home owners not being well educated on the proper process and materials to be used for long term success. I see all sorts of stuff, recent installations, done incorrectly and passing inspections. I get the "opportunity" to fix a lot of the stuff. This thread has been very educational, thanks to everyone for keeping it going.