I often hear people proclaim that a thick layer of cellulose will seal all of those air leaks below, thus eliminating the need to dig through, or remove, the old insulation before adding the new.  Unfortunately, 2 feet of cellulose is probably no better than 2 inches (my opinion) and even less so if it is over the top of an old layer of fiberglass.

If we look at a single 1/2" hole through the top plate, if it led to a 2' section of copper pipe filled with the same cellulose there would definitely be some level of resistance.  But above the hole, without the copper pipe, that air will diffuse in many directions, into literally hundreds of cubic feet of cellulose.  Even though cellulose can seem very dense, it will still provide an almost infinite number of parallel tiny paths that in effect greatly reduce the total air resistance.  And, if there is an old layer of fiberglass left at the bottom, that air flow is no longer coming from a single 1/2" opening,

Despite the inconvenience (and cost) of removing that old stuff and the effort to do a proper air sealing job, this is a once in a long time opportunity to access those leaks.  After 2' of cellulose is added, no one is going to dig through it to do any air sealing.  Today it's a best practice to air seal as much as possible.  Tomorrow it may be a requirement.


Tags: air, cellulose, sealing

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A long time ago in a galaxy far far away we did a bunch of field tests on this, maybe 1983 or 4.  This is what we did:

In perhaps 8-10 houses that had blown fiberglass, we did a blower door test, then brushed aside the insulation and sealed the top plates and penetrations with single part foam, then tested again.  In a typical attic of 1000 ft2 or so in the mid-Atlantic, we found we got a 200-300 CFM50 reduction.  

We did the same in a like number of cellulose attics and we got got about 50 CFM50 less difference on the average.  I think that shows that the cellulose did a slightly better job of sealing cracks and small holes.

I would never leave an attic unsealed and rely on the insulation to do the job.


Thanks for the reply Ed,

I agree that all cellulose is better than all fiberglass, it is the practice of adding cellulose over the top of existing fiberglass insulation that I feel yields fewer benefits than anticipated.  From what I have heard, never worked for the WAP, it has been a common practice for years to leave whatever is there in place and just cap it, faster and fewer materials, and lower costs is a strong motivator for them.  Since they do it, others have adopted this practice without consideration for performance.  Many of those others have moved from the WAPs to private practice.

The loss in performance comes in 3 categories:

1. Cleaning out the old makes it easier to do a better job air sealing.

2. Digging through the old leaves a mess and since all will be covered I'm sure they aren't concerned about being neat before the new cellulose is installed and neatness counts.

3. The benefit of air sealing only accounts for one component of the energy loss.  The mix of cellulose and fiberglass layers will result in a non uniform layer of attic insulation with a net lower r-value below that value implied by the total depth.

Add in that the old insulation left buried is often nasty in many aspects and home owners would be better served if that attic were cleaned and filled with all new.



Not sure I agree with you on this.

 The proper way to do this in my book is to brush aside the old insulation or move batts - regardless of what material it is - in the appropriate locations, seal the cracks, then move the insulation back to roughly it's original location.  We move aside blown material with 12" wide leaf rakes or scraps of 1/2 blue styrofoam that we use later to seal holes, then use the rakes to even out the blown, or use the rake handle to push down the edges of the batts to be sure they are making reasonable contact with the surface below.  Then blow over the top - always with cellulose unless someone wants to pay extra for fiberglass.

As you know, once you get over about R-35-40 (in my mid-Atlantic region) the benefits of additional material are very very small, so we make sure we shoot for R-49 because that seems to be the industry standard, knowing that if we get slightly less in some areas because fiberglass and cellulose might be mixed a little, that the loss can be measured in dollars annually, not tens of dollars.

Contrast that small potential loss to the cost of removing and disposing of the existing insulation, then paying for more material to replace it which might be in the several hundreds of dollars, sometimes close to a thousand dollars.

And as long as the nasty insulation is covered, what harm is it doing?

It all comes down to quality control of your team in the attic.


@ Ed "It all comes down to quality control of your team in the attic."

Doing the job right is definitely a key factor and assuming the existing insulation isn't totally disgusting it may be of some value.  But this is one of those areas where the quality of the job cannot be easily confirmed and whenever verification isn't possible, there will be some who will take the shortcuts.  Reports like, they only air sealed what they could easily reach from the attic hatch.  Or the contractor, back in the days when you needed at least a 25% improvement, who said he could make any home qualify.

I think my bottom line is, 12 inches of cellulose is better than 10 inches over 2 inches of old fiberglass and since no one will ever be going back to dig out that old stuff and or do more air sealing, despite some added cost, best to do it all at once.

I could only hope all the contractors in my area could stand at the level of ethics I suspect you hold yourself to.



That nasty insulation you are speaking of would be the squirrel,  mouse and rat feces / latrines which harbor deadly bacteria and viruses. Not counting the dead carcasses of all of the above. 

The dust from cellulose, fiber glass and the feces are constantly carried about the home on the air currents (drafts) causing a plethora of allergies and infections for all the occupants.

Removing the contaminated insulation is the ONLY solution then replacing it with a quality foam insulation.Foam does not settle, dust or harbor rodents. It does a complete air seal as well as being an apex insulation.


Any idea on how much time you spent with the can of foam air sealing the top plates and penetrations?

And what was the access factor to attic, both into the attic and in the attic (crawling, stooping, bending, on your knees, ect)



Degree of difficulty makes all the difference, and all of our auditors started out in attics so they understand this stuff to properly estimate time.

A 1000-1200 ft2 attic with no surprises and a soffit or two, 2 men, 2 hours = 4 man hours and it is ready for added insulation.

Complicating factors are - knee walls, pitch below 4/12, trusses making tight space, attic filled with ducts, zero clearance fireplaces with flue in the attic, etc.  In a worst case, it can take 12+ man hours to deal with everything in a 1000-1200 ft2 attic.

We use 16 pound cans of foam with 12 foot hoses and 21 and 36" long foam guns.  We just found a 36" gun with a 45°  bend at the last 6" which now makes those joints along eaves much easier to get to.

When we do this in new construction (we seal new homes as they go together) performing this task at the wall/ceiling joints (penetrations have been done because building inspectors want to see the work before drywall) it takes one man about 45 minutes in that 1000 ft2 attic - this is before insulation is blown.  We have tried everything to get around getting back up in the attic of a new house, but nothing works to our satisfaction - any suggestions??

What's your experience


I see a lot of fiberglass batts that were installed poorly in many ways.  The most common leave the fiberglass suspended some distance from the sheetrock.  Nailers, furring, the everyday bypasses we all see, wires, pipes, catwalks, etc.  all create small tents that fill with warm air, and the air moves.  Convective looping is the high brow term. 

Then there is wind wash through or under fiberglass batts along the soffit edge. 

Years ago I saw those flaws with IR from below and thought there is nothing in the attic only to enter the attic with a suspicious homeowner and find the floor is completely covered.  On one occasion I did throw the cellulose over it and look afterwards with the IR to find the same flaws persisted. 

Then I had one homeowner swear that adding cellulose over fiberglass made his house stink.

Now SPFA and others recommend removing all existing fiber or cellulose in an attic before you spray the roof. 

Too soon old, too late smart, I realize the stink is the leftovers from whatever inhabitants, bird , beast, or insect that were in the fiberglass for the past 20 years. 

There are a lot of reasons to pull out old batts before you add anything to an attic. 

Pat, we have been paid to remove old insulation in perhaps 4-5 attics out of the 10,000+ retrofits that we have completed,and this was always because we couldn't talk the HO out of it.  We have never had an odor complaint.  I would have to suspect that the odor complaint came from a dead animal as you suggest, but the odor got into the house because a crack or hole was not sealed.  Even then, the driving force for most of the year is towards the attic, not down into the house.

Bud, if 12" of cellulose is better than 10" of cellulose and 2" of fiberglass, why not just install 12" of cellulose over the fiberglass without the expense of removing the fiberglass?  This always assumes a sealed attic and batts pushed down for contact.

Even if it is crappy insulation, it is doing more good in the attic than in a dumpster.


Amen Pat!!!

Thanks for confirming Cellulose doesn't make a stellar air seal, always wondered.

Can't compete with those who've done 10,000 roofs and even make a living at it, but here's my ONE experience, which the pros from Dover might find amusing, if not informative....

Attic insulation can be covered with enough trash, splintered decking from re - shingling, animal wastes, water damage, to make you really wonder if leaving it is defensible health wise, much rather energy wise.  If these locked in wastes degrade indoor air quality in alot of homes being insulated without robust HRV systems being installed, codes might force the removal.  (I removed some, cleaned some, "tamping" and filling as needed to get the joist spaces completely filled)

When I started, the attic STANK of whatever the people under did..

When finished with insulating, not a smell in the attic, but occupants started leaving their windows open, negating heat savings from insulating.  (Years of "behavioral modification" has gotten past this - and yes, and HRV would help a lot, but that's about 2G for materials alone... someday.....)

Working conditions have ALOT to do with production and quality and from experience in other far more comfortable businesses, I can't imagine workers in attics always do what has to be done unless you all are using the most labor "sparing" techniques - like when the going gets tough, just spray foam all over.  (I worked under a 100 yr sea of roofing nails,  with a low pitch roof, and getting near top plates was suicide - I placed insulation from center attic with hoes and poles.)

Precision Air Sealing seems hopeless in a previously insulated attic, short of peeling most of it back so you're sure you find all the wall tops, lights, abandoned ceiling light JB's, pipe and wiring pentrations, along with holes for them never used, or abandoned, the odd missing piece of top plate, etc, etc....  I empathize with those that spray foam at perimeters and penetrations instead.  (I defaulted to installing tyvek over the whole attic floor and sealing down to the top plates, I felt this would also deal with inevitable air through cracking in the 100 yr old plaster.  Spray foam wasn't a good option as this was a two day at a time multi year task.)

Depth - It's got to be a tough market, reinsulating / insulating, if not impossible to get a client to go with 6 inches extra to compensate for the impossibility of a low bid installer getting all the details right.  It is possible when you can say, it's the code, nothing I can do.  Already codes are discounting "assembly" ratings, only "counting" the actual depth of insulation, and that's to compensate for the inevitable installation imperfections. (I lacked the money to pay for a first class job for an old house with many "issues", so I took the money and over insulated, to compensate not for skimping on time or details, but for not knowing the tricks of the trade)

Russell Higgins AIA


Ed, we do a lot with older farmhouses and more with 80+ year old general houses. In houses that old, the crack the intruders used has been well known for generations of their type and the trash they leave is significant. One church had 8" of bat poop throughout. Stuff like that has to come out, and in the case of the bat poop, it was done by someone specialized in that kind of work. When we foam a roof deck we take out the ballast below to avoid tht situation because most of the time if we are foaming a roof the atic is the mechanical room. That stuff can, and occasionally does, stink.

When we are simply doing an attic floor with cellulose, there is some consideration to partial removal. The decision then is partial vs total removal, not just burry it. That could have a lot to do with the equipment we use for blowin in though. I can easily blow1500 sf of R-49 cellulose in an hour. It is much more time consuming to try to lift a batt, fluff it properly, then reset it so it is in complete contact with the attic floor. During the fluffing the batt can get too lofted, and the weight of cellulose will not compress it uniformly or only to the proper density. Often the batts in 1940's and older houses are rock wool, which disintegrates whenever you touch it. I can spend time trying to put it back properly, or I can take it out and install cellulose without any concerns.

I think we aren't talking about the same situations because our generic housing stock and SOP's are different.


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