"Oh we have done it that way for years", "I'm a yankee at heart", "waste not, want not", "you're wrong, by compressing it you are increasing the R-Value", "I hate to throw away a good product", etc... I think I have heard every excuse in the book & it really does need to stop
First by compressing the batts, you are decreasing the effective R-Value (Yes, the proof is actually up on most of the manufacturers web sites)
Second if you are doing the job properly, you have already had to pull up the batts, cleaned the area & air sealed it, so you actually waste more time installing it properly than you would spend for material
Third, along the lines of properly installing it, are your installers going to take that time, or might you end up with voids where the FG isn't fully in contact with the Sheetrock (my area) or Vapor Barrier (up borth)
Fourth, if the material is still in good condition, why not donate it for use on a charity project
Just my .02 - anyone else?
When you condense fiberglass batts the overall R value decreases but the R value per inch increases. Compressing also decreases convection. Where compressing the batt is a problem is in the walls. When you see areas pressed below the stud surface you know that you have created an air path that will kill the installed R value. Its the old 6 sided rule.
My thoughts are that if they are going to be topped with cellulose then compressing them is not harmfull and actually beneficial.. For the most part you are going to know where the air leak are - top plates, light fixtures, plumbing, soffitts, etc. Those are the areas to pull the insulation back. Push it back in and it is better to compress to get full contact to the ceiling and joists. Cellulose over the top will create that monolithic layer that has a lower convection factor and higher insulation value.
When comparing fiberglass and cellulose in an attic the cellulose will have lower air permeability. Cellulose R value is less degraded by wind washing and convection than is fiberglass. When you top fiberglass with cellulose you make the fiberglass perform better as the cellulose blocks the wind washing and convection. The settling of cellulose is actually beneficial as it improves the Resistance to convection and wind washing.
If you look at high density fiberglass batts they have a higher R value per inch because they are denser. Compressing a standard batt increases the density per inch. New dense pack fiberglass like spider has a higher R value per inch than loose fill. Dense pack fiberglass performs more like dense pack cellulose in that is cuts down on internal convection and well leakiness into the cavity.
Niether fiberglass or cellulose is a radiant barrier but cellulose blocks radiant heat flow better than fiberglass. An often overlooked factor for attic insulation.
This is a complex issue and I don't believe there is an absolute, one size fits all, solution.
Robert is correct regarding the compression issue, not an issue. The issue of full contact with the ceiling below is debatable, particularly in the northeast where we use strapping to hang ceilings. And if the batts are humped up over wires and vent pipes etc. then it gets harder to reinstall properly.
A major issue is how hard is it to get the material out of the attic. If the only access is through a living space you have to bag in the attic, in bags that fit through the often tiny scuttle, and carry them out. It can be quite labor intensive.
As to donating it, if it isn't good enough for you why would you give it to a charity project where it is less likely to be installed properly? Our habit of saddling low income people with high energy costs is a social issue but I think it needs to stop.
Often I find the determining factor is vermin control. After years of seeing lots of attics I have noticed this trend: Fiberglass attics, lots of vermin. Cellulose over FG, lots of vermin. Cellulose only, no vermin. I'm sure that there are examples of vermin in cellulose but I have never seen them.
Bottom line? Good judgement still matters.
Bruce Harley's well accepted standards for grading of insulation include that compressing an R-13 wall batt by one inch decreases its R-value to R-10. Doing the math shows that the uncompressed batt has R-3.7 an inch, but the compressed batt is at R-4 an inch, higher. The problem is that the increase happens slower than the thickness loss, therefore an overall loss and added convection in the wall cavity to boot. So compressing the batt by an inch then adding more material makes the total R go up - an R-15 batt is denser than an R-13 is denser than the old R-11 batt. The problem, as has been stated, is the air pocket that is left below the batt by wires, etc. But it takes MUCH less time to get rid of most of that than it does to take the batts out. We leave them, push them down with a stick, and blow over them. And you guys in New England shouldn't be strapping ceilings anyway - costs money and material, energy for the life of the building, and has no purpose.
I'm doing what I can to keep builders from putting strapping in but it's an amazingly tough sell. I talked one guy into it but when the drywall guy showed up he went ballistic. He had ordered his sheets to go the other way, he assumed that there would be strapping. Lots of yelling ensued, the builder concluded that it was a dumb idea, "Ya gotta put strapping, my daddy did".
Of course there is that fact that all the existing buildings have strapping, that's not coming out anytime soon. That's why I only recommend people who own blowers. No batt product is going to deal with that.
The new Energy Star standards include mandatory sealing of the wall/ceiling joint (top plate/drywall joints from the attic) in new houses. The strapping technique makes these joints leak even more because the installer is even less likely to nail near the top of the sheet. Of course, in a retrofit, try to get the blown fiberglass away from these joints for sealing in a graceful and efficient manner - impossible. I have been told by a builder (Pittsfield MA) that it evens up any unevenness in the joists, but it doesn't, and just laying them out on horses and graduating the crown during installation does that better.
We have done several projects with this issue; as I see it, the best answer is to gets the old material in halfway decent shape, then blow over the top. (Actually, in the MF buildings we do, this is frequently going over blown FG, not FG batts, but the issue is the same.)
1) The FG has value where it is.
2) Removing it is labor-intensive, dirty, and has at least some associated hazard.
3) If we pull it, it won't get re-used, it'll get landfilled. Then it has no value to anyone.
Interesting points all & thanks for jumping in. I do have one quick overall point & a few corrections I see that should be made – for everyone that believes that compression is good, can you explain why all the FG manufacturers don’t promote that line of logic?
RobertH – quick point, the high density FG is made from doubling the fibers per inch, not from compression.
Bill – I wish I hadn’t lost my last camera - birds nest and mouse droppings everywhere in a cellulose insulated attic, the issue isn’t the material but the overall air-sealing & workmanship (this had full chases open to the crawl space, basement, etc… I love the Bottom Line point you made
I am not a fan of fiberglass particulay batts. If you have batts in the attic and they havent been infested with bugs or animals then I think there is some value in reusing. You want to get the most out of them if you are going to reuse. in this situation you need a working understanding of what you are trying to accomplish. Normally you want to avoid compression but in this case I dont think you need to worry about. In this situation you want to avoid voids along the bottom and sides and compression will not penalize you.
In walls compression results in voids and promotes convection which severlycomprimises the performance. Now if you take a batt for a 2x6 wall and insert is in a 2x4 wall as long as there are not voids it wil work but I would not recomend it. The R value will be less than if installed in a 2x6 wall but would perform better than a standard R13 batt. Again I would not recommend it. I think you are better off using a bibs or dense pack. Batts are for DIYers that are fooled into thinking they are doing a great job. I have read that 40% of stud cavinties are not the standard 16" width.