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Location: Visitation between Wal-Mart & Home Depot
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Quote:
Originally Posted by destiny826
Is it okay to have blown cellouse on top of fiberglass insulation in the attic?
My way of looking at it is that 18" of insulation is 18" of insulation regardless of what's on top and what's on bottom. I'm not aware of any chemical or physical incompatibility between paper and fiberglass. I'm not convinced that cellulose is all that great for my climate, but it sure as hell is a lot nicer to work with. If the expense and effort of removing the old fiberglass is substantial, I would weigh that carefully and plainly ask myself and/or my contractor what good it would do.
Don't forget the rest of the job:
Be sure that the eave vents don't get blocked (they may even be blocked now).
Those foam things are fine or make up something of your own. Just be sure it's there.
Know that the rest of the attic venting airflow is adequate (volume) and functional.
Insect screens, motors/controls for vent fans, etc. Consider putting the ridge vent in now.
Fix your bathroom and kitchen venting.
I'd bet that 90% of homes don't have this done right.
Identify where ALL of your electrical junction boxes really are.
Something as simple as some masons twine held tight from the box to the rafter above will do.
If you contemplate adding a ceiling fan or would really rather having the light/fan controlled from a switch... or you would like some more CATV outlets... or stereo speakers... whatever... do that work or at least the prep for it now.
Make doing all this stuff and any future work easier by installing a catwalk.
Nothing fancy is needed but 2 sheets (5/8" osb? mdf?) ripped to 24" will give you 32 feet of KNOWN solid footing up there. One strip down the center should be enough (you can put insulation on top of it too)
They are not incompatible, but why cellulose? Cellulose eventually breaks down and turns to dust which ends up in your house. I've been fighting dust for years because of this. The original insulation in my home was cellulose. I made the mistake of leaving it and going over it with fiberglass. I should have removed it altogether (expensive) and would have spent the money if I could get rid of all the dust.
Is it okay to have blown cellouse on top of fiberglass insulation in the attic?
Yes. The advantage of the blown cellulose is that you will be able to cover the areas where the fiberglass has settled, extend that well over the ceiling framing,and stop the thermal bridging. If your upstairs closets are cold in the winter or hot in the summer when the rest of the house is not, it is likely you have thermal bridging in your attic.
Those are very solid links above, but I do agree with others that stress the various differences in the long term insulation stability / qualities.
I have been in MANY attics of homes I have owned and have been shocked at how ineffective both the venting and insulation can be -- settling, moisture retention, wildlife infiltraton are things the government labs / university researchers take as an afterthought. Real world it is far and away NICER to have a well sealed attic carefully insulated with quality materials. From a "neatness" perspective that makes the additional cost of fiberglass batts something to seriously consider. If you can justify the even higher coast of foamed-in-place insulation that is even better -- air sealing is just so much better!
If you do use cellouse you really ought to make the effort to get EVERYTHING sealed up -- any moisture / air paths needs to be fully addressed. The fact is the blown fiberglass is not as good as actually keeping its "fluffiness" and R-value declines over time but it also very moisture resistant. It really shows in climates in the midwest where you do have wet spring/ humid summer / dry fall / bitter cold winter....
Proper attic ventilation and roof sealing should address the problem of moisture. I've never heard of people experiencing dust inside of a house due to attic insulation. If someone has a HVAC unit up in the attic he/she must be changing the filters as required or a tad more often. I don't see fiberglass bats as being able to effectively address thermal bridging when compared with blown cellulose or other such products. Spray foam is another option but it will run about 2-3x as much in price.
I put in an attic heat pump for the upstairs zone and had the blown cellulose heaped over that already insulated ventwork so as not to lose efficiency.
But it is necessary that you be able to move a sufficient volume of air through the attic on a regular basis. Often this can be accomplished by convection alone if the eave vents are sufficient in number, placement, properly sized and unobstructed... and you have an outlet for all that hot air to escape through near the top of the roof.
Batts /blankets of fiberglass run OVER existing attic floor joists PERPENDICULAR to bays...
By lining up the material 90 degrees to the existing insulation you effectively covers all the joists in the whole area. Thermal bridging is all but eliminated.
It costs more, but by using 8" nominal thickness unfaced batts / rolls of high density fiberglass you end up ADDING R30 to the attic with is much neater than blowing in additional insulation... Unless you are 100% sure you have sufficient airflow and ZERO water infiltration this is my favorite way to add insulation to an attic. Payback in pretty much any climate where you run a furnace or air conditioner for more than three months of the year is VERY short -- typically less than five years.
Quote:
Originally Posted by lifelongMOgal
Proper attic ventilation and roof sealing should address the problem of moisture. I've never heard of people experiencing dust inside of a house due to attic insulation. If someone has a HVAC unit up in the attic he/she must be changing the filters as required or a tad more often. I don't see fiberglass bats as being able to effectively address thermal bridging when compared with blown cellulose or other such products. Spray foam is another option but it will run about 2-3x as much in price.
I put in an attic heat pump for the upstairs zone and had the blown cellulose heaped over that already insulated ventwork so as not to lose efficiency.
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Batts /blankets of fiberglass run OVER existing attic floor joists PERPENDICULAR to bays...
