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I have an attic with blown in fiberglass insulation, probably original and 27 years old. The attic appears to have had some moisture issues in the past (bathroom vents being vented out of the soffit vents, which have been corrected) and the insulation is a little clumpy and a little dirty, but not terrible.

I want to add more insulation, but I'm not sure whether or not to use more blown in fiberglass or cellulose on top of it. I've read cellulose is the superior product. But I've read and saw videos of cellulose just being blown in on top of the fiberglass. I've read conflicting articles that says it's fine to do that and others say the cellulose will compact the fiberglass which isn't good for the existing fiberglass.

What should I do? Thanks!

Here is a pic of my current attic insulation:

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Add the cellulose right on top, I have seen folks remove the old but this is crazy both materials are insulation and combined the provide a higher R value. I would caution if all the moisture issues are not taken care of cellulose will pick up moisture and hold it longer than fiberglass.

  • Yep. I've done it in my own home. Any insulation will compress, and you just plan for it. – isherwood Feb 9 at 19:39
  • I'd like to "supplement" the current insulation with the cellulose. If it compresses, will the fiberglass lose it's existing R-value capabilities and thus require more cellulose? – mang Feb 9 at 19:45
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    Sure extra inches of fiberglass will also cause some compression, can't say how much but go ahead and see how it works. – Ed Beal Feb 10 at 1:05
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I would add more fiberglass insulation to the top of what you already have as it will not compress as much as cellulose does. But if you have clumpy fiberglass insulation in your attic now, you may want to check it with a moisture meter before adding anything on top of it. Also, cellulose is a dusty product, whereas fiberglass blows in pretty clean. I've been insulating for over 30 years and we have never added cellulose over fiberglass. Insulation works better when you add the same type to existing material. Just piling material up in an attic doesn't always make for a better insulated space.

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Thanks for all the help guys. I just blew the cellulose over the fiberglass and it seems to have worked just fine. Upstairs is definitely warmer. Just some cellulose install tips from my experiences:

  • It's friggin dusty. Make sure you put a tarp or something over your attic hatch once you're up there and tape it shut so the rest of your house doesn't have a layer of dust on it. Be careful when taking the hose back down as it will flake dust everywhere. If you have anything in the attic, take it out or cover it in plastic.
  • You will come out of there like someone dumped the vacuum cleaner bag on your head. Wear a long sleeved shirt and jeans, and a hat that you don't mind getting messy.
  • Get a quality dust mask. Get the best paper one you can find or maybe even upgrade to the fancy respirator type. My dust mask looked like a lint trap when I was all done. Don't forget safety glasses for the same reason so you can keep dust out of your eyes.
  • I had a cheap work light from Harbor Freight I wore on my head. That really helps as the light is always where you are looking. It gets so dusty that you can't really see that far in front of you, but this does help.
  • Buy more bags than you think you need. I wish I would have put more down. I know I was supplementing my existing insulation and not blowing in totally new, but still buy more bags than you think you need. You can always return them.
  • The blower you rent from the store isn't very good. I thought it would throw more insulation at a time than it did. It throws more dust than insulation. Lots of air. I had the slide on the machine pulled out to the recommended 1/4 of the way open.
  • Because of the blower being not very good, installation does take a long time. My receipt when I returned the machine showed that I had it checked out for 7 hours. I blew 20 bags of green leaf cellulose insulation and only had 2 or 3 ten minute breaks.
  • Get walkie talkies to communicate to the person loading the machine instead of using your cell phone. At one point, the hose came apart and it took me a minute or so to get my phone out of my pocket and call my friend to tell him to turn off the machine. Luckily it came apart in the attic. We tried leaving the line open too, but the reception was bad in the attic and the call kept dropping.
  • Do all of your bays first, then do the middle section. That way you can have your boards over the joists to walk for the majority of the install. Then you can move your boards out of the way as you blow the insulation into the middle part (that's also a pain).
  • Don't get lazy with your depth guides. Make sure you staple a bunch of them. More is better, and you can also cut out pieces of cardboard and use it for the same purpose. It's hard to gauge when you're not blowing right next to a depth guide, especially since it's so dusty. I know it's a pain to crawl back into the bays to staple them, but it helps.

Hope my experience helps others installing for the first time!

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I'm a insulation installer and any time you compress insulation you lose R-value our company I work for wouldn't blow over the fiberglass we would remove it, if it was the other way around you might be fine. But I personally would just add more fiberglass blow and not have to deal with removal, also fiberglass doesn't settle. You should have at least a R-45 in your attic the normal use to be a R-38 but new energy standard is a R-45. That should be around 22-24 inch hight depending on manufacturer. You should remove any fiberglass that has had gotten wet just to be safe.

  • Take it all out that costs and now add more sounds like a typical contractor answer insulation is insulation yes if compressed it won't work as well but take it out sounds like $ in your bosses pocket and a few extra hours work. – Ed Beal Feb 10 at 11:57
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I have also heard not to add cellulose over blown fiberglass because cellulose is heavier and will compress the fiberglass. When fiberglass is compressed it loses R-value (is a poorer insulator). Makes sense to me.

I think the best option would be to blow more fiberglass on top as it will not compress the existing stuff too much. I know Home Depot will sell blown in fiberglass and the machine so it should be about the same as doing cellulose yourself.

If you are set on blowing cellulose, you could probably put it right on top but just know it will compromise the effectiveness of the existing insulation. If you blow enough cellulose as if you had no existing insulation it will give you the R-value you need.

I agree that trying to remove the existing blown in will be hard and not worth it. Just leave what is there regardless of what you add on top.

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    Any new insulation will compress what's underneath. That's not a reason to not do it, and it's not a reason to not use cellulose. – isherwood Feb 9 at 19:39
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    @isherwood but cellulose is heavier than fiberglass. When you calculate how much of either blown-in to use, the manufacturer with take into account how much it compresses. Since fiberglass is so sensitive to the voids inside it (that is how it works as insulation), compressing it too much is bad. – auujay Feb 9 at 20:05
  • Cellulose is fine to use. I used it in my attic, the OP is asking if mixing them with fiberglass on the bottom is not recommended. I don’t have personal experience with mixing them but the argument for not putting cellulose over blown fiberglass makes sense to me. – auujay Feb 9 at 20:07
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    It may make sense but removing is silly, even if pressed to 1/2 it's original thickness it still has R value taking time and effort to remove it is a total waste of $ in my book. – Ed Beal Feb 10 at 12:00
  • Agreed, which is why I don’t think it is worth removing the existing insulation. I don’t know what R-value you would get out of the compressed fiberglass, I am sure it is greater than 0 but I think it gets compromised fast. The really conservative approach I suggested was to calculate the needed cellulose as if you were not getting any benefit from the fiberglass. It may be overkill but it is important to take some amount of compression into account. That is why my main suggestion is to just blow more fiberglass and avoid the issue entirely. – auujay Feb 10 at 15:19

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