Blown-in fiberglass on top of batt insulation in attic
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It’s my understanding that batt insulation is usually not recommended for attics because it can leave a lot of gaps.
What about if there is fiberglass batt insulation, and then blown-in fiberglass on top of it? Would this work okay, or is it recommended to remove the batt insulation and just use blown-in insulation?
The photo below shows a thermal image of our ceiling with only R11 batt insulation installed. Blown-in insulation will be added on top.
I’m trying to determine if using blown-in insulation is going to adequately fix all the thermal voids. The attic has been air sealed.
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Replies
There is no issues with blown in on top, it is done all the time and it works.
The important part before any blown in is to air seal. This is near impossible to do afterwards without taking all that insulation out.
You want to pull back the batts and air seal any light fixtures and wall top plates (both interior and exterior). Sometimes older houses have framing that is completely open to the attic especially if there are ceiling height transitions, these are the big ugly details you want to seal up. Since it is cold and you have a thermal camera, you should be able to find most of them.
Once you have a nice air tight lid on your house, you can blow in the insulation to top it up to the r value you want.
As Akos mentioned, you can blow in additional insulation over batts without trouble and it's done all the time. You can blow cellulose or fiberglass over the top of the batts. Batts are fine if they are installed well, but attention to detail is sometimes lacking and thats when you can have problems. Loose fill (blown) insulation can fill in any gaps and put a blanket of additional R value over the batts too. Note that you can also roll out unfaced rolls of fiberglass insulation over the existing batts and perpindicular to them which will also help if you have the access. For small areas, insulation rolls are often simpler than bringing in a blowing machine.
Bill
Thanks. Do you know if one inch of blown-in fiberglass insulation provides the same R value as 1 inch of fiberglass batt insulation?
Our fiberglass batt insulation is a little rough and there are a few areas where batts are missing. I'm wondering if adding blown-in on top of all this will level everything out and give the same R value, or if I should fix the areas missing batt insulation then do blown-in
Loose fill fiberglass is slightly lower density than high density fiberglass batts, which means a bit less R per inch. I have no idea what density batts you already have though. My guess is you have low(ish) density batts, which was common for older batts, so you'll probably see similar R values between those and the loose fill stuff.
The issue you'll have is you'll never get an even 1" layer of blown fiberglass installed. I would try for a minimum 3" thick layer, since it would be really difficult to do a decent job of covering everything with a layer much thinner than that.
Bill
Thanks. I was just using 1 inch as an example. The batts are R11 and R27 is being blown-in on top of that.