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Adding Blown Insulation Over Existing Attic Batts

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Jennifer asks:

Can I install cellulose blown in insulation over batt insulation with tears in the vapor barrier? I was air sealing my attic and had to tear the vapor insulation to reach some of the bypassing. Do I have to remove all of the batted insulation or can I just lay the insulation on top.

Answer:

Fiberglass-Batt-Insulation-Section-Vapor-Barrier
If the vapor retarder is correctly positioned, that is under the insulation directly against the ceiling, you can blow insulation over the batts.  In fact, the batt and blow approach is a recommended and common technique for overcoming the endemic problems of batt installation. The fact that you had to cut the batt facing (vapor barrier you mentioned) suggests to us the batts may have been installed with the facing up.  If so, blowing insulation over the batts would compound an incorrect installation by sandwiching a low perm layer between two layers of insulation. If the batts are correctly positioned with the facing down and are in good condition, blowing insulation on top of the existing insulation is a good idea.

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2022-02-16 18:53:49
Ray says:

When my house was built in 1978, blown in insulation was used. Out at the end of the roof rafters where it meets the soffit, a 10 inch piece of faced batt was placed on it’s edge to block any air movement from the soffit into the attic. I had vented vinyl installed over the original wooden overhang with holes being cut into the original wood before the vinyl was installed. I also had a ridge vent installed when the new roof was put on. The construction foreman told me to go in my attic and remove these small faced batts so the attic would have proper ventilation. Should these small faced batts be removed? Thanks

2022-03-23 11:31:07
CIMA Admin says:

The first consideration here is the matter of “proper ventilation.” Some background should be helpful: Per most codes “proper ventilation” is 1 square foot of vent space (called net free vent area) per 150 square feet or attic space if this, this, and this is true, or 1 square foot of NFVA per 300 square feet of attic space if that, that, and that, is true. The 1 to 150 and 1 to 300 ratios are firmly imbedded in standard practice and are usually mandated by code. They must be based on hard science and experience. Right? Well, not exactly. About 25-30 years ago a building science professor decided to look at the literature and come up with the documentation for the ventilation requirements everyone knew were essential. All he could find was a 1939 study that recommended attic ventilation, but gave no specific requirements. The first reference to a 1 to 300 ratio was in a 1942 Federal Housing Administration document that gave no supporting references. Studies in the late 1940s were inconclusive as to what constituted “proper ventilation,” but by then 1 to 300 and later 1 to 150 were carved in granite, never to be disputed, until the 1990s when some well-credentialed building scientists began to question whether we should be venting attics at all. Most experts didn’t go to that extreme, but by the turn of the century there was general agreement that the simple 1 to 150 or 1 to 300 mandate wasn’t a good way to address a rather complicated subject. A paper by two leading building scientists published by ASHRAE in 1999 gave six recommendations for healthy attics. Not found on this list was a prescriptive mandate for a specific level of ventilation or even a mandate for any ventilation under some conditions.

Now to answer your question, since there is no generic answer to what constitutes “proper ventilation” the best course of action is to do what works. It would seem that the attic in question has been working just fine since 1978. The new roof and ridge vent have not materially changed the air flow in the attic. I don’t think there is any good reason to remove the batts, but I don’t think pulling them out would create any problems. I just wouldn’t go to the trouble of doing that.