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Cellulose Insulation In Attics With Foam Under Roof Sheathing

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Justin asks:

My home currently has spray foam insulation on all outer walls. (including under my roof in the attic.) I wanted to add blown in cellulose in the attic for more soundproofing from outside noise. Would there be any issues doing this? I contacted a local company for a quote, and they refused service saying that it would create a \”double insulation barrier\” and cause moisture issues. I cannot find anything on the internet that says this will happen, and I’m a bit confused now.

Would having existing spray foam, and adding blown cellulose cause any problems with moisture or mold in the attic? It’s a new construction home less than a year old. Is there any documentation on this i can read on?

Thanks so much for your time!!

Regards

Answer:

They may be thinking of the very real issues that can result when two vapor barriers are installed in a single assembly or a vapor retarder is sandwiched between two layers of insulation.  That sometimes happens when someone puts faced batts on top of existing insulation with polyethylene or batt facing against the ceiling drywall.  You propose to install insulation as sound absorptive material within the building thermal envelop.  That’s done all the time in floor/ceiling assemblies, especially in multi-family buildings. It should have been done in an apartment complex I lived in many years ago. There were four units – two up and two down – in each building. We could hear everything that went on in the apartment below ours. Interior walls and floor/ceiling assemblies in the 2010 NAHB New American Home demonstration home were filled with cellulose insulation for sound control purposes after the roof and exterior walls were insulated for thermal efficiency.


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