Is it safe to use faced insulation between internal walls/ceilings for soundproofing?

Submitted by Kortalh t3_xx8ifg in DIY

Hi All,

I've been working on converting my unfinished basement into a home theater & guest room, and I'd like to add some level of sound reduction between the basement theater and main level, and between the theater and guest room.

Due to various factors (particularly HVAC duct placement) it's not practical to have an ideal soundproofing system, so spending top dollar for Rockwool doesn't seem like a good investment.

My understanding is that fiberglass insulation is a "good enough" alternative that provides some amount of soundproofing, which should be sufficient.

The total area I need to cover is about 930 square feet. Based on local supply costs, the rough prices would be:

  • $550 for faced rolls of fiberglass insulation
  • $5,160 for unfaced batts of fiberglass insulation
  • $11,160 for rockwool

Spending $550 is a lot more appealing than $5,160 given that the product is effectively the same.

My main concern is moisture. The basement gets humid in the summer, and we have a dehumidifier running regularly. If I use a faced vapor barrier between the walls & ceiling, will I be causing humidity issues in the basement? And could this lead to mold or rot?

Thanks for any insight!

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pm_me_broken_stuff t1_irb4q75 wrote

I'm super confused how/why unfaced insulation is almost 10x the price. Are you sure you were comparing similar products?

Worst case get the faced insulation and pull the facing off it. Boom, unfaced insulation.

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Kortalh OP t1_irb82mt wrote

As far as I can tell, yes -- R-13 fiberglass in both cases, same brand. I've checked the 3 major big-box stores and they're all similar. My only guess is that, since faced comes in compressed rolls and unfaced only comes in uncompressed batts, the cost must be due to the extra price of shipping.

Pulling the facing off is definitely an option. Not super appealing, but given the price difference, I'm willing to do it, ha.

But I'd rather not if I can avoid it, especially since it's a lot easier to staple in with the facing; if faced won't cause any moisture issues, that is.

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strangr_legnd_martyr t1_irb9dym wrote

Make sure you're reading your product descriptions closely.

Just a quick search at Home Depot says $18 for R13 15" x 32ft faced insulation. That's $18 for 40 square feet, or $0.45/square foot.

Same site says $634 for 10 bags of R13 15" x 93" unfaced insulation, but unless you read closely you won't notice that each bag is 11 pieces, not 1. So that's 106.56 sq ft per bag, 1065 square feet total per pallet, which gives $0.60/square foot.

It seems faced is still cheaper, but not 10x cheaper. Probably because the faced stuff is a regular stock item and the unfaced stuff has to be shipped in.

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Kortalh OP t1_irbcpvx wrote

Doh, I can't believe I missed that. That makes a lot more sense. In that case, I'll just go with unfaced and not take the risk of moisture issues.

Thanks!

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noreasters t1_irbqfo3 wrote

Add a second layer of 1/4” drywall (with caulk or sound dampening epoxy on top of old drywall).

Mass slows sound wave propagation and drywall is more dense than insulation, adding the material between old and new drywall forces sound waves to energize both boards with little direct transfer of that energy…highly effective.

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elemenohpee t1_irclroc wrote

Why only 1/4"? You're trying to add mass to the wall, so wouldn't 5/8" be more effective?

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noreasters t1_ircm7mt wrote

The real deadening is in the slowing of the sound waves.

Sound waves must vibrate the material to pass through.

2 layers of drywall with flexible material between means sound wave must vibrate outside panel with inside panel resisting and the filler material acting as a spring damper…BEFORE the sound can begin to vibrate the second board, now fighting the motion of the first.

After the sound passes that material it will have MUCH less power and ability to travel distance or vibrate other walls.

Edit: this means 1/4” drywall, or most any hard material, can be used; 1/4” drywall is cheap and relatively easy to work with.

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sdfree0172 t1_irboesc wrote

Your numbers are off. Probably misread the quantity per role or something.

Between conditioned rooms, it doesn’t matter which way a vapor barrier points - wall can still breath. If going up against an outside wall, it matters a lot and varies by where you are in the country.

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