damp spray cellulose vs fiberglass batting

Hey guys, a newbie with my first question I am close to starting a home building project in central Virginia, and with a little luck there will be some money left at the end so I can start phase 2 - a decent home studio in the basement. So there are a couple of things I want to get right if I can during phase 1, the building of the main house, so that phase 2 is set up for success. For the purposes of this discussion, I am primarily concerned with reducing sound leakage to the outside so as not to disturb my neighbors. I will at times have full rock bands in the live room for recording or rehearsal, so I'd like to have as much isolation as I can reasonably afford. The neighbors are not too close, maybe 200 or 300 feet away, but its out in the country so sound carries, not a lot of background noise out there. I hope to have a good sized live room and separate control room in the basement, all the details of those to be provided later when I get to phase 2 and ask a million questions. For now I am focusing on the exterior basement walls -- I will get to the interior control room walls in phase 2. For phase 1, I will have a poured concrete foundation wall. I am planning to use that wall as the first layer of mass, then a one inch air gap, then a 2x4 framed wall with two or more layers of drywall. The size of the air gap can still be changed at this point if that helps, we're still in design phase. Question 1 - assuming that I want my thermal insulation to provide as much help as possible for my sound isolation goals, am I generally better off using fiberglass batting or wet/damp sprayed cellulose between the concrete wall and the first layer of dry wall ? My house designer is pushing for the wet spray cellulose, she believes its better for both thermal insulation and sound isolation [fills all the nooks and crannies, more dense], but I can't find a lot of info on the pros and cons, other than this post by Rod Gervais on this forum in 2009. viewtopic.php?f=2&t=13712&p=95098&hilit=wet+spray+cellulose#p95098 Looking for any more information on why one is better than the other, and why related Question 2 - it is my understanding that its ok to have fiberglass batting completely fill the air space between the two mass surfaces in this scenario. Is it ok to do the same with wet spray cellulose, or is it better to only spray one wall surface and leave an air gap ? if it is better to leave a gap, how thick should the wet spray cellulose be applied. Please let me know if I have left out any basic info needed to provide a helpful response. I've been reading posts lately, but still definitely a newb. thanks tim

Hi Tim. Welcome! :) A couple of comments, no particular order:

For phase 1, I will have a poured concrete foundation wall.

And a separate poured concrete slab too? In other words, the foundation wall, a small gap, then the slab? Is that what you have in mind?

I am planning to use that wall as the first layer of mass,

Then maybe I'm misunderstanding what you mean by "foundation wall". To me, a foundation wall is just that: a low wall built as part of the foundation, on top of which you then build your actual walls. But it seems you are referring to an actual entire wall of poured concrete, floor to ceiling?

The size of the air gap can still be changed at this point if that helps,

If you are talking about the air gap in a two-leaf MSM structure, then yes it does help to increase the gap. It lowers the MSM resonant frequency, and improves isolation.

am I generally better off using fiberglass batting or wet/damp sprayed cellulose between the concrete wall and the first layer of dry wall ?

I reckon I'd go with the fiberglass batting. Especially after reading Rod's comments on the potential problems with flanking. It doesn't take much to cause a flanking path, and I don't see how you can avoid it with the sprayed cellulose, no matter how careful you are when spraying.

it is my understanding that its ok to have fiberglass batting completely fill the air space between the two mass surfaces in this scenario.

IT is more than OK! It's an excellent way of increasing your isolation. The difference between two identical walls, one with an empty cavity and the other with the cavity filled with insulation, can be as much as 16 dB. That's huge! (Partly filling the cavity will give you around 8 to 10 dB over the empty wall.)

Is it ok to do the same with wet spray cellulose, or is it better to only spray one wall surface and leave an air gap ?

I don't know how you'd be able to do that! How can you prevent it from creating flanking paths? If you were only looking for thermal isolation, then that cellulose would probably be fine. But your issue is acoustic isolation, which is a whole different ball game. And I don't see why using ordinary fiberglass or mineral wool would not be a good option for thermal insulation anyway. - Stuart - - Stuart -

Stuart -- thanks for the quick response. here are some follow questions and answers yes, the poured concrete wall is a full wall. it is both the foundation for the house and the full wall of the basement. Poured concrete floor as well. re the air gap in a two-leaf MSM structure, how do I determine the optimal size of the air gap ? I'm not sure I understand the flanking issue wrt wet spray cellulose. Seems like the key phrase in Ron's post is "in double frame walls it can only be used in the outer most face (spanning the air space will couple the wall frames) and it is virtually impossible to do the inner face with the air space intact." Seems like he is saying that wet spray cellulose is too dense to fill the entire space between walls [as you would with fiberglass] -- it significantly reduces isolation in that case because it will transmit sound between walls, not block it. Ok, I get that. But not sure why it is "virtually impossible to do the inner face with the air space intact". Admittedly I don't know anything about spraying on cellulose, but I'm not clear on why that would be difficult. Seems like it would be straightforward to spray it on the concrete wall, leave an air gap, then put up the framed 2x4 wall. If this isn't ridiculously hard to do, then it seems the wet spray cellulose offers an advantage in terms of coverage and sealing any air gaps when compared to fiberglass batting. So it seems it would be possible to spray on enough cellulose to give you your desired thermal insulation [eg equal to R19 fiberglass], then add your air gap and second wall. if anyone has more info on why its hard to do that kind of wet spray cellulose install, that would be helpful thanks tim

slight update -- looks like the plan would be to put a fine mesh web behind the 2x4 framed wall, then spray the cellulose on to that to fill the space [approx 3.5"], then close the other side with dry wall. No spraying of cellulose onto the concrete wall. So seems like there would be no issue with cellulose intruding into the air gap between the 2x4 wall and the concrete wall and creating any flanking issue since it would be constrained by the mesh. Does that make sense ? Looking for feedback so I can have the designer update her plans as needed.

Tim, I assume that if you install the netting properly - and it's fine enough - you might be able to maintain the air space - at which time the bridging issue would be gone - however I do not see any real added benefit from an isolation point of view with this material - although there is a huge insulation benefit - so if you want to spend the extra cash just go for it - although I don't know that in a basement you will realize enough of a energy savings to ever pay for it - but there is no advantage related to isolation. I made a factory rep meet with me on this issue once - and when they claimed that it was better than simple fiberglass insulation I made them produce the actual test results in their entirety - so not only the TL values but all of the other pages that go along with it - and imagine my surprise when I found that they use a very small wall sample - one without seams in the field (a 4'x8' sample versus the 10'x8' samples tested in IR761) which skews the results in their favor. If you examine IR761 you will find this insulation used in a number of their wall tests - no advantage...... What you do is your choice -but me - I would never throw my money away on this material for isolating walls........ Good luck with your build, Rod

Rod, thanks very much for your response. I guess I'll need to look at the cost numbers and determine whether there is sufficient thermal insulation benefit to justify the additional cost if I were to go this way. tim

timryan257 wrote:

Rod, thanks very much for your response. I guess I'll need to look at the cost numbers and determine whether there is sufficient thermal insulation benefit to justify the additional cost if I were to go this way. tim

Tim, just remember - the majority of the thermal loss in a basement wall is in the top couple of feel of the wall - the backfill against the wall outside is a pretty decent thermal barrier all by itself (which is why even here in New England where we have pretty cold winters - the basements in heated structures tend to remain at or slightly above 50 degrees F even if they aren't heated.) So the real question when it comes to something like this is exactly how much additional energy am I going to save using this material versus standard insulation. The difference between the 2 on a framed wall exposed to the elements is large enough that the payback (of the difference in insulation costs) in energy savings is made in a relatively short period in time. However the biggest bang for the buck there is the total seal that creates not only an efficient thermal barrier - but a complete air barrier as well - and a backfilled wall is one hell of a major air barrier. You have to look at these things very closely - have your insulation contractor provide you with a long term energy analysis - to determine whether or not like this makes sense. I have a project going on right now in a large commercial building (nothing to do with studios - just new boilers/ heat pumps/air conditioning and replacement of light fixtures for new energy efficient lighting) and the building has just 5 1/2" (so R-19) insulation in the ceilings..... we looked at beefing that up to R38 and found that the owners would never save enough energy to pay for the upgrade within the life of the equipment we were installing - so even though it would save quite a bit of energy it just doesn't make sense financially. Good luck, Rod

Rod yes, good points. which brings me to point out that I forgot to mention a key item -- when I said "basement", I didn't really define that. Probably relevant to this discussion that more than half of of the exterior wall space in question will not be up against backfilled earth, as this will be a walk out basement. So thermal insulation effectiveness is more important than if it were a completely below ground basement. I'm still working on getting some comparative installed cost numbers for the damp spray cellulose vs the fiberglass, but if the additional cost for the cellulose is not too great, it seems like a better solution all around due to the better air seal and thermal insulation, assuming that I can have it installed in such a way as to avoid the potential bridging issue raised earlier. thanks for all the feedback, it has been extremely helpful tim