air gap or more insulation?

I have several inches of space between leaves, enough to have an air gap even with appropriate insulation attached to each leaf. I was wondering if it was better to leave an air gap or just fill it in with fluffy insulation. I think I understand that an air gap would be better, but I'd like to be sure. Thanks! :mrgreen: :mrgreen: Scott

Air is cheaper. As long as both leafs are well deadened with the insulation, I don't think the added insulation will give you much bang for your buck.

plus6vu wrote:

I have several inches of space between leaves, enough to have an air gap even with appropriate insulation attached to each leaf. I was wondering if it was better to leave an air gap or just fill it in with fluffy insulation. I think I understand that an air gap would be better, but I'd like to be sure.

What means "appropriate insulation"? It's a non-linear process (the first bit helps significant more than the last bit), see also lilith_envy But that an airgap, rather than fluffy insulation, should be better is not true.

Eric_Desart wrote:

What means "appropriate insulation"?

The exterior wall has 3" of insulation already attached and the interior wall will have 6" attached. With that much insulation, there will still be about a 3" air gap. I priced batted insulation and "blow-in" type insulation yesterday. With the price difference, I can buy the blown-in type and completely fill the air gap for about the same price or less and it might be easier and faster to install. I'm just wondering what would be best. Thanks for the help! Scott

So Eric you think that (lets say he has 16mm plaster board on both leaves) MSaSM with a STC of 63 could be improved by MSSM (filling the gap) by how much? 1 or 2 dB. Even spray insulation would cost about $20 sq/m. How big is this wall? And more importantly is there a door or window between, that will the weakest link. Maybe the comment should be "air alone is not better than fluffly insulation" :D

lilith_envy, My studio is a lot like yours in that it's based on a steel outer structure. So the outer leaf is the steel and the inner leaf is drywall. I remember seeing your thread some time ago, but I don't know if I've seen all of it and I'll be going back to review and see how many ideas I can steal. :mrgreen: This wall is about 25 ft long by about 10 ft high and has 3 windows and a door. I'll be working on solutions to make plugs for those if the need arises. I'm a bit in the country, so noise is not generally an issue, but I may find that could change once I start recording in here. Blown insulation here is about 1/2 of what batted insulation costs. That's why I'm thinking of going this route. Besides, the last exterior wall I did I had a little trouble keeping the insulation close to the inner wall since there was nothing to put pressure on it from behind. This time I'll have a solution for that if I used batted insulation. Scott

lilith_envy wrote:

So Eric you think that (lets say he has 16mm plaster board on both leaves) MSaSM with a STC of 63 could be improved by MSSM (filling the gap) by how much? 1 or 2 dB. Even spray insulation would cost about $20 sq/m. How big is this wall? And more importantly is there a door or window between, that will the weakest link. Maybe the comment should be "air alone is not better than fluffy insulation" :D

I hope to respond to this without offending Eric - I say this because the question was addressed directly to him and not the rest of the world. Eric is certainly more qualified to answer anything regarding acoustics than I - but this one I (for some reason) can't seem to ignore. Lilith, First off - spray insulation is no great friend of isolation - filling between leaves with it creates a flanking bridge between the leaves.... which is a bad thing - (which is why it is scrapped back beyond the face of the stud when used for acoustic purposes) then you take into account that it is many times the cost of cheap fluffy insulation - AND then add to that the fact that you can get slightly better results with fluffy insulation than with the spray (in general) - and you quickly come to the conclusion that the spray isn't really worth consideration (heck even if the costs were the same it wouldn't be). One thing I point out to you is that your presentation of this is inaccurate - you present the air space in question as this MSaSM In fact - the air space is the entire region from back face of wallboard to back face of wallboard......... the insulation is present within the entire air cavity, creating the effect of a deepening the air cavity - it is not a substance that exists outside of that cavity acting on it's own. Any way you look at it - without adding another physical leaf - it is still MSM. Eric's response to the original poster was a question that indicated he did not yet have enough information from the original post to formulate an intelligent answer......... He then pointed out that he supported your response in (in general terms) although your response was perhaps a little too concrete without more data. Now that the original posted has responded that the air space is 3" even after filling the stud bays with insulation (the depth of the studs themselves)...... then answer is then 3" of additional insulation (the fluffy fiberglass kind) would be a fairly inexpensive way of achieving some additional isolation without the need of making expensive changes to the wall itself. To support this I would point you to read the following test id's in IR-761. Specifically TL-93-261, TL-93-265 & TL-93-263 All 3 walls are constructed with the same frame construction (double walls with 90 mm studs @ 406 mm on centers, with 16 mm gypsum type CX on the outermost faces. 261 has no insulation in the wall cavity - 265 has fluffy fiberglass in one cavity bay only(90 mm) - 262 has fluffy fiberglass in both cavities (90mm). for 261 STC 45 for 265 STC 55 for 262 STC 58. Now - although I will grant you that it is the case (As Eric stated - and you suggested) that the largest gain for the buck is the initial investment (one bay in this case) for a gain of 10 STC points - with only an additional 3 STC points for the same material added to the 2nd bay...... I would point out to you that the real analysis is not in the STC value - but rather in the individual frequency ranges and what you gain or loose dependent upon what your needs are. Within that 3 STC point difference between 2 walls with insulation versus a 2 walls having only 1 of the 2 insulated - one loss is about 1 dB at 50Hz,,,,,,,,,, but a gain of 5 dB @ 500 Hz and 8 dB @ 1250 Hz. Some real gains exist around the coincidence range of the wall - for example - 17 dB @ 2000 Hz, It is this information that becomes meaningful when deciding on which course you are going to proceed on (at least from a design point of view). When Eric speaks - one does well (meaning they can learn a lot) if they become thoughtful about what it is that he is saying - he's a fairly brilliant man - and anyone who wants to become knowledgeable in the field of acoustics would do well to pay attention to his questions as well as his answers.. Sincerely, Rod

Scott - it appears to me that the insulation in the cavity between the outer steel and the inner drywall is wasted. The outer steel would not be airtight therefore it can't be considerd as a skin. The place for the insulation is in the airspace between the two layers of drywall.

So Eric you think that (lets say he has 16mm plaster board on both leaves) MSaSM with a STC of 63 could be improved by MSSM (filling the gap) by how much? 1 or 2 dB.

You should get a much better improvement than just 1 or 2 dB! I'm looking for the research paper where I have the exact figures, but I seem to remember that the difference between an empty 4" air cavity in an MSM wall, and filling that cavity fully with rock wool, is a difference of something like 8 or 10 dB, and if it is a 8 inch gap, that goes up to something like 16 dB. (Don't take those numbers as gospel: just what my poor old brain recalls.) There are a few reasons for the improvement: The first is that adding the insulation drives down the resonant frequency of the entire wall. The second is that the insulation damps standing wave resonance inside the wall itself (similar to flutter echo), third it damps the "S" in the MSM equation: It damps the spring. There are a couple of other minor reasons too, (it even adds a tiny amount of mass to the wall, but that's practically negligible), but the overall effect makes a big difference. Leaving out the insulation inside an MSM wall is not a good idea. Air by itself is OK, but air plus insulation is MUCH better. Now, in this specific case, there's already a lot of insulation in there, so that wall should be isolation pretty well already (especially considering the large air gap). So I don't know how much extra isolation you'd get from filling the remaining 3 inches of air gap with insulation, but if it were my wall, and I had the extra money, and I needed maximum isolation, I'd probably do it. - Stuart - Edited to add: Damn! I gotta learn to type faster! In the time it took me to write that, both Rod and John already replied!

Rod, Thanks for elaborating! It's amazing how much I can learn from the details of a response to someone else. :D Honestly, the information you provided helps a lot in, if nothing else, realizing that there is always more to a question/answer than there may appear. The question now might be - what, if any, would be the resultant flanking between walls with the entire cavity filled with cellulose http://greenfiber.com/homeowners/related_products.asp So, unless I hear a reason not to, it looks more like I'll fill in that air with cellulose (blown-in) insulation if that continues to look like an easier and at least equally expensive alternative. My only concern now would be settling. :( Thanks! Scott

[quote="John Sayers"]Scott - it appears to me that the insulation in the cavity between the outer steel and the inner drywall is wasted. The outer steel would not be airtight therefore it can't be considerd as a skin. The place for the insulation is in the airspace between the two layers of drywall. John, I don't have two layers of drywall. I only have one. The outer wall/layer is steel. This is an older diagram, but if you look at the wall, there is a white space that is empty. That is the space I am thinking of filling - and instead of with pink fluffy fiberglass, I would use cellulose. Thanks! Scott edit: I've attached a file, but it says "not yet downloaded". Did I forget to do something?

I think you mis-understood me Rod. In fact I think were all saying the same thing. :D My comment was about money vs result and to eric about his comment "air alone....." plus6vu has insulation against both walls, so I just felt that it was misleading. plus6vu is also asking about spray insulation, so the answer is no. I would be worried about bugs nesting in that area as well, gross My shed has roddent barrier but those little buggers still get in!

not yet downloaded means no one has looked at it yet. Well hadn't..... now I have! :mrgreen:

Soundman2020 wrote:

So Eric you think that (lets say he has 16mm plaster board on both leaves) MSaSM with a STC of 63 could be improved by MSSM (filling the gap) by how much? 1 or 2 dB. You should get a much better improvement than just 1 or 2 dB! I'm looking for the research paper where I have the exact figures, but I seem to remember that the difference between an empty 4" air cavity in an MSM wall, and filling that cavity fully with rock wool, is a difference of something like 8 or 10 dB, and if it is a 8 inch gap, that goes up to something like 16 dB"

The point is that he has insulation. steel-insul wool-air-insul wool-plaster. I think?

[quote="lilith_envy"] I would be worried about bugs nesting in that area as well, gross My shed has roddent barrier but those little buggers still get in![quote="lilith_envy"] Yep, they do get in, but I don't have too many rodents. The snakes keep the population down pretty well. :mrgreen: I keep looking for ways to seal it off better from the little critters, but I've lived in houses with worse problems. I'm not overly concerned about it. [quote="lilith_envy"] not yet downloaded means no one has looked at it yet. [quote="lilith_envy"] Thanks! I was expecting it to pop up in embedded in the post. As long as it can be seen... Scott

Soundman2020 wrote:

So Eric you think that (lets say he has 16mm plaster board on both leaves) MSaSM with a STC of 63 could be improved by MSSM (filling the gap) by how much? 1 or 2 dB.

You should get a much better improvement than just 1 or 2 dB! I'm looking for the research paper where I have the exact figures, but I seem to remember that the difference between an empty 4" air cavity in an MSM wall, and filling that cavity fully with rock wool, is a difference of something like 8 or 10 dB, and if it is a 8 inch gap, that goes up to something like 16 dB. (Don't take those numbers as gospel: just what my poor old brain recalls.)

Heck! If I could pick up anything like 8-10 db more isolation by filling that cavity with insulation, that would be well worth it! Scott

plus6vu wrote:

John Sayers wrote:

John, I don't have two layers of drywall. I only have one. The outer wall/layer is steel.

Not preserved: wall detail.pdf

This is an older diagram, but if you look at the wall, there is a white space that is empty. That is the space I am thinking of filling - and instead of with pink fluffy fiberglass, I would use cellulose. Thanks! Scott edit: I've attached a file, but it says "not yet downloaded". Did I forget to do something?

OK so you already have a 3 1/2" layer of insulation against the steel and you have 6" insulation between your studs that extends into the cavity by 2". That's heaps - adding more insulation won't make the ISO any better - the only was is to add mass in the drywall.

John Sayers wrote:

OK so you already have a 3 1/2" layer of insulation against the steel and you have 6" insulation between your studs that extends into the cavity by 2". That's heaps - adding more insulation won't make the ISO any better - the only was is to add mass in the drywall.

As long as I'm not hurting myself by filling the gap with insulation, that's really all I'm looking for. There's just a possible ease of installation and time savings that might be had with Cellulose as opposed to batts. Scott

Heck! If I could pick up anything like 8-10 db more isolation by filling that cavity with insulation, that would be well worth it!

Yeah, but hang on a sec! Those numbers I mentioned are for going from an EMPTY cavity (no wool at all) to a full one. In your case, it's something like 80% full already, so your gain won't be anywhere near as spectacular. With nearly 10" of insulation as it is, you are well into the zone of diminishing returns: Like john said, you aren't going to gain much from extra insulation in there at this stage, since you already have so much. Use the money for more sheetrock, I'd say. An extra layer or two of sheetrock on each leaf will get you much more more isolation for your money at this stage. You've got stacks of insulation in there, and stacks of air gap, but not so much mass. Go for more mass, I'd say. IMHO - Stuart -

There's just a possible ease of installation and time savings that might be had with Cellulose as opposed to batts.

But did you notice what Rod said about cellulose? It doesn't sound like your best bet. In fact, if it does flank, you'd lose all the benefit you gained from such a big air gap and the existing mountain of wool that you have in there. - Stuart -

Soundman2020 wrote:

But did you notice what Rod said about cellulose? It doesn't sound like your best bet. In fact, if it does flank, you'd lose all the benefit you gained from such a big air gap and the existing mountain of wool that you have in there. - Stuart -

Stuart, Yes, I did see what he said, and maybe I'm a bit confused. The term he used was "spray insulation" and referred to its cost as being many times higher than fluffy. That doesn't really describe cellulose to me. I thought that he may have been referring to a more foam-type insulation. I could be wrong. I even asked Rod

plus6vu wrote:

The question now might be - what, if any, would be the resultant flanking between walls with the entire cavity filled with cellulose http://greenfiber.com/homeowners/related_products.asp

Again, the goal is not necessarily to gain isolation, though losing it is certainly out of the question. The goal is an installation time/cost issue. Here fluffy, batted insulation is about 2x the price of Cellulose. So even buying the extra insulation needed to fill the gap, the cost would come close to even. Maybe Rod can clarify. Scott

plus6vu wrote:

Soundman2020 wrote:

But did you notice what Rod said about cellulose? It doesn't sound like your best bet. In fact, if it does flank, you'd lose all the benefit you gained from such a big air gap and the existing mountain of wool that you have in there. - Stuart -

Stuart, Yes, I did see what he said, and maybe I'm a bit confused. The term he used was "spray insulation" and referred to its cost as being many times higher than fluffy. That doesn't really describe cellulose to me. I thought that he may have been referring to a more foam-type insulation. I could be wrong. I even asked Rod

plus6vu wrote:

The question now might be - what, if any, would be the resultant flanking between walls with the entire cavity filled with cellulose http://greenfiber.com/homeowners/related_products.asp

Again, the goal is not necessarily to gain isolation, though losing it is certainly out of the question. The goal is an installation time/cost issue. Here fluffy, batted insulation is about 2x the price of Cellulose. So even buying the extra insulation needed to fill the gap, the cost would come close to even. Scott

I started studying the IRC NRC data regression by Warnock. Regression of this data means that based on all these measurements one searches for (regression or trend) formulas which aren't based on physics but on statistical logic/relationship of the real measurement data and their resulting single-number ratings versus their influencing parameters. In these measurements are used: fibreglass batts 11 kg/m3, somewhat denser rockwool batts 35 kg/m3, blow-in cellulose fibre 23 kg/m3 and wet-sprayed cellulose fibre 50 kg/m3. The difference between all of them calculated as a single-number dB(A) weighting down to 50 Hz (=ALD50) is extreme low (in the order of mostly 0 or 1 dB). This dB(A) weighting down to 50 Hz is better for our purpose than STC (still doesn't cover music). I still have to study this more in-depth, but for now I should say that it only very limited matters. I think one can safely use blown-in cellulose fibre as an alternative (if one chooses to for whatever reason). This wet-sprayed stuff as cavity filling I don't know very well. But as John said: With the amount of blanket you have already, 100% filling versus the amount you planned already will hardly make any difference.

plus6vu wrote:

Rod, Thanks for elaborating! It's amazing how much I can learn from the details of a response to someone else. :D Honestly, the information you provided helps a lot in, if nothing else, realizing that there is always more to a question/answer than there may appear. The question now might be - what, if any, would be the resultant flanking between walls with the entire cavity filled with cellulose http://greenfiber.com/homeowners/related_products.asp So, unless I hear a reason not to, it looks more like I'll fill in that air with cellulose (blown-in) insulation if that continues to look like an easier and at least equally expensive alternative. My only concern now would be settling. :( Thanks! Scott

Scott, the same report I referenced earlier (IR-761) tested wall assemblies with blown cellulose - and their report was as follows:

Most of the materials used in these assemblies are manufactured products with clearly defined dimensions and other properties. The exception to this rule is cellulose fibre, for which the base material has controlled properties, but the installation permits some variation, as it is blown or sprayed into the wall cavities on site. This permits variation in density, which affects air flow resistance, total weight, and stiffness of the resulting cavity fill. The density was measured when each wall specimen with cellulose fiber was taken apart, and is presented in the tables. The weight of absorptive material was recorded for each specimen tested. These values were used to calculate the average weight per unit area, and the density. For wet-sprayed cellulose fibre, the actual thickness was sampled at many positions to determine the average. For blown-in cellulose fibre, the thickness was taken to be the depth of the stud cavity. Airflow resistance was measured in accordance with ASTM C522-87 for samples of each type of absorptive material (square samples 150 x 150 mm). For airflow measurement of loose fill cellulose fibre material, the material was compressed in the specimen holder to approximately the same density obtained when it was blown into the wall cavities. Several tests with different compression showed the airflow resistivity was quite sensitive to compression for this material. Values should therefore be recognized as rather uncertain; significant changes might occur for the range of densities observed in wall specimens.

I would avoid cellulose insulation if I were you........ unpredictable behavior of the assembly is a huge problem. I would also consider adding mass at the inside face of the steel sheathing - and caulking the joints to create an air tight assembly on that face...... a lot of work to get it good - but a lot of added value in isolation. Rod

Rod, Thanks, I hadn't read that comment about cellulose (or forgot it). You indeed must see that you have the appropriate density. Edit: Rod, nowhere they put a direct relationship with this variation in density and the single number ratings. Hence the question remains, how important is this? :) Damned means (for myself): going more in-depth (more work).

Rod, You are truly a blessing. Thanks for going to so much trouble to look this up. Not only would I have not even thought about density inconsistencies, but I would not have even thought to ask. Like most folks, I suppose, I figured that since cellulose was so popular, then it must be fine to use. Insulation is insulation right? Well - apparently not. Thanks again. Regarding the wall assemblies and adding gypsum. As per a much earlier post (2-3 years ago ?) they are (will be) 3/4 OSB glued and screwed to the studs (some of the studs are pretty warped - getting good lumber was relatively impossible at the time) and then an additional layer of 5/8 gypsum with all seams staggered. They have backer rod and are caulked as per your instructions both here and in your book. I am thinking of using channel between the OSB and the gypsum, but I don't know if I have the funding or the time to do that. You and others here have been a huge help with this project. I can not thank you all enough. Scott