Hi everyone,

I've been reading these forums for quite some time now. There is a lot of very useful information and I thank you so much for that. However, I have not yet stumbled on the answer I am seeking for.

I will be building a house next spring in southern Canada with 2x10 walls (I know, I could go with larsen wall, sip, or some other wall technology, however contractors here do not seem very happy with new ways of doing thing... using 2x10 is a mostly painless adaption and I really want to limit the use of non renewal resources like what is needed to create foam )  that will be insulated with dense packed cellulose insulation (around R35) with a polythene vapor barrier on the warm side. On the warm side of the polythene I will be using 2x3 acting as firring, meaning there will be a 1.5" space between the drywall and the vapor barrier.

Now here is my question. I would like to do something useful with that space, in addition to passing wires and pipes. I am wondering if I can insulate this cavity too with some kind of batt (roxul or something else, about R6) without causing some mold growth. I am not sure but I think that this could be ok because the dew point should be high enough, having at least R35 on the other side of the vapor barrier. The kind of paint of the drywall might also be issue if it is a vapor barrier...

Am I a fool? Is it worth it? Would you do it?

Thanks a lot,
Guylain

With 10" of dense packed cellulose on the cold side of the polyethylene, yes, you CAN insulate it with R8 rock wool. The dew point of winter time conditioned space 20C 30-25% RH air is between 3-5C. The average temp at the vapor barrier has to be below the dew point for the predominance of hours for at least a week or two before that becomes a mold problem. Even if you tipped the 2x3s the other way to put R8 batts on the warm side of the poly with R35 outside the poly that puts the poly more than 80% inside the thermal envelope- the average outdoor temps would have to be WAY into negative numbers to make that a condensing surface of any consequence (colder than -50C for an average). With 1.5" of space you only add ~R5 with standard batts, compressed.

Vapor retardent paints have much higher permeability than poly, but there's no good reason to use them (and some reason to avoid them) in your wall system.

In fact, with cellulose you have huge hygric buffering, and SKIPPING the poly would give the wall much higher drying capacity if you then used 1.5" (R9-ish) of closed cell SPF in the cavity between cellulose & gypsum, but you'd probably get an argument from the inspector. With a 10mm rainscreen on the exterior and a 10" cellulose cavity fill, in most of southern Canada you could get away with using only standard latex paint one the interior as the vapor retarder, but the building code hasn't fully kept up with how high-R houses are built.

Hi Dana1,

Thank you so much for your answer. This is what I expected but wanted a second opinion.
I am not a contractor, only someone that wants to learn and do the right thing. Errors are costly to fix once the walls are finished.

I've tried to find rockwool batt that are 1.5" thick but I cannot find less that 3.5". I read that it is possible to compress a batt but in the case of rockwool, it deeply fear that the material won't be compressible enough. Is there any provider that would have 1.5" thick rockwool? If not, I might need to install standard pink style batt and compress them, say R8 whch usually fits in 2.5" should  not be too hard to fit in an 1.5" cavity. Or as you suggested, I could tip the 2x3 and install without compression. I would prefer to keep the same wall width though.

Best Regards,
Guylain

I'm not sure about the compressabilty of rock wool batts, but another option is available: Wet sprayed or dry blown cellulose/fiberglass. If you went with JM Spider or Certainteed Optima you'd get about R6 out of it, with low density cellulose you'd get ~R5.5, and either solution would be higher performance than compressed rock wool (not that it would be easy to measure in an ~R40 wall.) Dry blown from the interior after the gypsum was up would require 2 holes ~3cm each per bay, but it could be done in such a way that they get covered by the top/bottom trim. (Blow from the bottom hole first until it's coming out the top or backing up the blower, then top-down. Normally this is done with the holes ~30cm from the floor & ceiling, but good result can still be had (if slightly more difficult to install) putting them at the very ends of the cavites.

If you're using it just as furring, 2x2s will have much lower thermal bridging than a flat-sided 2x3, and would fit standard batt widths using standard stud spacings rather than running them 1.5" wider than "normal" stud-center spacings.

Here's three useful resources, all are free and available as .pdf files, just Google the citation:

1. Swinton, M.C., Kesik, T., Performance Guidelines for Basement Envelope Systems and Materials, Final Research Report, Institute for Research in Construction, National Research Council Canada, October 2005 
2. Straube, J., Smegal, J., Building America Special Research Project: High-R Walls Case Study Analysis, Research Report – 0903, March 11, 2009 (rev. 8/7/09) 
3. Straube, J., Smegal, J., Building America Special Research Project: High-R Foundations Case Study Analysis, Research Report – 1003, 20 August 2010.
   

I am not sure I correctly understand what you meant when you are wrote about doing holes at the floor and ceiling. You seems to assume that the firring will be vertical? I will probably put them horizontally to reduce the thermal bridge to the absolute minimum. This would make it much more cumbersome to use dry blown material on the cavities since holes will be much more apparent.

The idea about wet sprayed cellulose would be perfect but unfortunately here, the only installer that did this stopped doing it a few years back and reverted to dry blown cellulose. Out of luck! :( I guess I will need to use standard R8 pink batt... I'll see. At least, it should be quite cheap and I will be able to did it my self.

I will also use 2x2s as you suggested instead of 2x3 as firring.

I really wish to thank you for your help. You have been very helpful.

Thanks,
Guylain

Hi Robert, thanks for the tips. The documents are really interesting. Really good idea for some other things I am currently working on.

Guylain

Posted By Guylain Lavoie on 21 Oct 2010 10:37 PM
I am not sure I correctly understand what you meant when you are wrote about doing holes at the floor and ceiling. You seems to assume that the firring will be vertical? I will probably put them horizontally to reduce the thermal bridge to the absolute minimum. This would make it much more cumbersome to use dry blown material on the cavities since holes will be much more apparent.

The idea about wet sprayed cellulose would be perfect but unfortunately here, the only installer that did this stopped doing it a few years back and reverted to dry blown cellulose. Out of luck! :( I guess I will need to use standard R8 pink batt... I'll see. At least, it should be quite cheap and I will be able to did it my self.

I will also use 2x2s as you suggested instead of 2x3 as firring.

I really wish to thank you for your help. You have been very helpful.

Thanks,
Guylain

Yes I was assuming vertical cavites, but if they're horizontal, that works too.  In a vertical studwall cavity the common low-density dry-blown method is to make two holes in either the sheathing (if installing from the exterior) or gypsum board (if installing from the interior), one hole near the end of each end. 

If your furring cavites are horizontal, drilling a series of holes ~ 2-2.5m apart along the cavity similar densites could be acheived.  It would also outperform low-density vertical cavities a bit, since air onvection would be blocked by the furring, not just the cellulose.  Itf there are few obstructions, it might be possible to drill a single larger hole near the center of the cavity and snake a narrower "dense-packing" tube  close to the ends of the cavity, reducing the number of holes to patch. Most installers use tubing with an inner diameter betwen 25-35mm in standard stud bays, but building  up a  ~20mm version for the project is not difficult. Narrower than that 20mm might be both too flexible and have issues with frequent clogging if it needs to be longer than ~2m though.  But if it can reach to within 1 of the end of the cavity you would still achieve densities at least as high as with 2-hole method, but not quite up to dense-packing specifications.

Posted By Guylain Lavoie on 18 Oct 2010 09:24 PM
Hi everyone,

I've been reading these forums for quite some time now. There is a lot of very useful information and I thank you so much for that. However, I have not yet stumbled on the answer I am seeking for.

I will be building a house next spring in southern Canada with 2x10 walls (I know, I could go with larsen wall, sip, or some other wall technology, however contractors here do not seem very happy with new ways of doing thing... using 2x10 is a mostly painless adaption and I really want to limit the use of non renewal resources like what is needed to create foam )  that will be insulated with dense packed cellulose insulation (around R35) with a polythene vapor barrier on the warm side. On the warm side of the polythene I will be using 2x3 acting as firring, meaning there will be a 1.5" space between the drywall and the vapor barrier.

Now here is my question. I would like to do something useful with that space, in addition to passing wires and pipes. I am wondering if I can insulate this cavity too with some kind of batt (roxul or something else, about R6) without causing some mold growth. I am not sure but I think that this could be ok because the dew point should be high enough, having at least R35 on the other side of the vapor barrier. The kind of paint of the drywall might also be issue if it is a vapor barrier...

Am I a fool? Is it worth it? Would you do it?

Thanks a lot,
Guylain

I'm a little surprised that no one has said anything about your statement above (bolded). 

My wife & I built a house earlier this year & dealt with some of these issues as well, but I had done my research, knew what I wanted, & wasn't going to be swayed.  Don't ever forget, YOU are the customer - YOU are writing the check.  If your contractor is unwilling to do what you want, let him know that you'll find someone else who will.

In my case, I provided our builder with documents, links to websites, etc., explaining what I wanted to do & why I wanted to do it.  The funny thing is, on several items that he didn't want to do, he has since told me that he's incorporating those things into all of his homes going forward.  Builders, as a lot, are pretty resistant to change.  You may need to force the issue, but it's YOUR house.  Make sure you get what you want.

I second this.  You are the customer.  You will get resistance.  On a different issue, but same concept (non-traditional energy efficient construction methods) I pushed back repeatedly when my builder balked, and won.  Since then the builder has admitted incorporating some of these methods as standard for all houses going forward.

In general, one should be aware of the rules relating to the selling of unconventional products in a competitive and conventional - consumer based marketplace. One of the rules states, it takes a buyer and a seller of like mind towards unconventional products for an offer and acceptance to take place. Why bring this up? We’ve seen people build unconventional homes who end up sitting on them because the house was too ‘special’ or too ‘unique’ for the general marketplace. This may not apply specifically to the readers in this instance, but generally it’s always wise to think about future buyers for ones assets...frequently in housing it’s possible to achieve similar thermal performance using modifications to conventional practices that don’t look on paper like they're from a popular science experiment...SIPS = standard, ICF = standard, 2 x6 advanced framing w/ 2” Rigid exterior = app. standard…slight mods such as 2 x 2 interior strapping wouldn’t create raised eye brows….again...this may not apply specifically to readers…I just toss it out as a general consideration for everyone to consider when ‘experimenting’ with ones assets.

Just for giggles, one of my final papers in 1983 (27 years ago) was on foam block walls with sprayed gunite…Larsen truss walls were considered ‘advanced’ framing, solar thermal was in a heyday and ERV's/HRV’s (Fan-X...a Swedish ERV product imported by PM Wright) had already been around for a number of years…All were good and became conventional except Larsen truss walls…maybe someday…who knows?

Robert;

it certainly applies to log cabins, A frames, earth ships and geodesical domes that are limited not so much because how they are built , but more by how they look,  but an alternative building method that looks the same as any other home will not be affected at the resale. SIPs and ICFs are well accepeted in the market place

Posted By Guylain Lavoie on 18 Oct 2010 09:24 PM

I will be building a house next spring in southern Canada with 2x10 walls ... I will be using 2x3 acting as firring, meaning there will be a 1.5" space between the drywall and the vapor barrier.

I would like to do something useful with that space, in addition to passing wires and pipes.

Guylain, two comments. Are you planning to use 2 x 10 lumber for the studs? If so, give consideration to a double 2 x 4 wall. I'm sure the studs will be less expensive and you will avoid thermal bridging from inside to outside. That will improve the R value of the wall.

Second, double check your code requirements for electrical wiring and plumbing. I seriously doubt you will be allowed to put wiring and plumbing in the 1.5" space. The IRC requires wiring and plumbing to be a minimum of 1.25" behind the edge of the framing member or furring strip, or be protected by 16 ga steel plate.

Posted By dmaceld on 23 Oct 2010 09:59 PM

. The IRC requires wiring and plumbing to be a minimum of 1.25" behind the edge of the framing member or furring strip, or be protected by 16 ga steel plate.

the 1-1/4" only applies when the wiring / plumbing is attached or passes thru a framing member. Therefore wiring or plumbing held away from furring with a "standoff bracket" will only needs protected with a "can't nail" plate where it passes thru the furring strip

Hello, I am now in search for a more open-minded contractor. If I find such rare bird, that's fine. Otherwise, that's true, the total wall R will be a little bit lower, but according to an article published in 2009 by the Consortium for Advanced Residential Buildings, a difference of about R-1 is to be expected. I can email you the document if you are interested.

Thanks,
Guylain

Posted By jerkylips on 22 Oct 2010 01:13 PM

Posted By Guylain Lavoie on 18 Oct 2010 09:24 PM
Hi everyone,

I've been reading these forums for quite some time now. There is a lot of very useful information and I thank you so much for that. However, I have not yet stumbled on the answer I am seeking for.

I will be building a house next spring in southern Canada with 2x10 walls (I know, I could go with larsen wall, sip, or some other wall technology, however contractors here do not seem very happy with new ways of doing thing... using 2x10 is a mostly painless adaption and I really want to limit the use of non renewal resources like what is needed to create foam )  that will be insulated with dense packed cellulose insulation (around R35) with a polythene vapor barrier on the warm side. On the warm side of the polythene I will be using 2x3 acting as firring, meaning there will be a 1.5" space between the drywall and the vapor barrier.

Now here is my question. I would like to do something useful with that space, in addition to passing wires and pipes. I am wondering if I can insulate this cavity too with some kind of batt (roxul or something else, about R6) without causing some mold growth. I am not sure but I think that this could be ok because the dew point should be high enough, having at least R35 on the other side of the vapor barrier. The kind of paint of the drywall might also be issue if it is a vapor barrier...

Am I a fool? Is it worth it? Would you do it?

Thanks a lot,
Guylain

I'm a little surprised that no one has said anything about your statement above (bolded). 

My wife & I built a house earlier this year & dealt with some of these issues as well, but I had done my research, knew what I wanted, & wasn't going to be swayed.  Don't ever forget, YOU are the customer - YOU are writing the check.  If your contractor is unwilling to do what you want, let him know that you'll find someone else who will.

In my case, I provided our builder with documents, links to websites, etc., explaining what I wanted to do & why I wanted to do it.  The funny thing is, on several items that he didn't want to do, he has since told me that he's incorporating those things into all of his homes going forward.  Builders, as a lot, are pretty resistant to change.  You may need to force the issue, but it's YOUR house.  Make sure you get what you want.

Hi, yes you are right. However, the details are what is important in construction.  If you can get a contractor to do some incremental modifications to its normal work in favor of increase efficiency and quality, that is great and maybe better than forcing him to work with techniques that he is totally uneasy with and where he may endup doing errors. In the later case, you better be well informed and be able to follow each step of the construction. I am not sure I am such person. My only background is coming from a few years of googling... nothing really formal. Nonetheless, I am now in search for a more open-minded contractor. Maybe I'll find one that is closer to my expectation.

Thanks for the tip.
Guylain

Posted By Guylain Lavoie on 25 Oct 2010 10:27 PM

Posted By jerkylips on 22 Oct 2010 01:13 PM

Posted By Guylain Lavoie on 18 Oct 2010 09:24 PM
Hi everyone,

I've been reading these forums for quite some time now. There is a lot of very useful information and I thank you so much for that. However, I have not yet stumbled on the answer I am seeking for.

I will be building a house next spring in southern Canada with 2x10 walls (I know, I could go with larsen wall, sip, or some other wall technology, however contractors here do not seem very happy with new ways of doing thing... using 2x10 is a mostly painless adaption and I really want to limit the use of non renewal resources like what is needed to create foam )  that will be insulated with dense packed cellulose insulation (around R35) with a polythene vapor barrier on the warm side. On the warm side of the polythene I will be using 2x3 acting as firring, meaning there will be a 1.5" space between the drywall and the vapor barrier.

Now here is my question. I would like to do something useful with that space, in addition to passing wires and pipes. I am wondering if I can insulate this cavity too with some kind of batt (roxul or something else, about R6) without causing some mold growth. I am not sure but I think that this could be ok because the dew point should be high enough, having at least R35 on the other side of the vapor barrier. The kind of paint of the drywall might also be issue if it is a vapor barrier...

Am I a fool? Is it worth it? Would you do it?

Thanks a lot,
Guylain

I'm a little surprised that no one has said anything about your statement above (bolded). 

My wife & I built a house earlier this year & dealt with some of these issues as well, but I had done my research, knew what I wanted, & wasn't going to be swayed.  Don't ever forget, YOU are the customer - YOU are writing the check.  If your contractor is unwilling to do what you want, let him know that you'll find someone else who will.

In my case, I provided our builder with documents, links to websites, etc., explaining what I wanted to do & why I wanted to do it.  The funny thing is, on several items that he didn't want to do, he has since told me that he's incorporating those things into all of his homes going forward.  Builders, as a lot, are pretty resistant to change.  You may need to force the issue, but it's YOUR house.  Make sure you get what you want.

Hi, yes you are right. However, the details are what is important in construction.  If you can get a contractor to do some incremental modifications to its normal work in favor of increase efficiency and quality, that is great and maybe better than forcing him to work with techniques that he is totally uneasy with and where he may endup doing errors. In the later case, you better be well informed and be able to follow each step of the construction. I am not sure I am such person. My only background is coming from a few years of googling... nothing really formal. Nonetheless, I am now in search for a more open-minded contractor. Maybe I'll find one that is closer to my expectation.

Thanks for the tip.
Guylain

As someone else mentioned, cost is also a factor.  Typically a 2x6 is twice as much as a 2x4.  I would assume that going from a 2x6 to a 2x10 is a similar jump in price.  That makes a 2x10 wall roughly double what you'd pay for a double stud 2x4 wall.  Tell the builder that you'll gladly go with his plan, if he covers the difference in cost between the 2.  There's no reason that you should pay twice as much for the same product (same r-value) just because he isn't comfortable with it.

You're right in saying that there is value in keeping the builder in his comfort zone, but a good one will educate himself to get to that comfort level.

If you engineer the place, you'll find that you can get the same structural capacity with 2x10s that you can with 2x6s or 2x4s with fewer boards, ending up at about the same material cost, and less labor (fewer boards to cut.)

But with 2x10s you're stuck with signficant thermal bridging. Using engineered I-beams for studs rather than raw 2x timber you can cut the thermal bridging by ~ 60-70%. (An approach commonly taken these folks: http://www.e-colab.org/ecolab/Projects.html http://www.e-colab.org/ecolab/SmithHouse.html )

Larsen trusses and double studwalls typically have even lower thermal bridging.

2 x 10 walls????????? Just remember, the bigger the board, the harder it is to find a straight one!

single 2x4 wall with 3.5" of spray foam & 2" of blue board outside would give you R 33, a thermal break, and pretty sealed up walls (no air movement within the cavity). I know spray foam isn't cheap, but I'd guess that you could still be ahead of the 2x10 wall on price.

For comparison, a 2x10 wall with dense pack cellulose (R4/inch) would give you R38 but no thermal break.

I don't remember if it was mentioned before, but a 2x10 wall is going to give you really deep window openings. The thicker the wall, the bigger your windows need to be to let in the same amount of light. Just stuff to think about...


-edited - didn't catch that OP was using cellulose vs. fiberglass..