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Building a shed, challenging code with walls.
Old 3 weeks ago
  #1
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Funky Animal's Avatar
Building a shed, challenging code with walls.

I am building a shed in my garden, due to it's size I needed a permit. Now I'm having a challenge getting the best STC with the wall I needed to build.

I wasn't allowed to put up an OSB leaf on the outside of the building, which was a bummer. So I'm now figuring out what's the best way to pursue insulation.

Following code I needed to the following:

Slats > Damp open foil > Insulation / Framing > Vapor barrier > OSB (or Gypsum)

I'm building an extra room inside, and looking to get the best STC possible in this situation. Maybe someone can help me out what's the best way to pursue this?

Preferably I wanted to build the walls inside out, but with the situation changed, it's probably not the best way to go down that road.

I have made 2 options, I'm open to better suggestions.
Attached Thumbnails
Building a shed, challenging code with walls.-option2.jpg   Building a shed, challenging code with walls.-option1.jpg  
Old 3 weeks ago
  #2
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Avoid the first option as that will give you 3 leaves. Option 2 is the route to pursue.

When you said you cannot put OSB on the outside of the building, I am not sure whether you mean on the outside, visible (where you would not want it anyway), or just that you cannot have it on the outer wall, where the slats and the 22x44mm layer is. Adding OSB on the outer wall but inside the water-repelling foil would help add mass to the outer leaf.

The gap (which includes the insulation) between the two leaves will ideally be as large as possible but without decreasing the internal volume of your studio. The larger the gap, the lower the pressure of the spring - air between two airtight layers acts as a spring - and that will lower the resonant frequency of the 2 walls and the gap as one system.

Beefing up the inner leaf, where you have plans for 2 layers of 12.5mm gypsum, would also add mass and help improve the transmission loss.

The calculations quoted in post 5 of the topic New construction 2-ch listening room build questions will enable you to calculate the approximate isolation of your studio's wall system for three frequency bands before you start building.
Old 3 weeks ago
  #3
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Quote:
Originally Posted by Starlight View Post
Adding OSB on the outer wall but inside the water-repelling foil would help add mass to the outer leaf.
I'm no building scientist, but as I understand the problem with that is that OSB has pretty low vapor permeability. Without some way to vent the cavity, and a significant difference in temp and then RH, you could have condensate building up.

I think that possibly the outermost leaf here, the siding may be vented, meaning it possibly isn't a leaf in a mass-spring-mass system.

As I interpret it the "22x44mm placed horizontally each 60mm" are vertical furring strips that are spaced 60cm on center? Is there any additional venting of the exterior walls or roof?
Old 3 weeks ago
  #4
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Quote:
Originally Posted by Starlight View Post
When you said you cannot put OSB on the outside of the building, I am not sure whether you mean on the outside, visible (where you would not want it anyway), or just that you cannot have it on the outer wall, where the slats and the 22x44mm layer is. Adding OSB on the outer wall but inside the water-repelling foil would help add mass to the outer leaf.
Preferably I wanted:
Slatwall | Furring strips | Foil | OSB | Frame with insulation | Vapor barrier

But I wasn't allowed to put the OSB on the framing on the outside. I wanted to create good sealed outer leaf, but I can't now.

Quote:
Originally Posted by Starlight View Post
Avoid the first option as that will give you 3 leaves. Option 2 is the route to pursue.

The gap (which includes the insulation) between the two leaves will ideally be as large as possible but without decreasing the internal volume of your studio. The larger the gap, the lower the pressure of the spring - air between two airtight layers acts as a spring - and that will lower the resonant frequency of the 2 walls and the gap as one system.

Beefing up the inner leaf, where you have plans for 2 layers of 12.5mm gypsum, would also add mass and help improve the transmission loss.

The calculations quoted in post 5 of the topic New construction 2-ch listening room build questions will enable you to calculate the approximate isolation of your studio's wall system for three frequency bands before you start building.
Ok, so with option 2, you mean option1.jpg, right?...I see GS switched the images around.

Is the slatwall considered as a leaf, since it has a lot of airgaps in it?

I can beef up the inner wall with sandwiching OSB between and the 2 gypsum sheets, would that help?
Old 3 weeks ago
  #5
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Quote:
Originally Posted by RyanC View Post
As I interpret it the "22x44mm placed horizontally each 60mm" are vertical furring strips that are spaced 60cm on center? Is there any additional venting of the exterior walls or roof?
Should be vertically, sorry.
Yeah, it provides and airgap between the slats and foil.

The outside has SIGA Majvest type product on it:
https://www.youtube.com/watch?v=xkFHp2V1yYg

Inside I have the SIGA Majpell on it:
https://www.youtube.com/watch?v=UTSqM3QN-2E
Old 3 weeks ago
  #6
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Here's a photo of the slats, foil, futting strips.

Don't mind the sand, I cleaned it up ;-)
Attached Thumbnails
Building a shed, challenging code with walls.-outside.jpg  
Old 3 weeks ago
  #7
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Quote:
Originally Posted by Funky Animal View Post
Ok, so with option 2, you mean option1.jpg, right?...I see GS switched the images around.
Yes, that is what I meant but RyanC's post #3 is a better assessment of the situation so ignore my first comment. The need to ventilate is possibly the reason the outer wall is not allowed to have any OSB added.

Quote:
Originally Posted by Funky Animal View Post
Is the slatwall considered as a leaf, since it has a lot of airgaps in it?
As RyanC says, it will probably not count as a leaf because it will to be open enough to ventilate.

Quote:
Originally Posted by Funky Animal View Post
I can beef up the inner wall with sandwiching OSB between and the 2 gypsum sheets, would that help?
For the same thickness, gypsum is a little bit heavier but the difference is small. Gypsum is the usual material for the interior leaf but if you need to attach heavy things to the wall, such as an air conditioner, then OSB will be a better choice at that spot.
Old 3 weeks ago
  #8
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Quote:
Originally Posted by Starlight View Post
Yes, that is what I meant but RyanC's post #3
As RyanC says, it will probably not count as a leaf because it will to be open enough to ventilate.
So in this case...this would be a good option?
Attached Thumbnails
Building a shed, challenging code with walls.-option2.jpg  
Old 3 weeks ago
  #9
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I am, for sure, not qualified on this, and I think it really depends on what the whole venting system looks like. But here is a pretty good thread with a response from Rod on it that does tackle this sort of issue-

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=6054

Are there other vents in the roof or eaves as well?
Old 3 weeks ago
  #10
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Quote:
Originally Posted by RyanC View Post
I am, for sure, not qualified on this, and I think it really depends on what the whole venting system looks like. But here is a pretty good thread with a response from Rod on it that does tackle this sort of issue-

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=6054

Are there other vents in the roof or eaves as well?
From what I understand from that thread, I need to have my vapor barrier on the inside of my extra room.

So I’m thinking of going with this, ofcourse I’ll first check this with someone with more knowledge on this subject.

Slats > Furrier strips > House cover > Insulation + framing > OSB > Airspace 8cm > Insulation + Framing > Vapor barrier> Gypsum > OSB > Gypsum
Old 3 weeks ago
  #11
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Quote:
Originally Posted by RyanC View Post
I understand the problem with that is that OSB has pretty low vapor permeability. Without some way to vent the cavity ...
Quote:
Originally Posted by Funky Animal View Post
Quote:
Originally Posted by Funky Animal View Post
Slats > Furrier strips > House cover > Insulation + framing > OSB > Airspace 8cm > Insulation + Framing > Vapor barrier> Gypsum > OSB > Gypsum
Being neither a builder, architect or a studio designer, here are my two questions:

1. Will the outer OSB layer (orange text and visible in the picture) block the cavity from being properly ventilated?

2. RyanC mentions that OSB has a pretty low vapour permeability, certainly lower than MDF but as it is not impervious and is outside the vapour barrier, is it likely, over time, to become damp and slowly start to perish?
Old 3 weeks ago
  #12
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Quote:
Originally Posted by Starlight View Post
Being neither a builder, architect or a studio designer, here are my two questions:

1. Will the outer OSB layer (orange text and visible in the picture) block the cavity from being properly ventilated?
In my example it won't. Since it won't be airtight then. However, I can make vent the cavity space and avoid moist build up. The insulation probably won't be as effective as having it air tight, but it probably is better than having nothing.

Quote:
Originally Posted by Starlight View Post
2. RyanC mentions that OSB has a pretty low vapour permeability, certainly lower than MDF but as it is not impervious and is outside the vapour barrier, is it likely, over time, to become damp and slowly start to perish?
I think with condensation it is likely to become damp, and than it will start to perish. So I need to find a way around that, maybe on each wall I should have a possibility to vent the cavity.

But this might also be depending on where I put the vapor barrier.
Old 3 weeks ago
  #13
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Please let me confess another mistake on my part: I thought the outer OSB would be outisde the vapour barrier but looking again at image option2 in theOP I see it is in fact inside. That would mean it will not become damp from humidity from outside the vapour barrier. It might, as you sa, get wet from condesation though if there is a possibility of a large enough temoerature difference inside and outside the studio. Consider putting a thermal barrier bewteen the vapour barrier and the OSB but before you do, stop listening to me and talk with a builder or architect who will know how this can be done properly.
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