Location of vapor barriers?

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Old 09-05-15, 08:54 AM
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Location of vapor barriers?

Hello

Hoping someone can provide some clarity on some conflicting information I'm getting.

The I have a 2-story house that's about 50 years old. Moved in a year ago. The second story is clad with the original and poorly maintained cedar. The wood is full of wood pecker holes and its bad shape all around. Also, I guess the house was built when heating was cheap because the wall consists of the wood cladding, a layer of tar paper, wood stud, interior drywall. I've got to install siding before the winter.

OK - I'm no stranger to the siding process and have done it a bunch of times - albeit on new builds. My plan was to remove the cedar, stuff Roxul between the studs, apply another inch of rigid insulation over top of that. Then Typar (taped), 1x2 strapping and finally new vinyl siding.

Rough sketch and pic of the house below.

Chatting with some people who were supposed to be informed on this stuff I got some really strange recommendations:

-One guy told me that I needed a vapor barrier between the interior drywall and studs. Meaning I had to replace all the drywall on the exterior walls of the second floor of the house. Even if this is code now, this would be crazy. There's no sign of water damage or moisture on the existing drywall. Has anyone heard of of this ever being done? Seems crazy-extreme.

- Another guy said that I needed either a layer of vapor barrier poly between the stud and rigid foam. Wouldn't this tend to trap moisture between the house wrap and poly?

This is the last home I'm ever going to live in so I want to do this right. Any informed advice would be appreciated.

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  #2  
Old 09-05-15, 09:14 AM
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Hi TJ and welcome to the world of confusion.
You do want to avoid having 2 vapor barriers, but house wrap is specifically designed to not be one.

The intent of the standards is to stop the vapor movement at a point where all surfaces will be warm enough to avoid condensation. Traditionally that has become (in your climate) just under the drywall.

Then we started adding rigid insulation to the outside of our homes and introduced the risk of having a second vapor barrier. Since your home apparently does not have an interior VB, which is now good, you can add the rigid to the outside, IF IT IS THICK ENOUGH. It is actually the ration of rigid r-value to the other interior r-value. The objective is what I mentioned before, you need to be sure the inside surface of that rigid remains above the dew point. I'll let you read.
Calculating the Minimum Thickness of Rigid Foam Sheathing | GreenBuildingAdvisor.com

A quick look at the chart says you may need thicker rigid on the outside, but remember, local codes always rule, so no matter what we suggest, you need the ok from your local authority.

Bud
 
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Old 09-06-15, 07:05 PM
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YES! Thank you! I didn't want to appear foolish but the idea of two vapor barriers in my head seemed like a moisture trap.

Some news. I did some more poking and it seems I have a layer of tar paper between the drywall and the studs. Not quite a poly barrier, so it breathes - and I'm not pulling down the drywall on a whole floor of the house. I still think that's crazy talk.

OK - Given that I'm putting new batt in between the studs I'm apparently good with 1/2" Silverboard over top without building wrap even.

Now - this is turning out well. Here's the thing. Once I pull it all off and install the Silverboard and strapping I'm 1 1/4" further out than I was before.

Surely this has been done before but I'm not sure how to cap around the windows?

Check out the pics;

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I can't just do an F channel around the window and set the siding in anymore since its so far out, right. So how do I cap around this so no water gets in?

Thanks again. Very helpful.
 
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Old 09-06-15, 07:45 PM
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TJ, "I'm apparently good with 1/2" Silverboard over top without building wrap even."
1/2" foil faced rigid foam doesn't seen near enough to meet the requirements in that link. What am I missing?

More reading: Video: How to Install Rigid Foam Insulation Outside a House | GreenBuildingAdvisor.com

Bud
 
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Old 09-07-15, 07:16 AM
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Hey Bud - Just watched that video. They appear to use two separate layers of rigid. I've seen many a new build and I've actually never seen this done before. Perhaps a thicker rigid insulation but definitely not two separate layers. Perhaps that's poor practice on the part of the builders around here?

I did notice that they did only use the 2 layers of rigid and no batt. This is why (apparently) I am OK with the one layer of rigid. (or so I thought) The plan was to stuff R14 Roxul between the studs, then the rigid. You're correct that the 1/2" doesn't bring the R value requirements up to snuff but increasing to even 2" wouldn't do it....and makes the problem of what to do around the windows even worse.

I think exterior walls above grade around here call for something like in the neighborhood of R24. That would mean the Roxul plus 2 1/2" of rigid...If I'm not mistaken....That's HUGE!

I think it's me who's missing something.....I'm open to wisdom here.

Thanks
 
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Old 09-07-15, 07:57 AM
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The article on the ratio of exterior rigid insulation is designed to keep the inside surface of that exterior rigid above the dew point, especially if you use foil faced, it has zero permeance so all drying must be to the inside. Not sure where in CA you are located, coast or interior, but I suspect 1" or 1.5" would be required.

The reason they use two layers is to avoid any gaps where cold air can reach the inside layers. Unfortunately, rigid foam board shrinks and loses r-value over time. Overlapping laters with a good tape job helps to eliminate that.

Here's an alternate route. With no insulation in those walls, get a quote from a company that will blow in cellulose insulation. Given the poor shape of the exterior siding, it becomes easy for them to drill all stud bays and add insulation. Inside the walls is important. Then, a common practice is to cover the existing siding with 2" of foil faced insulation, one plus one if you wish, but it is more expensive so probably why the jobs you see are only using one layer.

I have added 3.5" of rigid to my exterior and it did create some challenges with window and door trim, but I have all the tools so not too bad. The advantage of adding a good layer to the outside is it covers all of the studs and reduces the thermal bridging losses. The added layer also covers the double plate at the top and the rim joist at the bottom.

Check your climate zone requirement and let me know your minimum exterior rigid thickness.

Bud
 
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Old 09-07-15, 09:35 AM
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Thanks Bud. Really good info. What did you end up doing around the windows? I'm not sure how I'd cap it being so deep.
 
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Old 09-07-15, 10:19 AM
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Mine was easy, I installed new windows . I also have a break to form trim where needed. They can be rented.

But I would cheat and frame the window with 2x material and cover that with trim material. Then install the J-trim on top to accommodate the vinyl siding. The small amount of heat loss due to the extra 2x material vs the benefits of a secure attachment is worth it.

There are several web references as to how to install trim. It takes a little practice but is certainly a potential DIY job.

Bud
 
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Old 09-07-15, 06:13 PM
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Build it out....Of course...That makes complete sense. Thanks!

I wish I had a break....
 
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