Do I need a vapor barrier in here?

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  #1  
Old 10-28-14, 08:45 AM
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Do I need a vapor barrier in here?

Hello all,

I recently purchased a house that was extensively rennovated and the work for the most part appears to be very well done. An area of concern, however, is a crawl space that is off the master bedroom. Here is the entrance to it:

Name:  Crawl Space 2.jpg
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The crawl space is probably 30 feet long and is sandwiched between the bedroom, an ensuite bathroom and the exterior roof. Inside, the crawl space has fiberglass insulation on both sides - on the wall on the interior side, and on the roof side as well. Here's the thing - I don't see any poly vapor barrier in there, either on the inside or outside wall. I pulled some of the insulation out to take a look and there is some strapping and an airspace on the roof side and on the inside wall (bedroom wall) there is some kind of dense particle board between the insulation and the sheet rock. I know the picture doesn't show this, so I'm doing my best to explain ...

Other important information - I live in southern Canada (British Columbia) in a somewhat temperate climate. Winters see a few months of snow, but summers get quite warm (90 degrees F, 30 C) for at least a few months of the year. The roof on that side of the house is south facing, so it gets most of the sun. The crawl space is not directly heated, although it will certainly get a bit of heat leaking in from the bedroom and the rest of the house. Normal practice in this area is to put a poly vapor barrier on the warm side (inside) of all exterior walls. I see poly in the rest of the house, so I find the omission here curious.

To get to the point, do I have a problem here? Should I put in a poly vapor barrier, or is it possible that the dense particle board on the inside wall will essentially be the vapor barrier? Should I wait and see if I have any condensation issues, and if so, what should I be looking for?
 
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  #2  
Old 10-28-14, 09:50 AM
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Hi jj and welcome to the forum,
@jj " Should I wait and see if I have any condensation issues?"
Unfortunately it could be a bit late once you discover the moisture issue.
I'll have to absorb your description a bit, but as a general rule, you need to define your thermal shell and keep the air barrier and moisture barrier at the same location. Example, having insulation in the rafters along with insulation in the interior wall would not be good. The reasoning for that is not entirely obvious, but it deals with partially heated spaces that can be filled with humid air and have surfaces below the condensation point. Keeping everything together and in contact proves less of a concern.

A side note, air leakage is the major transporter of moisture so wherever you define that shell it wants to be as air tight as possible.

Now, that side attic should be insulated on one side or the other. If the interior wall is your choice, then the floor needs to be insulated below and possibly in need of a vapor barrier. Possibly, because VB's are not always needed except in the very cold climates. In addition, that space would need to be vented to the outside both high and low.

What is currently available for venting?

If the rafters are selected for your thermal boundary, then the insulation on the warm side needs to be up to code, the space above it needs a vent path, and the side attic needs to be conditioned to some level.

There is a lot to choose from so I'll add a good whole house link that might help.
http://www.buildingscience.com/docum...build-renovate

I'm running long so I'll let you catch up.

Bud
 
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Old 10-28-14, 10:41 AM
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The venting is currently a ridge vent that runs most of the length of the roof and soffit vents down below under the eaves. There is air space of course between the insulation and the roof, I would say at least a few inches, but I haven't measured it. When I bought the home, the previous owner who did the renovations explained how the air space wasn't up to code before, so they removed the roof piece by piece and raised it up to allow for a proper air space in there.
 
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Old 10-28-14, 10:43 AM
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they removed the roof piece by piece and raised it up to allow for a proper air space
What? What exactly does this mean? From your description, it sounds like a lot more work than most people would put into something like this.
 
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Old 10-28-14, 10:50 AM
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Sorry, I'm not the best with construction related lingo, so let me try to explain better ...

The roof probably was a ton of work if it truly wasn't up to code. The rennovations on the house were quite extensive. Most of the house was gutted down to the frame and foundation and redone including the plumbing and the wiring. The house was subject to building inspection upon completion, so everything had to be up to code. My understanding is that when they realized that the roof didn't have sufficient air space, additional lumber was added to the outside of the trusses (or whatever you call the framing for the roof) to build them up to allow for the additional required air space. How exactly this was done or accomplished I don't know for sure. If it helps, I can try and get in there with better lighting later today and take some better pictures.
 
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Old 10-28-14, 11:05 AM
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You said in detail what I had inferred from your original statement - not a small project.
 
  #7  
Old 11-21-14, 10:09 AM
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Ok, so long story short, I have decided that the vapor barrier will go on the roof side and not the interior wall side. I need help determining what I should use for that vapor barrier as I have done some research and realized that there are options here.

Before I go into the options, let me outline what my roof construction looks like. From the outside-in, the roof is:

- asphalt fibreglass shingles (good ones - apparently rated for at least a 30 year life)
- tar paper
- OSB sheathing
- air space of nearly 2" (created by laying 2x4s under the OSB)
- 1x8" boards running accross the rafters (these were the old roof I think before the new one was laid over top, there is 1-2" of space between each adjacent 1x8 - in other words, you can see between them to the air space and the osb sheathing above)
- 2x8" rafters
- approx 6-8" of firbreglass bat insulation between the rafters

One other thing of note about the roof. The area I am working with here is ventilated, but perhaps not very well. There is a large chimney and a jut out section at the bottom of the roof (for an exterior door) that basically prevent an easy bottom to top vertical venting of this section of the roof. To that end, the roofers chose to make the vent channels in this section run horizontally accross the roof, rather than vertical from the soffit up to the ridge. They accomplished this by laying 2x4's to create air channels between the old roof (the 1x8s) and the osb sheathing they laid over top. Not ideal I know, but I can see why they did it in this case. The roof is about 50 feet wide, and this horizontally vented section is maybe a third of that. This side of the roof is also south facing and gets a significant amount of sun. During the recent cold snap we had, we got some snow and even though the temperatures were well below freezing, when the sun came out the snow on that side of the roof began to melt quite quickly. To bottom line it, I'm not confident that the venting will work well and combined with the sun exposure, its likely this part of the roof won't stay very cold, but I want to what I can to minimize the potential problems.

Given the less than ideal setup for that part of the roof, I'm considering three options for my vapor barrier:

1. Just add a layer of poly over the existing fibreglass and make sure its sealed up good. Advantage - cheapest and easiest. Disadvantage - there may not be sufficient insulation in this part of the roof.
2. Use polyisocyanurate ridgid foam - I am told this acts as a vapor barrier (when properly sealed at the edges) as well as having the advantage of adding some additional R value to the roof. Disadvantage - would be awkward to install and more costly.
3. Closed cell spray foam the underside of the roof. Advantage - good R value and acts as a vapor barrier. Disadvantage - very costly.

Option 2 seems to me to be the most feasible, but I welcome opinions/thoughts. If I do use option 2, the one question I have is how I would apply it along with the exisitng fibreglass. Could I simply apply 2" of it over top of the joists and seal it up (with the fibreglass still between the joists), or would it be better to go the other way, with this rigid foam between the joists and then the fibreglass bats over top of it? Either way, I know its important to get a good airtight seal.
 
  #8  
Old 11-21-14, 11:49 AM
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Once you decide to make the bottom of the rafters your thermal boundary and air barrier you need to be sure that side attic space will keep the air barrier above the dew point. Since the bottom of the rafters will in some way be sealed off from outside air, that space will essentially be filled with inside air and the moisture it carries. A lot to explain, so let me give you two articles illustrating the two options. The articles have the same origin but I'll let you dig through them, they are short.
Two Ways to Insulate Attic Kneewalls - Fine Homebuilding Article

http://www.finehomebuilding.com/pdf/021230088.pdf

Bud
 
  #9  
Old 11-21-14, 01:53 PM
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Thanks Bud,

That article is perfect. It pretty much describes my situation to a tee. I'm going to use the insulate the roof method - still a pain and a ton of work, but easier in my case than trying to do the floor and the knee wall.
 
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