Vapor Barrier

Old 02-03-17, 08:27 AM
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Vapor Barrier

As I prepare for finishing part of my basement I'd appreciate some advice on the vapor barrier. I have a low knee wall (15") with 2x6 framing on top, the framing has fiberglass insulation in place. I'll be covering the knee wall with 2" XMS then framing over it.

As it stands there is plastic sheeting stapled over the insulation but it is not sealed, I think it's really just there to hold it in and keep it neat, I'm planning to take it all out and replace it.

Questions :

1. I gather this really needs to be *sealed* i.e. I should tape all joins in the vapor barrier and top and bottom, same for the ceiling, correct?

2. If I glue the XMP to the knee wall and tape the joins do I need the vapor barrier to go over it or, if I understand correctly, the XPS is essentially a vapor barrier?

3. I'll be putting DriLock flooring on the concrete floor, do I need to "join" the vapor barrier on the walls to the flooring or just leave a gap?

4. My main issue - electrical boxes - I had planned on using the basic plastic ones, what should I do regarding the vapor barrier? I have seen in videos a soft, shaped sheath being used but don't know where to find them. Should cut and tape around the boxes? Use specialized ones with a lip/flange?

5. Can lights - I'll be putting cans in the ceiling that are IC rated, same question, do I need a vapor barrier (as the other side of the ceiling is not "cold" and, if I do, how should I seal round the cans?

Thanks again for all the great advice I have found here, I really appreciate it.

Old 02-03-17, 09:57 AM
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1). See #2.

2). The taped XPS is the only VB you need. Seal the gap where the foam meets the cement with a foam adhesive, like PL300. This assumes the walls are dry and there are no known moisture problems that would require the wall to have a drainage plane. (Leaving a small weep hole / gap in the sealant every 24" or so is often a good practice, in the event it needs to weep.)

3). I assume you mean DRIcore. Follow mfg instructions. DRIcoreĀ® Subfloor | Frequently Asked Questions They recommend a 1/4" gap around all perimeters, so it does not seem you could seal the perimeter. They also do not recommend a vapor barrier on the floor or any type of coating.

4). If that is something that concerns you, you can get vapor barrier inserts for your boxes, or you can get vapor-tight boxes. Carlon is one company that supplies them. (This seems pointless if the XPS is your vapor barrier, and you are framing in front of the xps. Your outlets will be in front of the vapor barier.)

5). You do not need a vapor barrier on the ceiling. It is pointless and probably counterproductive to install a vapor barrier between two conditioned walls or floors. When can lights protrude into an unheated attic space, you use IC / air-tight fixtures.
Old 02-03-17, 10:07 AM
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As usual, the internet can provide a wide variety of information and the reader is left with the chore of sorting it out. Here are some quick comments although they may not be what you want to hear.
1. All wood that contacts the concrete needs to be treated for that use and all fasteners into that wood also need to be special.
2. Vapor barriers are not always necessary and can cause mold issues if the fiberglass insulation is on the foundation side.
3. The rigid foam board (pink or blue) is typically referred to as a vapor diffusion retarder and will allow a small amount of drying to the inside. In NH I would suggest no VB is necessary unless your code department is still 20 years behind the building sciences. Do I Need a Vapor Retarder? |

Before the ceilings are sealed off you will want to air seal and insulate those cavities. Walls or ceiling the more significant issue is keeping the moisture laden air away from a cold surface. Those IC cans are probably also AT (air tight) and the installation should maintain that, if there is a cold zone above.

X beat me again.

Last edited by Bud9051; 02-03-17 at 10:08 AM. Reason: smiling
Old 02-03-17, 11:05 AM
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I think all of my outlets apart from one will be in the framing over the knee wall so, as you pointed out the XPS will be the vapor barrier, that's good news

I plan however to put one around chest level that'll be behind a wall mounted TV, I guess that's the one I need to think about whether to get a vapor seal box or just tape around it.

Great news and realization about the cans, I'd automatically thought about sealing everything, makes life a lot easier as it is indeed a warm/warm ceiling/floor.

And yes, I mean DriCore

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