Vapor barrier for a building
January 16, 2009 11:12 PM   Subscribe

What's solution for the vapor barrier on my pre-existing building: caulk, housewrap, other?

I have a building in the backyard (about 250 sq. feet) that has been there since the 30s. It is well constructed, but the shiplap siding (redwood) is nailed directly over the studs. Inside is bare studs.

I want to finish off the interior, with electrical, insulation, and sheetrock. I'm just concerned about moisture getting into the space between the sheetrock and the siding.

I figure either I can caulk all the horizontal seams from the inside to make sure moisture does not get in, but apparently you're not supposed to do this with shiplap siding. I've also considered using a vapor barrier like roofing felt or Tyvek, and just wrap it around the inside, around each of the studs and then up against the inside of the siding. Or is the actual answer that I don't need to worry about this?

We're planning on this being the "Rumpus Room" for the kids, not a living space, although we will put a space heater in. This is the SF bay area, so its a moderate climate, with rain for 4-5 months of the year and pretty dry the rest of the year.

Thanks for any help or suggestions.
posted by mach to Home & Garden (7 answers total)
Does moisture get in now? A lot?

If not, I'd go ahead and insulate, wire and then vapor barrier and sheetrock. That's the way we always did it rehabbing an older home where moisture was (mostly) outside.

If so, I'm not so sure. I guess you could go the route you suggest, which might be better anyway. I think I would use Tyvek and do it like you described after some thought.

Either way, if you're hanging drywall it's worth the extra dollars to throw something up as a vapor barrier.
posted by cdmwebs at 11:59 PM on January 16, 2009

You should have your vapor barrier on the warm side of the insulation so that warm moist air does not condense in the insulation. For this climate, I would put it on the inside face of the insulation. However, using closed cell foam insulation (such as expanded polyurethane) reduces the need if you tape the joints.

Is water getting through the siding? If it is, you need a water-barrier (which can be vapor-permeable) such as Tyvek housewrap. Unless your studs are very well preserved I think you would probably need to protect them from getting wet too so it probably means that you need to remove and replace the siding.

Personally, I am a fan of multi-foil insulation (which acts as an air/vapor barrier) such as Actis. It is quite expensive, but you can place this on your existing studs, put spacing battens for a 25mm air gap, and then attach your sheet-rock. It is the equivalent of 200mm of fiberglass and has a radiant barrier.

Are you going to do anything to the floor/ceiling? If not, it might not be worth insulating any of the room, but just to make it all air-tight. You can have a few vents/windows to ensure air exchange when required.
posted by a womble is an active kind of sloth at 3:01 AM on January 17, 2009 [1 favorite]

Against the siding, it's infiltration of liquid water that you need to worry about, i.e. if you regularly have wind-driven rain regularly coming through the siding and running down the insides of the wall. As has already been said, the vapor barrier goes towards the warm side of the wall in a climate like yours. You should put one in, under the sheetrock, because it's cheap and easy to do, but in a rumpus-room that has no shower, no kitchen and is only occasionally heated, in a climate where temperatures never get very extreme so there's never a big differential between indoor and outdoor temps, it's really not going to be doing anything.
posted by jon1270 at 4:02 AM on January 17, 2009

Does moisture get in now? A lot?
There's a few places where I can see there has been water damage in the past.

Are you going to do anything to the floor/ceiling?
Yes, I plan on insulating the ceiling as well. and putting down new laminate flooring.

Unless your studs are very well preserved I think you would probably need to protect them from getting wet too so it probably means that you need to remove and replace the siding.

That's the option I really want to avoid. The studs actually are pretty well preserved.

Another option is to just patch the areas that have seen water damage and use the Tyvek like I'd originally thought.
posted by mach at 10:14 AM on January 17, 2009

What type of insulation do you plan on using? I would be leery of using fiberglass because it will become a big sponge if water gets through to wet it. Then you start having problems with mold and rot. I would consider using closed cell foam board insulation cut and fit into the stud bays The material cost is higher but generally the R value is also higher and most importantly it will not turn into a big sodden mess in the wall. One method for getting a good seal with foam board is to cut the board a 1/4 to 5/16 smaller in height and width, wedge it into the space with a few nails to hold it centered and then foam the cracks with insulating spray foam. (Not the latex type it is not closed cell)

Are the clapboards moisture sealed on the outside surface? If they are not or the paint or whatever is worn they can become saturated to the point that they will be wet through to inside. So if that is the only water getting through reseal the outside surface.

Since you are not in a cold climate perhaps you don't need to insulate. That way any moisture that comes through the siding will be able to dry out in the stud cavity. I have seen windows that have been leaking for years in a uninsulated house that have not cause much damage because the water that infiltrated just either ran out or dried inside the wall.

If you need a bit of insulation you might consider using this stuff it is bubble wrap with a reflective layer ("U" value), on each side and can make a pretty big difference. Generally it is stapled up to the studs on the inside surface and often is use with insulation in the bays.

Fix the leaks now while you have the luxury of being able to see where the rain is coming in.
posted by flummox at 5:37 PM on January 17, 2009

How about insulation like this? Might solve a couple problems at once. I have no idea about cost, though.
posted by cdmwebs at 8:21 PM on January 17, 2009

This may be more vapor barrier and sheathing info than you want, but I found this site to be very educational, especially this pdf. Good luck with your project!
posted by orme at 8:41 PM on January 17, 2009

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