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Old 06-26-2011, 07:41 AM   #1
pwa
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yet another vapor barrier post


Hi to all, I have a home in the northwest built in the 30's with a crawlspace about 800 sq.ft. that is only half covered in 6 mil plastic,and insulated between the joices with R19.There are 2 6x10 vents(1 choked off by sidinding to 2x10)that I uncovered for summer.The dirt is bone dry and I am finishing the R19 but do I need the plastic if there is no moisture?There is no heat/air ductwork in the crawl space.Thanks in advance for any help!!
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Old 06-26-2011, 09:11 AM   #2
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yet another vapor barrier post


With insulation between the floors joists, and essentially an open crawlspace (where air gets in most of the year), then what you have is the floor joists themselves becoming colder than the actual flooring above.

Once the floor joists becomes colder, condensation from the outside air turns into moisture on them, they become wet and eventually turn black and rot...over time, not next week!

That's the big picture; however humidity i.e. vapour in the air can come from two places in an open crawlspace: the air, like I just said and the soil below. Ideally, you should be trying to reduce, as much as possible, the evaporation of humidity from everywhere - including the soil - and that's the reason people put a Level 1 vapour barrier (6 mil poly) on the soil. Some people even tape the seams together and tape the plastic to the crawlspace walls, by about 4-6".

So tell me are the bottoms of your joists, damp or wet at all? I don't see many crawlspaces up here - but there are some still in the older houses built in the 20's and 30's. Problems occur in these homes when a non-breathing flooring material such as vinyl rolls was put down in the 50's and 60's...then came the idea of insulating down there between the joists to warm the floor. Nowadays, we have even greater problems...

Nowadays, the concept of an "open" crawlspace makes little sense. Now we condition them like the rest of the house, i.e bring the crawlspace into the building envelope.

So, IMO, I'd cover the remaining half of your floor and tape the pieces together. Then, I'd look into doing something to warm up those floor joists I mentioned. I'd also look into increasing the square-footage of open ventilation access vents you have now with a view to making it as effective as possible.
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Old 06-26-2011, 12:32 PM   #3
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yet another vapor barrier post


Hi,No I have not seen any dampness.What do you mean by warming up the joices? The vents have been closed since I bought the house and this is the first summer I took them off,they were'nt really made to come off and I plan to cover them in the winter?I am putting in the r19 today.Got about 7" to work with except for a couple of doug out isles.yeah I know suck it in and suck it up
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Old 06-26-2011, 12:55 PM   #4
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yet another vapor barrier post


As soon as you put insulation between the joists, you prevent warm air from rising to the floor above - that's the whole point. But by doing that, you cool down the individual joists to where only the 1-2" that the joist sticks out over the insulation by get colder sometimes to below the dew point in there, and you get condensation on the ends of the joists.

Not a major thing, but in some climates it can quickly deteriorate; I can't tell you whether to open or shut your vents, seems like you should be doing what we do up here in re: cold rooms. Open during the summer to dry out, closed during the winter.
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Old 06-26-2011, 02:39 PM   #5
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yet another vapor barrier post


thank you for your reply,back to the dirt
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