Homeowner question for the braintrust

[herr_oberst]

My house has a crawlspace of approx 3 feet in height, the living area subfloor is fully insulated with batts, (the edges of the joists are not insulated), water pipes are insulated with foam sleeves, there is a very heavy-duty vapor barrier on the dirt floor plus all of the cracks around the sill and penetration areas have been sealed with spray foam. The walls of the crawlspace are not insulated.

This is a ranch house, built in about '72.

This is Portland, relatively mild winters but wet.

My question is, now that I have the subfloor in the crawlspace very well insulated should the vents be covered?

I used to cover them when there was no insulation (to keep drafts at bay and keep the pipes from freezing...) but now I think I should leave them open to prevent condensation.

Can I get your thoughts?

[Dantilla]

Leave the vents open.

The insulated floor is the conditioned "Envelope", the rest of the crawlspace is not.

The vents should be closed only if the foundation perimeter walls and floor were insulated so that the crawlspace was within the heated area. Then there is no need to insulate the floor joists.

[futurefun]

Let those vents do their job & breathe.

[RWebb]

ok to cover in winter; not summer

[herr_oberst]

Randy, great point, I meant to specify winter.

Isn't the build up of condensation in the winter an issue?

[on2wheels52]

A trusted home repair guy said to put a small fan in the crawl space and keep it running year round. That it doesn't matter if you have vents or not, the import thing is to keep the air moving.
Jim

[HardDrive]

Yeah, you do not want moisture building up in the space. I discovered an unvented area under a bathroom addition on our current house. Horrible black mold issues.

[unclebilly]

I run forced air from the furnace into my crawl soace with no return (I let it leak out). This keeps it warm in there in the winter and my pipes from freezing. The walls are insulated.

[RWebb]

I dunno what you are supposed to do, but "down" here everybody seals them in the winter & opens them back up in summer.

OTOH, Portland has light rail and we don't so maybe it a redneck thang...

[island911]

Webbs first post has some wisdom.

Try to understand what drives condensation - it collects on the colder surfaces first. Your floor will be warmer than the ground. On cold winter days the foundation walls (concrete I expect) will be the likely place for any moisture to accumulate. Is that a problem?

You can always experiment to gain more information. I would. I expect that where the foundation meets the floor/exterior wall will be the first place to look for any developing problems.

[island911]

Originally Posted by HardDrive

Yeah, you do not want moisture building up in the space. I discovered an unvented area under a bathroom addition on our current house. Horrible black mold issues.

That sounds like a bathroom leaking water thru the floor. A different problem, but still one to look out for.

That is, you can still have black mold even if fully vented.

[dad911]

I'd also seal the vapor barrier to the wall and any penetrations in the vapor barrier.

[tharbert]

Originally Posted by RWebb

ok to cover in winter; not summer

^^^This

[willtel]

I used to work for a company that installed these vents in crawl spaces. They have a metal coil that winds and unwinds with changing ambient temps and closes the vanes in the vent with it. Good for seasons like now with cold nights and warm afternoons.

[1990C4S]

Originally Posted by unclebilly

I run forced air from the furnace into my crawl soace with no return (I let it leak out). This keeps it warm in there in the winter and my pipes from freezing. The walls are insulated.

Don't tell Mike Holmes.

[A930Rocket]

My understanding is that you want a little bit of moisture, so leaving 15-25 percent of the vapor barrier off is ok. I've always done it without problems.

And my vents were always open year round.

Originally Posted by dad911

I'd also seal the vapor barrier to the wall and any penetrations in the vapor barrier.