We all should know the difference between an open and closed foundation/attic system but
just in case there are any questions, I’m going to provide a quick overview. Recently, an inspector
asked me a question about an encapsulated foundation system regarding gas line
installation. This made me think that maybe not all of you are looking for everything that is
important in the different types of moisture control system that can be in a house.
Open Foundation System
An open foundation system use to be the most typically observed foundation ventilation
system having a plastic vapor barrier over all the dirt grade surface (newer homes) or at
least coverage over some of it in older homes. Foundation vents are provided to allow for
air flow providing the ventilation. Back in the day, builders were allowed to have a
reduction in the number of vents required if they would provide a full coverage of plastic
inside the foundation. (Just to point something else out, it’s not the number of vents but
rather the sq. inches of open ventilation in the foundation that must be provided for)
Louvered slide type vents have much less clear open venting area which requires more of
these than the baffled type vents. As you know from doing inspections, the slide vents are
almost always frozen in place and the baffle vents are almost always broken having the
thumb slide broken off. If the builder chooses to keep to the code venting requirements,
only 40 – 60% of the dirt grade coverage was required to be provided. I think it’s
interesting to see how codes have changed over time.
A sealed crawlspace is not an encapsulated crawlspace, but it is a closed foundation system.
It’s kind of like the whiskey/bourbon debate in that not all whiskeys are bourbon, but all
bourbons are whisky. A sealed crawlspace has a fully covered dirt grade surface vapor
barrier and has all but typically one foundation vent completely sealed off. It must have a
mechanical system (typically a dehumidifier) to provide ventilation. The one vent left open
is to allow for heat and/or emission transfer from the mechanical system operating. In a
sealed crawlspace, insulation is still required in the floor system.
Now we get to the encapsulated system. This is also a closed foundation system, but it is a
closed system on steroids. It has no foundation vents at all. It also has a moisture barrier
covering not only the dirt grade but also the side foundation walls and the interior piers.
The interior side of the foundation walls must be provided with insulation prior to the
vapor barrier being installed and the vapor barrier must be sealed along the top edges at
both the piers and foundation walls. There must be 3-4” of exposed CMU block under the
sill plate to allow a visual area to look for evidence of termites. Don’t you know that the
National Wood Destroying Insect Association had a fit when they first started employing
encapsulated crawlspaces and did not provide this viewing space at the top of the
foundation walls! Insulation is not required in the floor system and the foundation access
door should have some sort of seal on it.
This is a Must!
Now comes the important part of this inspection reflection. The HVAC system must have a
supply vent opening (typically a dryer vent end cap at the air handler plenum. If it is a large
crawlspace, a minimum of two supply vents should be provided – one at the air handler
plenum and one at a distribution box deeper in the foundation area. There are no return air
openings – only supply air. The supply air provides positive air pressure inside the
foundation that prevents outside air from being able to infiltrate into the foundation.
Think of the crawlspace as being a balloon and the supply vents are trying to blow up the balloon.
The HVAC system is always trying to inflate the crawlspace to keep outside air out of it.
More and more older homes are converting open foundation systems to closed
encapsulated systems. In homes that have been converted (in all homes) you must make
sure if it has a 90% efficient furnace, that the combustion air vent extends to the exterior of
the foundation wall. This would not have been necessary with the original installation in
the open crawl space but must be provided in the encapsulated system.
Essentially an encapsulation of the crawlspace assimilates the crawlspace area into the same air
envelope as that of the house and you cannot draw combustion air from the interior of a house.
One more thing to look at and this is what the inspectors question was specifically about. You must
check to see that the gas line does not have a pressure regulating valve installed inside the
foundation area. Again, this would be allowed in an open crawlspace but when converted
to a closed encapsulated system the regulator must be relocated to vent to the exterior of
Whether it is a foundation or an attic (spray foam sealed) all the prior mechanical
I hope this provides a little more understanding about open and closed ventilation systems.
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