I don't understand

[Charley L. Bottger]

How one can pressurize a crawl space as I have never observed a crawl space with positive close foundation vents?????? Strange statement I found on a web site





"The supply duct openings must be closed off, or the crawlspace will be pressurized, which will blow crawlspace air into the occupied space. Also, the fact that supply air is entering the crawlspace means there is more return air leaving the house than supply air entering it; therefore, the house is being depressurized, making the problem worse. For this reason, supply duct leaks need to be sealed also.

[James H. Bushart]

The article you are quoting from explains it, fully. Do you not agree with the building scientist who wrote it?

[David A. Andersen]

The issues that he pointed out to be addressed are correct but his assessment is not.

Adding supply air from the HVAC system into the crawlspace will not pressurize the space greater than the living space above as the supply air duct also discharges into the house.

Air cannot move across the building envelope opening without a pressure differential. The supply air system cannot pressurize the crawlspace greater than it is pressurizing the living space.

He stated that closing the foundation ventilators with the HVAC running depressurized the crawlspace by one Pascal. How does negative pressure air enter a positive pressurized house?

I am also confused!

Increasing the size of the air-conditioning unit will not remove more moisture, the unit must be undersized to remove moisture.

Air leaks in the return will pressurize the interior of the house and draw in contaminants from the crawlspace. Adding supply air ducts to the crawlspace will depressurize the interior of the house that the return is trying to pressurize. It's not as big a deal as he's making it.

Blowing air-conditioned air into the crawlspace that does not have a cooling load causes increased amounts of condensation to occur within that space. He claims evidence to support a water source in the crawlspace. ??

When you seal off the crawlspace you want to pressurize it by taking in air from the exterior, passing it through the HVAC system to remove moisture and discharging it into the house/crawlspace.

Attempting to depressurized the crawlspace with a fan, sucks in moisture from the exterior leaks in the crawlspace and promotes moisture to accumulate in the crawlspace still.
　
He even said; "Clearly, even with the blocked vents, the crawlspace was relatively leaky and would need air sealing. "
　
He made some changes in the environment, but he wasn't 100% successful because of these inaccurate perceptions.

Why did he not properly seal the air duct system before he went out and excavated the foundation and added insulation (?) To the exterior of the house?
They still have a 150 CFM air leak after all the work was done!

He sealed up the penetrations in the floor from the house. This was a waste of time as pressurizing the duct supply needs a path to enter the crawlspace anyway. Another waste of time and money.

Why is it even necessary to seal the crawlspace air from the house interior when you're depressurizing the crawlspace? The air is not going to enter the house with the fan running.

I see nothing about installing a vapor barrier in the crawlspace to prevent moisture from the soil. Does he think drainage tiles are going to keep the moisture out of the crawlspace?

He talks about air pressure, but he doesn't seem to understand vapor pressure. You change the vapor pressure in the crawlspace and it doesn't matter what you do to the drainage on the exterior of the house, you're still going to pull moisture in from subterranean sources.

This is not a structural issue, it's an indoor air quality problem. I can guarantee you they spent more time/money digging up the foundation than addressing indoor air.

Did they resize the HVAC system which is now grossly oversized and cannot control humidity within the house (especially when they removed the latent heat loads in the crawlspace from the system)?

What is your assessment Jim?

[Gary Farnsworth]

There are varying opinions about crawl spaces, and will also vary to climate and area of the U.S.

My parents bought their home in Overland Park in 1951, and still live there. The home has a 3 and a half foot crawl space. The floor joists still look like the day the home was built. The concrete blocks look new; no cracks or mortar gaps. The HVAC venting ducts are in the attic. There are no return air ducts in the crawl space.

How did it stay this way for years, and it still looks like new? You figure it.

Crawl space is vented every spring through fall, and the crawl space entrance is screened. Crawl space vents are closed every winter, along with the entrance. There is 6 inches of gravel throughout the ground of the crawl space. (no plastic sheeting). There are no spaces, holes or openings between the crawl space and the living areas. Gutters, terracing, vegetation are all very well maintained for proper drainage and terracing away from home foundation areas.

I have seen many dehumidifiers set and running in crawl spaces, but my parents do not have one.

[Brian A. MacNeish]

Originally Posted by dandersen

The issues that he pointed out to be addressed are correct but his assessment is not.

Adding supply air from the HVAC system into the crawlspace will not pressurize the space greater than the living space above as the supply air duct also discharges into the house.

Air cannot move across the building envelope opening without a pressure differential. The supply air system cannot pressurize the crawlspace greater than it is pressurizing the living space.

Ever check differenct areas of a house with a micromanometer using a long hose running back to a central reference point? Yes, the crawlspace can be pressurized with supply air if there is no balanced return serving the crawlspace. Think about stack or chimney effect in a house....this is a very small pressure created by weak natural forces- no 1/3 or 1/2 hp fan used- but it causes air to move from the crawlspaces upwards to exit into the attic in looser houses.

He stated that closing the foundation ventilators with the HVAC running depressurized the crawlspace by one Pascal. How does negative pressure air enter a positive pressurized house?

I am also confused!

Increasing the size of the air-conditioning unit will not remove more moisture, the unit must be undersized to remove moisture.

Air leaks in the return will pressurize the interior of the house and draw in contaminants from the crawlspace. Adding supply air ducts to the crawlspace will depressurize the interior of the house that the return is trying to pressurize. It's not as big a deal as he's making it.

Blowing air-conditioned air into the crawlspace that does not have a cooling load causes increased amounts of condensation to occur within that space. He claims evidence to support a water source in the crawlspace. ??

When you seal off the crawlspace you want to pressurize it by taking in air from the exterior, passing it through the HVAC system to remove moisture and discharging it into the house/crawlspace.

Attempting to depressurized the crawlspace with a fan, sucks in moisture from the exterior leaks in the crawlspace and promotes moisture to accumulate in the crawlspace still.
　
He even said; "Clearly, even with the blocked vents, the crawlspace was relatively leaky and would need air sealing. "
　
He made some changes in the environment, but he wasn't 100% successful because of these inaccurate perceptions.

Why did he not properly seal the air duct system before he went out and excavated the foundation and added insulation (?) To the exterior of the house?
They still have a 150 CFM air leak after all the work was done!

He sealed up the penetrations in the floor from the house. This was a waste of time as pressurizing the duct supply needs a path to enter the crawlspace anyway. Another waste of time and money.

Why is it even necessary to seal the crawlspace air from the house interior when you're depressurizing the crawlspace? The air is not going to enter the house with the fan running.

I see nothing about installing a vapor barrier in the crawlspace to prevent moisture from the soil. Does he think drainage tiles are going to keep the moisture out of the crawlspace?

He talks about air pressure, but he doesn't seem to understand vapor pressure. You change the vapor pressure in the crawlspace and it doesn't matter what you do to the drainage on the exterior of the house, you're still going to pull moisture in from subterranean sources.

This is not a structural issue, it's an indoor air quality problem. I can guarantee you they spent more time/money digging up the foundation than addressing indoor air.

Did they resize the HVAC system which is now grossly oversized and cannot control humidity within the house (especially when they removed the latent heat loads in the crawlspace from the system)?

What is your assessment Jim?

I wonder how the work actually decresed "ice damming" unless by airsealing/change of upward air flows to reduce heat entering the attic that led to snow melt.

[Charley L. Bottger]

Originally Posted by jbushart

The article you are quoting from explains it, fully. Do you not agree with the building scientist who wrote it?

Nope I don't agree with it at all sounds like something a third grader would write not a scientist

[Charley L. Bottger]

Originally Posted by gfarnsworth

There are varying opinions about crawl spaces, and will also vary to climate and area of the U.S.

My parents bought their home in Overland Park in 1951, and still live there. The home has a 3 and a half foot crawl space. The floor joists still look like the day the home was built. The concrete blocks look new; no cracks or mortar gaps. The HVAC venting ducts are in the attic. There are no return air ducts in the crawl space.

How did it stay this way for years, and it still looks like new? You figure it.

Crawl space is vented every spring through fall, and the crawl space entrance is screened. Crawl space vents are closed every winter, along with the entrance. There is 6 inches of gravel throughout the ground of the crawl space. (no plastic sheeting). There are no spaces, holes or openings between the crawl space and the living areas. Gutters, terracing, vegetation are all very well maintained for proper drainage and terracing away from home foundation areas.

I have seen many dehumidifiers set and running in crawl spaces, but my parents do not have one.

I have inspected 100's of raised floors with the duct in the crawl space the soil and lumber was just as dry as could be and others water dripping everywhere each one has to be considered individually

[James H. Bushart]

Originally Posted by dandersen

What is your assessment Jim?

It's an interesting case study that has the potential for many variables as it may be applied in different areas of the country...

As for anyone's disagreement with the air pressures, however, they were measured and made a matter for record in this study. In the absence of a similar study done under the same conditions by another building scientist conducting the same method of analysis, I have no reason not to believe that the crawlspace was pressurized at the same time the house was depressurized. I believe that this was the point of contention that initiated this thread.