Thread: Humidity in raised crawl space

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Very nice "raised" crawl space about 4 feet high. Only thing in it is a sump pump and the HVAC. There are several vents to the outside around the periphery. Moisture measurements of the joists and ground were around 16%. Condensation was dripping off the AC ducts. Outside temp in low 80's with high humidity.

My questions are if it is ventilated from the outside is the high moisture simply from the outside humidity and there is nothing that can be done about it? Will a dehumidifier help or would we be simply trying to dehumidify the whole city since it is vented to the outside. Should the ducts be insulated - bu then I am concerned about the copper water supply pipes freezing during the winter.

Your guys expertise would be greatly appreciated.

Firstly where in the USA are you located? America is a big place with different weather conditions.

1. Install minimum 6 mil plastic on ground and lap up sides of concrete block and secure, overlap seams by six inches and seal.

2. Insulate duck work to stop sweating

3. Alternatively seal off vent openings to exterior, place 6 mil plastic over ground and don't insulate duct work. And make it a conditioned space.

Between outside humid air being drawn into a cool space and moisture coming up from exposed earth floor you will always have a high humidity level. This can lead to mould, and deterioration of wood framing in crawlspace over time if the conditions are not brought under control.

More info can be found here
Building Science Information

Originally Posted by Raymond Wand

Firstly where in the USA are you located? America is a big place with different weather conditions.

1. Install minimum 6 mil plastic on ground and lap up sides of concrete block and secure, overlap seams by six inches and seal.

2. Insulate duck work to stop sweating

3. Alternatively seal off vent openings to exterior, place 6 mil plastic over ground and don't insulate duct work. And make it a conditioned space.

Between outside humid air being drawn into a cool space and moisture coming up from exposed earth floor you will always have a high humidity level. This can lead to mould, and deterioration of wood framing in crawlspace over time if the conditions are not brought under control.

More info can be found here
Building Science Information

I am in Columbus, Ohio. Hot humid summers and cold winters.

Originally Posted by Tom Cissell

I am in Columbus, Ohio. Hot humid summers and cold winters.

No to the dehumidifier.... Do not add any fans....

I like what www.crawlspaces.org has to say about crawlspaces.

With wood moisture content around 15% that means the relative humidity in the crawl space is about 80%. You should shoot for keeping is below 60%. A moisture barrier on the ground should help a lot. I would seal the vents and use a dehumidifier. Without sealing the vents or covering the ground don't bother with a dehumidifier. You will use a lot of energy and accomplish nothing.

The ducts should have been insulated anyway

The pipes should have been insulated anyway.

Therre is a lot more to sealing a crawl space than mentioned above. Half ass seal will do nothing. If the only concern is with the ducts sweating and the pipes sweating or freezing. follow my first choices. 16 percent is not high at all. It will bother nothing. Homes where folks keep their AC temp high as in not turned down can have 12% in the drywall and home goods. Humidity will and does fluctuate over the summer where you are. It is not always high. A return duct or supply duct in a crawl should always be insulated. As well as the pipes.

I repeat. Unless you are going to do a complete professional crawl space sealing don't seal it at all. You will still have moisture in the crawl and maybe more as it will build up to higher levels. The duct will always be sweating. The copper pipes will always be dripping.

Have an HVAC company insulate the pipes and you can guy pipe onsulation and insulate the pipes yourself

Originally Posted by Raymond Wand

Firstly where in the USA are you located? America is a big place with different weather conditions.

1. Install minimum 6 mil plastic on ground and lap up sides of concrete block and secure, overlap seams by six inches and seal.

2. Insulate duck work to stop sweating

3. Alternatively seal off vent openings to exterior, place 6 mil plastic over ground and don't insulate duct work. And make it a conditioned space.

Between outside humid air being drawn into a cool space and moisture coming up from exposed earth floor you will always have a high humidity level. This can lead to mould, and deterioration of wood framing in crawlspace over time if the conditions are not brought under control.

More info can be found here
Building Science Information

This makes the most sense. "And make it a conditioned space," is key.

Originally Posted by Scott Patterson

No to the dehumidifier.... Do not add any fans....

I like what www.crawlspaces.org has to say about crawlspaces.

Thanks Scott, Great source of info on crawl space.

Originally Posted by Scott Patterson

No to the dehumidifier.... Do not add any fans....

I like what www.crawlspaces.org has to say about crawlspaces.

Living next to ocean, I swear by this webpage's advice. A great source of info for crawlspace management. I've seen the results - Excellent!

Originally Posted by Ken Amelin

Living next to ocean, I swear by this webpage's advice. A great source of info for crawlspace management. I've seen the results - Excellent!

It is my first stop when I have a question about sealed crawlspaces or if I just have a question about moisture management in crawlspaces..

This is also a good reference: BSI-009: New Light In Crawlspaces ? Building Science Information

There is another item not directly addressed in this thread. Duct sealing. It is most likely referred to in the Building Science articles that others have referred to, but I thought it worthwhile to bring out in the thread itself so it's not unintentionally overlooked.

Sealing of any ducts which may be leaking should take place before insulating the ducts. Any sealing should be done with mastic for longevity of the job.

This will help to:
1. prevent Return ducts from pulling air from the crawl and distributing it into the house and (potentially) pressurizing the house. This is an IAQ measure as well.
2. prevent Supply ducts from pushing air into the crawl and (potentially) depressurizing the house.
3. Save energy spent on conditioning that air that may be leaking.

Best to all!

Seal foundation vents, install vapor barrier using radon requirements, insulate duct work and water supply lines, and then install a constant running exhaust fan through a closet or void in home. These are techniques being used in high radon areas but also work in high humidity situations

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Seal foundation vents, install vapor barrier using radon requirements, insulate duct work and water supply lines, and then install a constant running exhaust fan through a closet or void in home. These are techniques being used in high radon areas but also work in high humidity situations

Originally Posted by Tom Cissell

crawl.JPG
Very nice "raised" crawl space about 4 feet high. Only thing in it is a sump pump and the HVAC. There are several vents to the outside around the periphery. Moisture measurements of the joists and ground were around 16%. Condensation was dripping off the AC ducts. Outside temp in low 80's with high humidity.

My questions are if it is ventilated from the outside is the high moisture simply from the outside humidity and there is nothing that can be done about it? Will a dehumidifier help or would we be simply trying to dehumidify the whole city since it is vented to the outside. Should the ducts be insulated - bu then I am concerned about the copper water supply pipes freezing during the winter.

Your guys expertise would be greatly appreciated.

Here in Florida I have a high humidity crawl space with ducting. I am slowly remodelling this 1950's house and have planned the following for the crawl space:

1. Insulate the underfloor between the joists using R13.
2. Install thermal reflective barrier under the joists taped and sealed, wrapped down all around and sealed to foundation to assure no air leakage, this is non-perforated vapor barrier type of reflective barrier.
3. Install heavier insulation around ductwork with all new duct and hangars.

If the vapor wrap seals the wood of the house then it should not matter how much humidity is in the block or concrete foundation within reason. Too much would be a bad thing and would need assessed later for other "fixes" regarding the foundation being moist.

This method should end or severely reduce moisture at the wood and stabilize the moisture year round as well as add insulative values. If water lines are inside the joists or insulated and inside the thermal barrier they should not freeze. It will also give better insulative values and stop moving air through the floor and walls from under the house.

Here in Florida, many houses have fully vented soffits, full ridge vents, sometimes turbine vents (powered or passive) added as well as gable vents, we also see many with a lot more crawl space ventilation than "code minimum".

Sometimes I have seen vents every other or every third block around a foundation. These crawl spaces usually feel a lot less humid overall. Then again we dont get cold weather (below freezing for more than a few hours) or snow.

Dirk, If I understand you correctly you want to install a vapor barrier on the bottom side of the joists after you insulate the bays. This, in my opinion, would create a moisture issue where moisture will form between the floor above and vapor barrier. Some of the worst moisture issues I have seen in a crawl space were the result of this type of installation. Let me clarify that in my area we have low humidity, it may be much different in your area. Crawl spaces should be designed for the area and other factors. I like the idea of a conditioned crawl space: BSI-009: New Light In Crawlspaces ? Building Science Information Just my opinion.

Dirk, After I wrote my post I found this: http://www.crawlspaces.org/ some good information on crawl space design for the Southeast.

Originally Posted by Tom Rees

Dirk, If I understand you correctly you want to install a vapor barrier on the bottom side of the joists after you insulate the bays. This, in my opinion, would create a moisture issue where moisture will form between the floor above and vapor barrier. Some of the worst moisture issues I have seen in a crawl space were the result of this type of installation. Let me clarify that in my area we have low humidity, it may be much different in your area. Crawl spaces should be designed for the area and other factors. I like the idea of a conditioned crawl space: BSI-009: New Light In Crawlspaces ? Building Science Information Just my opinion.

Dirk, After I wrote my post I found this: http://www.crawlspaces.org/ some good information on crawl space design for the Southeast.

Thank you for the information I will look into it. There are different questions in rework to new install.

It might be that the 3 or 5% open thermal barrier would work better. I can look into that as well.

One large question comes to mind, sealing (vapor barrier) the crawl space when well ventilated is NO different that sealing walls in Florida. Stucco walls have ventilation between the stucco and the vapor barrier, this is a condensation point. The interior of the walls are stabilized by the air conditioning of the house. All condensation is outside the vapor barrier if that happens. Also any kind of siding will have some air movement between it and vapor barrier in most circumstances. This assures condensation is does not degrade the siding.

Unlike many areas we are in fine sand here. We have virtually no clay or rock. When a water line failed and thousands of gallons of water went under the house, it was dry in a couple days. Most of the water percolated in minutes to hours.

I understand that condensation on the vapor barrier may happen in the crawl space at times, but not inside it, in fact the vapor barrier will assure that humid air does not permeate into the oak flooring and through the house (as it does now fairly readily) as well as from under the house through the walls and into the attic space.

Thanks for the information and I will see what the differences are on how this property can be made more efficient and keep moisture from entering the house and floors from underneath.

1. Remove the wood scraps (termite treats) that are on ground. Conducive debris such as wood scraps, paper, and cardboard should be removed to prevent attracting wood destroying insects.

2. Install 6 mil black plastic (vapor barrier) on ground. Overlap seams and cover all exposed soil.

3. Not sure what do about the humidity, it's a dry climate here in Eastern Washington. If the primary source of humidity is coming from the weather and climate, you might want to consider making it a "sealed and conditioned" crawlspace.

4. Before considering a sealed and conditioned crawlspace I would first test for Radon.

5. Under the right conditions a sealed and conditioned crawlspace has many benefits and can be a cost effective solution to improve energy efficiency.

6. With a sealed and conditioned crawlspace you would only have to insulate the perimeter foundation walls and joist band. There is no need to insulate HVAC ducts, water supply pipes, or below floors.