Thread: CRAWL SPACE VENTS

recently had a presentation by a crawl space contractor---he stated that when they are called in to restore a wet crawl space and put a vapor barrier in they seal all crawl space vents--he said they all should be sealed and HI should recommend that

now i was always told that the juror was still out on this---open vents in summer--close in winter--what does everyone think--and what if crawl has heat/ac vent
charlie

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Originally Posted by CHARLIE VAN FLEET

recently had a presentation by a crawl space contractor---he stated that when they are called in to restore a wet crawl space and put a vapor barrier in they seal all crawl space vents--he said they all should be sealed and HI should recommend that

now i was always told that the juror was still out on this---open vents in summer--close in winter--what does everyone think--and what if crawl has heat/ac vent
charlie

This from Building Science.
RR-0401: Conditioned Crawl Space Construction, Performance and Codes —

Here in northern Nevada the argumant is always whether or not to open or close the vents. Most get closed and stay that way forever or until an HI or pest guy comes along.

I usually tell them to open the vents always to allow for ventilation. Many of our crawlspaces are damp.

Recently a company in Carson City has been selling the "Clean Space Crawlspace System". This system does require closing and sealing of all the vents as well as any other gaps or holes to the exterior. A heavy guage vinyl moisture barrier is installed and sealed at the seams and to the perimeter foundation wall. A sump pump with battery back-up is installed as is an automatic fan system. I hear its really spendy to install.

I've only seen one in the Reno area and it was a pleasure to crawl around in - really clean. The home above was a disaster but thats another tale.

Jonathan

jonathan
clean space is who gave our presentation
thanks

Beware of seminars or presentations given by persons or companies pushing their products or services.

Most are professional salespersons and could sell you tits off a bulldog and make you think you needed them. And got them for a good deal.

rick

rick

i know the sales pitch stuff--does texas have crawl spaces and what do you tell your client---open close--stuffed---what--that is what i'm asking for--country wide
thanks
charlie

Charlie,

In my 20+ years of crawling foundations, I could count on my hand the number of homes I've seen with a vapor barrier.

Those that did have them were not installed properly. Large puddles of water is seen on top of the vapor barriers that I've seen or pictures of.

Here in our area, most HI's recommend leaving the foundation vents off all year to allow ventilation of the crawlspace.

I've seen many homes that were soaking wet and condensation dripping off the floor joists due to having no ventilation. You take the vents off, place a fan under the house and the crawlspace would dry out in just a couple of days.

We used to do this all the time doing termite treatments. Some houses would have water ponding along the inside walls of the foundation. Most were due to improper drainage. Regardless, we could not termite treat or inject chemical into standing water so we had to take measures to get the soil dried out. AIR MOVEMENT worked great with any moisture problem.

There is some homes I've seen that actually had some type of fan system that was designed to fit in the foundation vent opening to push the air. We used to see them advertised in a PC magazine all the time. Some PC companies installed them in the slow times of the year.

Rick

Here is North Carolina we have 90% humidity with 90 degree heat in the summer. The crawl space area is cool. Guess what happens. The humid air goes into the cool crawl space and condenses. The air circulation attempts to dry the space out but the air is so humid, it is difficult for it to absorb more.

The best idea is to close the crawlspace, insulate the walls, vapor barrier on floor and walls, and condition the space.

Most of the homes around here have 75%-95% of the crawl space floor covered with vapor barrier. New construction is approaching 100% coverage.

If the client is not going to close and condition the space, then ventilate. At least some of the moisture will evaporate.

In the South I would advise folks to open the crawl space vents.

A sealed crawlspace is only good if it was designed to be sealed. I have seen numerous problems with crawl spaces that have been sealed and they were not originally built to be sealed.

If the ground is prone to being damp, then you don't want to seal that moisture in.

As for vapor barriers, the IRC requires them so I would hope that all new construction has one.

Uncle Joe Lsitburek has some good advice on the topic.

http://www.buildingscienceconsulting...s_moisture.pdf

I like his Top Ten Lists

Building Science Corporation - Joe's Top Ten List of Dumb Things to do in the South

Building Science Corporation - Joe's Top Ten List of Dumb Things to do in the North

Obviously, he's more passionate about the South!

My wife and have a 1,450 sq. ft. house in the Lakewood area in Dallas, TX. Our house was built in the 1920’s and we have a pier & beam foundation. When we purchased the house our crawl space had some drainage and ventilation issues, but since then a French drain with a sump pump has been installed around the entire outside foundation wall, so now any runoff/rain water is pumped out away from the house. We also had to take out all of the original hardwood floors because they were cupping due to moisture that was caused by a severe leak, which has since been fixed. Since the old owner had a leak, this moisture caused mold, so we had to have a mold remediation company come out and properly clean the crawl space. Since all of this work has been done, we now have brand new hardwood floors and a fairly clean crawl space, but I want to make sure our floors do not cup or get damaged from any future moisture and I want to make sure we do not have any mold come back in the crawl space. We have 4 foundation vents and I am wanting to install 4 more vents myself to help ventilate the crawl space more, but there are so many different opinions online and I would like to have a professional opinion on the following questions:

1. We only have 4 crawl space vents for a 1,400 sq. ft. house and I wanted to install about 4 more vents, but the internet shows a ton of people saying to not ventilate your home and others that say to put plastic sheeting down. So my first question is what is the best way to ventilate my crawl space to avoid future wood rot problems and to avoid our brand new hardwood floors from cupping?

2. Our sub floors and crawl space are not insulated, should we insulate them? If so, what needs to be done in order to properly insulate my crawl space?

3. Are there any precautions that I need to take before cutting out four new 16”X8” holes for the new vents (other than electrical and plumbing lines)?

I would greatly appreciate any answers and/or advice that you have. I want to make sure that I only have to do this once and I don’t want to have to worry about this again. Again, I really appreciate your valuable time.



Thanks,

Bradley Gleaton

The best advice will come from someone around your area but up here in soggy Oregon the answer would be install more vents - get them near the corners if the other ones aren't near them already. Then, cover the ground with 6-mil black plastic sheeting (overlapping the seams by 6" or so). Insulation is up to you.... of course it's a good idea energy efficiency wise. I've never noticed an impact on moisture one way or another.

Again, wait for someone local as things might be different down there.

Originally Posted by Matt Fellman

Again, wait for someone local as things might be different down there.

...in the meantime you might want to read this, I'm pretty sure I got this from someone on this site.

An electric baseboard heater can help a lot, it's dry heat. I see a lot of crawls, and the newer ones always have a concrete skimcoat over (hopefully) a plastic vapour barrier. A drain should be installed at the lowest point in the floor, but there should never be that much water in there, so the drain is for emergencies like plumbing leaks.
The baseboard heater should have a thermostat near the hatch, otherwise it is practically useless, as in turned off and too hard to get to, (or turned up too high and too hard to get to). But with the concrete over plastic, most crawls are pretty dry, and used for storage a lot of times.
Sometimes, a bath exhaust fan is installed with a humidistat. This seems like a good idea, but it can be very bad. If no incoming air is provided, moist warm air will be sucked down from the living space.
Usually, you need those vents open around the perimeter so wind can blow in one side and out the other. But I see unventilated crawls that are working well, with a bit of heat and good perimeter drainage. Insulation, either foil blanket or foam board, goes on the perimeter walls. I think the key is a really tight vapor barrier, because the damp comes up from below, at least it does so here in my area.

This is a familiar sight here in the humid south. Single story 3000 sq. ft. with vented crawlspace. Makes me wonder if we have been putting the vapor retarder on the wrong side of the insulation? I've become a big fan of closed crawlspaces.

I read the attachment, "Crawl Space Moisture Control", and by the looks of that reading there is no really good way to ventilate a crawl space. Again, I live in a 1,450 sq. ft. house in Dallas, TX that was built in the 1920's and the crawl space only had 4 vents. I have added two other vents and I plan on adding at least four more. I am installing the 16"X8" vents that can be purchased at Lowe's. These vents will automatically close at 40 degrees or lower. Do you suggest any other vents? After reading the attachment, it makes me think if I need to install a powered crawl space ventilator. What do you think? My last question is my house has the vents on two sides of the house, do I need to install vents on every side of the house? If so, you would be able to see the vents on the front of the house from the street and I was afraid of that. Please let me know what you think.


Thanks,

Bradley Gleaton

Crawlspace moisture control actually begins at the perimeter of the structure. It will not matter how much ventilation you create if you have poor drainage on the exterior.

Does the soil around the perimeter have proper slope away from the structure?
Do you have guttering on the home with downspouts that direct the flow away from the structure?
Is guttering clear of debris to assure drainage?

rick

We just had a french drain installed around our entire foundation. The french drain pipe drains into a sump pump in the backyard which then pumps all of the water out to the back alley. The only area that we have gutters are on the back side of the house where our wooden deck is and the downspout goes right into the sump pump. Therefore, we do not have any drainage issues or at least I don't think we have any drainage problems. I just want to see if I need to do anything else other than installing 4 to 6 more foundation vents (total of 8 to 10) on all 3 sides except the front of the house. Please let me know what you think. I greatly appreciate all of your input.

Thanks,

Bradley

Outside where the deck is located at the corner of the house, is there any drainage control under the deck?

Yes...the company that installed the French drain and sump pump removed about 15 of the top boards on the deck and installed the French drain underneath the deck as well. Therefore, we should have all of the drainage corrected since we moved in unless I am wrong, which could very much be the case. Please let me know if I need to do anything else for drainage and any other precautions that I can take or products to use/install to make our crawl space moisture level acceptable so our new hardwood floora do not cup or get damages. Please let me know what you think. I really appreciate all of the input.


Thanks,

Bradley

Do you not recommend me using these temp vents for my crawl space in Dallas, TX? I have already installed 4 of these and I was going to instll 4 to 6 more. I just liked the fact that it will automatically close if it is 40 degrees or colder, so I don't have to worry about our hardwoods getting socold in the winter time. I noticed on the owners manual that I can stick a piece of wire in the vent to open it even if it is under 40 degrees so I can still ventilate in the winter time if I need to. Please let me know what you think. I appreciate all of your input.

Thanks,

Bradley

I don't see where they'd would be an issue but I can't see why you would need as many as you mention.

Are you able to get access under the home? How is the clearances?

rick

Originally Posted by Bradley Gleaton

I just want to see if I need to do anything else other than installing 4 to 6 more foundation vents (total of 8 to 10) on all 3 sides except the front of the house.

From the IRC:

SECTION R408
WALL VENTED CRAWL SPACES

R408.1 Space moisture vapor control.

Vented crawl space foundations shall be provided with

foundation vent openings through the exterior foundation walls.

R408.1.1 Foundation vent sizing.

The minimum net area of ventilation openings shall be

not less than 1 square foot (0.0929 m2) for each 150 square feet (13.9 m2) of crawl space

ground area.

Exception:

The total area of ventilation openings may be reduced to 1/1,500 of the

crawl space ground area where the required openings are placed so as to provide

cross-ventilation of the crawl space. The installation of operable louvers shall not be

prohibited.

R408.1.2 Foundation vent location.

One foundation vent shall be within 3 feet (914mm) of

each corner of the building. To prevent rainwater entry when the crawl space is built on a

sloped site, the uphill foundation walls may be constructed without wall vent openings. Vent

dams shall be provided when the bottom of the foundation vent opening is less than 4 inches

above the finished exterior grade.

Every study has their own conclusion about what is the best way to ventilate a crawl space. I believe the best method depends on your area of the country. I think a good place to start would be to install your vents per code then you can decide when is the best time to open and close em.

According to my quick calculations (in other words double check my math) you will need at least 12 8"x16" vents and at least 2 of those should be on the front of the house within 3' of the corners.

The crawl space clearance is about 3 feet. According to the code of 1 sq. ft. of ventilation for every 150 sq. ft. of crawl space we would need around 18 vents which is way too many. I am planning on having around ten or so. 4 on both sides of the house and 2-3 on the back side of the house. What do you think about that? I appreciate all of your time.

Originally Posted by chris mcintyre

According to my quick calculations (in other words double check my math) you will need at least 12 8"x16" vents and at least 2 of those should be on the front of the house within 3' of the corners.

8 x 16 = 128 x 0.70 (for metal louvers to account for net free vent area) = 89.6 square inches net free vent area per 8" x 16" vent

If talking about vent blocks, they will provide even less net free vent area, wood louvers provide about 0.25 instead of 0.70 net free vent area.

Using 1,450 sq ft of crawlspace, the net free vent area required would be 1/150, so, to make it easier, let's round the 1,450 sq ft up to 1,500 sq ft.

1,500 / 150 = 10 sq ft net free vent area is required (with the vents properly located within 3 feet of the corners and cross ventilation from each other)

10 sq ft = 144 sq inches x 10 or 1,440 sq inches of net free vent area

1,440 sq inches / 89.6 sq inches per vent = 16.07 vents (based on standard metal vents which give approximately 70% net free vent area of the gross vent area)

Now, if the metal vents are screened, the net free vent area drops even further, meaning even more vents would be required - all based on the 1/150 vent area/crawlspace area ratio.

Originally Posted by Bradley Gleaton

The crawl space clearance is about 3 feet. According to the code of 1 sq. ft. of ventilation for every 150 sq. ft. of crawl space we would need around 18 vents which is way too many.

Way to many as in 'Way too many for what you want.', but not 'Way too many for what is required.'? Define "Way too many."

Originally Posted by James Risley

is the "reduction to 1/1,500th with cross ventilation" correct? sounds like an error

That is not in my IRC, but it may be in some state edition so I cannot comment on that. It would be nice to know what that is from, though.

What is the "net" opening of an 8x16 vent? 8x16=128 less 10%?

Basically, if a vent is a metal vent and the net is not stated on the vent, then the code presumes a 70% net free vent area. If the louver is wood, the code presumes a 25% net free vent area.

Think of it this way: Take a screen on a window or a door, measure off a 1" square, envision sliding all the threads to one side and to the bottom of that 1" square, what is left is the "net free" area.

Let's say there are 13 vertical and 13 horizontal threads in that 1" square and that each thread was 0.015".

That would be 0.015" x 13 = 0.195" for the threads *in each direction*, which means if all of the vertical threads were slid to the left you would have an opening 0.805" wide x 1" high with 13 horizontal threads in it. Now slide the horizontal threads to the bottom and you have an opening - net free opening - of 0.805" x 0.805" = 0.648 sq inches net free opening for that 1" square screened area - the rest of the opening is really blocked by the screens thread. If 1" sq inch is 100%, then 0.648 sq inches is 65%.

Because the crawlspace vents should be screened, take the net free area of 70% for metal louvers time 65% (in the above example, it may well change for your screen) and you would have the net free area of a screened metal louver if screened with the same screening as is on my screened back porch (which is where I got my measurements from for this example ).

I have attached a drawing of the outline of my house and i included a key on the bottom left hand corner to let you know what is what. Please take a look at this and let me know if this looks okay or if you have any other suggestions. I am going to finish up this project this weekend, so any other suggestions would be greatly appreciated. Again, I want to thank you all for all of your input.


Thanks,

Bradley

My vents say 50 NFA on the box and in the literature. Do you think installing 15 of these vents is overkill (15X50NFA = 750 NFA)? Are there any disadvantages from doing this? Please let me know what you think.

I am located in an area that has major crawl space issues related to moisture / humidity. Structural damage due to excessive moisture and the related wood destroying fungus are common.
Some observations I have made over the years.

1) never had a moisture problem that damaged framing in an uninsulated crawl. No fungus related problem on wood structural members. Older cottages in this area were built as summer vacation homes and crawls were not insulated.

2) batt insulation installed between the floor joists is a major problem.The condensation which accumulates on the underside of insulation tends to run down the insulation, soaking the lower areas of the joists and often causing fungus to take hold on framing components.

3) adequate amount of ventilation installed as per code helps, but does not totally eliminate the problem. High humidity levels and the cooler temps of the crawl space will still have unacceptable levels of condensation on the underside of the insulation.

4) It is common for the insulation to be installed face down. I realize this presents a fire safety concern - code violation - etc.. Insulation installed face down does , however, cure the condensation problem. Knowing this might open the door to acceptable methods and insulation products that can be installed in crawls with this type of moisture problem.

5) A vapor barrier helps if there is excessive and/or high levels of moisture in the soil, but has no effect on the condensation due to high humidity levels.

I would be greatly interested in ideas on how to deal with this problem , that do not entertain total encapsulation of the crawl area.

Originally Posted by James Risley

Bradley will have to take into consideration the space occupied by the louvers when the vent is open.

That is what the 70% does - takes into consideration the space blocked by (taken up by) the metal louvers. If the vent is marked with the net free vent area then you can use that, otherwise metal louvers default to 70%.

Originally Posted by Bradley Gleaton

My vents say 50 NFA on the box and in the literature. Do you think installing 15 of these vents is overkill (15X50NFA = 750 NFA)? Are there any disadvantages from doing this? Please let me know what you think.

Originally Posted by Jerry Peck

Using 1,450 sq ft of crawlspace, the net free vent area required would be 1/150, so, to make it easier, let's round the 1,450 sq ft up to 1,500 sq ft.

1,500 / 150 = 10 sq ft net free vent area is required (with the vents properly located within 3 feet of the corners and cross ventilation from each other)

10 sq ft = 144 sq inches x 10 or 1,440 sq inches of net free vent area

1,440 sq inches / 89.6 sq inches per vent = 16.07 vents (based on standard metal vents which give approximately 70% net free vent area of the gross vent area)

Do it this way (see above calculation to get this far):
1,440 sq inches / 50 sq inches per vent = 28.8 vents (which means you would need 29 of those vents for proper ventilation)

Originally Posted by James Risley

So if any of these vents provide cross ventilation an adjustment can be made for a lower number of vents based on whatever formula can be used in Dallas.
JR

???

I must have missed something, that 1:150 IS based on cross-ventilation layout and with vents within 3 feet of corners.

Did you or someone post something I missed?

Originally Posted by James Risley

So if any of these vents provide cross ventilation an adjustment can be made for a lower number of vents based on whatever formula can be used in Dallas.

You said Dallas, he is in S.C., besides, we questioned that 1/1,500 part and are still waiting for clarification on that.

Unless Dallas is different than Texas as a whole, then it is as I posted in the IRC.

Originally Posted by Jerry Peck

That is not in my IRC, but it may be in some state edition so I cannot comment on that. It would be nice to know what that is from, though.

I did not have a code book here, I found the pdf with a google search and now I can't find it so I am not sure where it is from... but I'll keep looking.

Found it:http://www.ncdoi.com/OSFM/Engineerin...es/040914c.pdf

Originally Posted by chris mcintyre

I did not have a code book here, I found the pdf with a google search and now I can't find it so I am not sure where it is from... but I'll keep looking.

Found it:http://www.ncdoi.com/OSFM/Engineerin...es/040914c.pdf

It is in the 2002 NC Building Code 1202.3.2 I haven't looked to see if it is in the new codes.

I am a woman. I am not an engineer nor a home inspection expert. I do have a good eye, and a great nose. I can spot mold, and I can smell musty. I also don't need to go to the sun to know it's hot.

As to vent or not to vent a crawl space I am deferring to Craig DeWitt PhD, PE

"Fill a glass with ice and beer. Condensation will form on the outside. You can blow on it all you want, and the condensation won't go away until the ice melts and the beer warms up."

I rest my case.

I have been selling and installing Temp Vents along with a moisture barrier since 1974. When left alone the moisture barrier and vents do the job of lowering the moisture content of the wooden members in the crawl space. I have never had a failure with this method.
When a customer gets talked into sealing his crawl it voids his termite warranty because the wooden members and ground area we inspect annually are covered
Tempvents were invented by a doctor in Gastionia NC in the early 70s. I was a salesman for Orkin at the time. The manager of our local office came back from a meeting and handed me a case. and told me to go sale these. I went through the material and looked at the sample vent. In my mind I thought of all the houses I had seen that could use some help with moisture. I sold four cases of these vents along with moisture barriers that week. The inventor came to our office and brought me and another salesman who had sold a lot of them also, a case of tempvents free. I put them in my own home and told a real difference.

For moisture control there is nothing better IMO.

I have attended "sales presentations" at our quarterly TN association of Inspectors meetings (HITA) and talked with two other contractors at a trade show. HITA members have discussed this at length and the general consenses is that the issue is not venting or not venting, the issue should be conditioned or unconditioned crawl spaces.

My current understanding:

1. Unconditioned crawl spaces should be vented at a rate of 150 sq. in. for every 100 sq ft of crawl space. I see this in less than half the homes I inspect.
2. A vapor barrier (4mil minimum) should cover all bare ground. Lower ground moisture will always wick to the dry surface and contribute to the RH% in the crawl space. I see this in about 20% of homes I inspect. Most vapor barriers cover 30% to 70% of the floor space.
3. In my experience, water in the crawl space is not a factor of having or not having a vapor barrier. The water is coming from the usual culprits; plumbing leaks, ground drainage and condensation from uninsulated plumbing and HVAC duct work.
4. Conditioned crawl spaces have installation standards that seem to be very consistant amoung suppliers. I have been told the state is reviewing and considering adding to building code. Its a "green thing". I have seen one conditioned crawlspace. The vents are sealed and the 20 to 24mil, reinforced vapor material covers 100% of the floor, seams overlap 4' to 6' and are sealed. The vapor barrier covers all colums and the side walls to within 3" of the sill plate. The 3" of exposure is to insure wood distroying insects are not given a hidden access to the home. The final item is a dedicated dehumidifier for the crawl space. This was one nice crawlspace to inspect.

Here is that Building Science.com link: BSI-009: New Light In Crawlspaces — Building Science Information

Hi, ALL &

* Up here, at least (Lower Mainland - British Columbia, Canada) on the WET West Coast, some form of crawl space conditioning is the recommendation. With a forced-air furnace, introducing forced-air all year around (heated minimally in the heating season & not heated otherwise) via the furnace blower-fan, into a well-insulated crawl. Keeps things dry & the lower floor warmer. If an older home, recommended to close-off those foundation vents for good. New homes are no longer built with foundation vents.

Not the same /not so easy with electric heat or some other means, as moving air is much more involved...

Need to 'move' air down there with some form of directional fan(s) & introduce some heat - against frigid floors and risk of pipes freezing.


Cheers !