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Old 01-06-2012, 09:45 PM   #1
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Ventilated or Unventilated Crawlspace?


I am trying to make sense of the MA building code (which defers to the IRC as far as I can tell) regarding ventilated and unventilated crawlspaces. I am chasing the idea of fixing up my crawlspace in such a way that it both 1) makes sense and 2) satisfies code.

Here is what I am reading:

Quote:
R408.3 Unvented crawl space. Ventilation openings in under-floor spaces specified in Sections R408.1 and R408.2 shall not be required where:
1. Exposed earth is covered with a continuous Class I vapor retarder. Joints of the vapor retarder shall overlap by 6 inches (152 mm) and shall be sealed or taped. The edges of the vapor retarder shall extend at least 6 inches (152 mm) up the stem wall and shall be attached and sealed to the stem wall; and
2. One of the following is provided for the under-floor space:
2.1. Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7m2) of crawlspace floor area, including an air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.9;
2.2. Conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m2) of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.9;
2.3. Plenum in existing structures complying with Section M1601.5, if under-floor space is used as a plenum.
Satisfying 1. is plain enough. However, the items 2.1 and 2.2 indicate that I need to exchange air (either with a fan or an hvac system) with the "common area" which I read as "the living area of my house". In 2.3, the definition of "plenum" is vague, and it seems that if it is a "plenum" then you are just satisfying either 2.1 or 2.2.

Frankly speaking, no matter how well installed a vapor retarder might be, I really am not happy with the thought of deliberately exchanging house air with the funky old crawlspace, though I understand the intent is to condition the crawlspace.

If I were to go with a ventilated crawlspace, then this is the rule:

Quote:
R408.1 Ventilation. The under-floor space between the bottom of the floor joists and the earth under any building (except space occupied by a basement) shall have ventilation openings through foundation walls or exterior walls. The minimum net area of ventilation openings shall not be less than 1 square foot (0.0929 m2) for each 150 square feet (14 m2) of under-floor space area, unless the ground surface is covered by a Class 1 vapor retarder material. When a Class 1 vapor retarder material is used, the minimum net area of ventilation openings shall not be less than 1 square foot (0.0929 m2) for each 1,500 square feet (140 m2) of under-floor space area. One such ventilating opening shall be within 3 feet (914 mm) of each corner of the building.
This is somewhat BS as my house is only 640 sq. ft. This is essentially the state of my crawlspace now, with one 1 sq. ft vent. Strangely enough, air doesn't seem to want to flow through it. This vent thankfully does not exchange a lot of air, so my pipes don't get too cold in winter.

There doesn't seem to be a mechanical ventilation option. Would it not make sense to have vents between my conditioned living area and the crawlspace, and a single mechanical vent fan that pulled air out of the crawlspace? This would pull a little vacuum on the house, pulling fresh air into the house, and would pull conditioned air through the crawlspace, flushing out any humidity and keeping the pipes warm.

Am I missing something?

I think my next house will be a yurt - much easier to maintain.

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Last edited by Thunder Chicken; 01-06-2012 at 09:51 PM.
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Old 01-07-2012, 07:40 AM   #2
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Ventilated or Unventilated Crawlspace?


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Originally Posted by Thunder Chicken View Post
Satisfying 1. is plain enough. However, the items 2.1 and 2.2 indicate that I need to exchange air (either with a fan or an hvac system) with the "common area" which I read as "the living area of my house".
The area "common" to both the crawl and the rest of the house is "outside".

Am I missing something?
That the "architect speak" vocabulary they insist on using used is nearly Orwellian.

Quote:
Would it not make sense to have vents between my conditioned living area and the crawlspace, and a single mechanical vent fan that pulled air out of the crawlspace?
Yes it would (and does):
Tempered House Air --> Unvented and otherwise untempered crawl --> outside

hth

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Old 01-07-2012, 12:21 PM   #3
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Ventilated or Unventilated Crawlspace?


So I can have an unventilated crawlspace, so long as I mechanically ventilate it? The person that wrote this must be a real bore at parties.
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Old 01-07-2012, 12:33 PM   #4
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Originally Posted by Thunder Chicken View Post
So I can have an unventilated crawlspace, so long as I mechanically ventilate it?
As I understand things:
You can create a hermetically sealed off space... with plastic on ground, close off all gaps, insulate walls and joists above, seal doors and windows etc... so that any naturally occurring unchecked ventilating airflow is blocked off... so long as you then provide for known air movement through the space.

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The person that wrote this must be a real bore at parties.
I don't think he gets invited to many parties.
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Old 01-07-2012, 12:40 PM   #5
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Are you still required to insulate the floor joists even if you insulate the foundation walls? I might be confusing the definitions of a "ventilated" space vs. a "conditioned" space.
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Old 01-07-2012, 01:29 PM   #6
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Ventilated or Unventilated Crawlspace?


I imagine you are trying to make your home warm in winter?
With most homes the warm air inside rises, escapes through the walls, ceilings, doors, windows etc and continually pulls in cold air from outside, that then has to be warmed. Called the stack effect .....expensive.
A lot of this cold air enters from the ventilated crawl space.
The way to deal with this is to box in the joists with sheets of blue board or similar insulation, making sure the insulation is at least air tight and trying for water vapor proof.
Once this is done, it doesn't matter if the crawl space is ventilated, insulated or not.
The problem is, that about 200 times a year a warm wet mass of air will arrive, that is drawn under your home through all the holes and gaps and this warm wet air can condense onto any cold exposed wood work etc making it wet with the possibility of mold and wood rot.
Once the joists are boxed in and kept warm and dry the problem goes away as water vapor only condenses on surfaces that are below the dew point, closed cell insulation is happy to be both cold and wet on one side and warm and dry on the other. At this point it does not matter what happens in your crawl space as it cannot effect your joists, it will keep your feet warm and lower your heating costs.
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Old 01-07-2012, 10:44 PM   #7
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I imagine you are trying to make your home warm in winter?
I basically just want a crawlspace that doesn't make me want to drink heavily when I look into it. It has insulation between the joists now and the house is pretty tight and warm. The big issue is that there really is no structure in the crawlspace holding the floor up other than some old cinder blocks, rocks, and some temporary jack posts.

I plan to get some beams and proper columns installed and replace a section of the floor that was done badly. Removing this floor is actually a blessing as it means we can get all the materials, beams, etc. in right in through the front door into the crawlspace. We can also dig and grade the earth more easily. The floor is over a very shallow section of the crawlspace - if I belly crawl under it my shoulder blades are against the joists.
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Old 01-08-2012, 07:50 PM   #8
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Ventilated or Unventilated Crawlspace?


This is a very contentious subject.
I am dot going to disagree with anyone here. Everyone always has their feelings about it.
My thoughts are that if you know you have proper grade and a non/baloon framed home, that vents left wide open year round are the way to go.
In your case, I would add some so you have good cross ventilation.
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Old 01-08-2012, 07:58 PM   #9
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Ventilated or Unventilated Crawlspace?


My personal feeling with crawlspaces is that they should be treated like a basement, just that it has lower ceilings. Seal that up nice and tight, clean up the floor, and ensure there's at least one return and one supply so the air gets circulated properly using indoor air only. Clean up spider webs and other nasties and it makes a nice storage space for Christmas decorations, and other stuff of that sort.

Good idea to run a dehumidifier in summer too if it gets humid down there. Keep in mind they also generate heat though.
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Old 01-08-2012, 08:15 PM   #10
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Originally Posted by Red Squirrel View Post
My personal feeling with crawlspaces is that they should be treated like a basement, just that it has lower ceilings. Seal that up nice and tight, clean up the floor, and ensure there's at least one return and one supply so the air gets circulated properly using indoor air only. Clean up spider webs and other nasties and it makes a nice storage space for Christmas decorations, and other stuff of that sort.

Good idea to run a dehumidifier in summer too if it gets humid down there. Keep in mind they also generate heat though.
Thats a new one on me. Are you saying; run a supply and return duct off from the homes forced air system?
The crawl spaces here in Va typiclly are about 32" high, surrounded with a brick curtain wall that is never tight up to the framing, and foundation vents every 6'. If a brick home, the 1" air space between sheathing and back of brick, allows for plenty plenty of air from the cornice/soffit. All on a 4 mill poly covered sand floor.
I think you must be talking about something different.
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Old 01-08-2012, 10:31 PM   #11
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Ventilated or Unventilated Crawlspace?


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Originally Posted by Red Squirrel View Post
My personal feeling with crawlspaces is that they should be treated like a basement, just that it has lower ceilings. Seal that up nice and tight, clean up the floor, and ensure there's at least one return and one supply so the air gets circulated properly using indoor air only. Clean up spider webs and other nasties and it makes a nice storage space for Christmas decorations, and other stuff of that sort.

Good idea to run a dehumidifier in summer too if it gets humid down there. Keep in mind they also generate heat though.
All well and good, but my 640 sq. ft. crawlspace was established by Bubba and his drinking buddies back in 1941 and is about 18" deep on average. The house supports are concrete filled sewer pipes, concrete blocks, and a couple of boulders randomly shoved under joists (no beams). The perimeter wall whistles when the wind blows. I think the spider webs and other nasties are the only things holding it all together.

If you can tell me how to get this crawlspace into shape without losing $10k and my sanity, you'll be my friend for life.
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Old 01-09-2012, 12:13 AM   #12
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You will just have to build a sill system with footings, block piers and treated triple 2xs. Anything in that 18" has to be treated and 6x6s or 4x6s are not allowed anymore.
Check with your locality to see how deep the footings have to be. They only have to be 4" below grade here in Va.
Two weekends
I know in Vt, they have to be deeper.
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Old 01-09-2012, 10:08 PM   #13
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Thunder Chicken,
please, please, please post pictures!

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