Crawlspace ventilation help


i have a question about crawlspace ventilation…

in the course i took last year, the ratio for crawlspace ventilation was 1:300 , but in the NACHI course it’s 1:150…

can anyone pls direct me in the right direction?:shock:


I believe the 1:150 is the maximum and that can be reduced by region, vapor barrier installed, etc.

Check with your local AHJ/code.

Even if it has a VB, I look for a lot of vents or note the potential for high moisture content in the wood.

Local conditions prevail.

Here’s an example from OBC - Ontario Building Code
**[FONT=Arial,Bold]size=4 [/size][/FONT][FONT=Arial][size=4]Where an unheated crawl space is ventilated by natural means, ventilation shall be provided to the outside air by not less than 0.1 m² of unobstructed vent area for every 50 m² of [/size][/FONT][FONT=Arial,Italic][size=4]floor area[/size][/FONT][FONT=Arial][size=4].
[FONT=Arial,Bold]size=4 [/size][/FONT][FONT=Arial][size=4]Vents shall be,
(a) uniformly distributed on opposite sides of the [/size][/FONT][FONT=Arial,Italic][size=4]building[/size][/FONT][FONT=Arial][size=4], and
(b) designed to prevent the entry of snow, rain and insects.
[FONT=Arial,Bold][size=4] Ventilation of Heated Crawl Spaces
[/size][/FONT][FONT=Arial,Bold]size=4 **[/size][/FONT][FONT=Arial][size=4]Heated crawl spaces shall be ventilated in accordance with Section 9.32.

I advise my clients not to ventilate the crawlspace.

Who would want humid air condensing on the cold floor / walls.

Great infos guys!!


Exactly I too advise my clients to not ventilate the Crawl space .
Here is some Info more if you need it

Thx Roy!! very helpful links indeed!:mrgreen:

Glad to help any questions any time do not hesitate …

Although we can see there is some mixed opinion here, several points need clarification.

  1. Is it a “heated” crawlspace versus “unheated”?
  2. What would the building “standard” be at the time of construction?

My point being does an engineers “opinion” trump “building code” as may be suggested; or perhaps be careful of how the inspector reports an opinion beyond the standards of practice.

Claude how about you give us your thoughts and answer your own questions … Roy

I have seen too many damp from ventilation full of mould CS and cold rooms will give my thoughts and ideas to my client how to help have a healthier home .

Patrick posted the question. I simply reponded attempting to indicate what the code states in Canada, offering help.

Seems you take exception to my response - that is unfortunate!

How would I know if Patrick’s question is related to a heated or unheated crawlspace? My point was simply there may be other “conditions” that equally make crawlspaces not as damp. This includes proper/adequate ventilation.

It goes back to simple logic of the intent of building code, and considering the conditions encountered by the inspector.

NO Claude I did not take exception to your post sorry if it came across that way .
("Seems you take exception to my response - that is unfortunate ") .
I and others are always looking on how to educate our selves and others .
If you have some information or thoughts I was just asking you to share them .
For you to not give facts or ideas is unfortunate for all.

Good point Claude!

I omitted to say that I also recommend that the perimeter walls be insulated, a sealed VB on ground and perimeter walls be installed and a source of heat provided being we are from the great white north.


ok here is the situation :

surface : 30 feet x 24 feet
floor : dirt (irregular 2 to 8 feet of clearence)
VB : covers only half of the ground surface
Ventilation : 2 (on opposite walls) size 2 inches X 2 inches
Heat : one big a** electric heather (220) takes alot of energy to run
moisute : alot
insulation between joists (floor) : missing half of it
insulation on walls : none

Fix :
-insulate between joists
-VB everywhere (min 6 inch over laped…taped )
-insulate walls (fiberglass boards or spray foam + fire retardant)
-get rid of the big a** heater
-block the ventilation holes
-can’t really add gravel over the VB because of the irregular ground
-insulation tubing over the copper pipes

i"m i on the right track? i would think yes , but if i"m missing something pls add it up!! lolll


I have a neighbour who had a similar problem with their basement / crawlspace. They had the foundation walls sprayed with ( poly-urethane ) foam and as a lot of their basement was exposed limestone bed rock they had that foamed too. The rest of the mud floor ( about 30%) was covered with a VB. They also had heat ducting from the furnace opened into the area and a cold air return installed.

They are now able to store cardboard boxes, books and clothing in their basement and the floors of the house are reportedly much warmer too.

Roy, i appreciate your point of view. However, the building code states and makes it emphatically clear. It sets the minimum “legally” acceptable building standard.

One must also consider the age of the house, the conditions present and what the impact is on the client.

I do not discount what others say, but the point being I always like to the CYA principle (cover your ***) - what will hold up in court? Opinion or fact, acceptable building standard or a home inspectors opinion.

Patrick what you have described in the original condition is deemed an unheated crawlspace. I would recommend ventilation, along with proper moisture protection. If it is really wet - I have often seen sump pits and sump pumps in the crawlspace.

If the foundation walls and header area is properly insulated, now it can be deemed a “heated” crawlspace, and that changes the ventilation requirement.

I would be cautious with insulating between the floor joists, particularly if a vapour barrier is installed on the cold side (bottom) of the floor joists, and where the insulation is provided on the perimeter foundation walls. My point being I have often found condensation collected in the cavity. Example wet insulation and wet areas in the bottom of the floor joists.

I hope this helps.

Look at the time you’re posting here!!! Do you go to bed late or get up early?

To minimize exposure to liability - home inspectors should stay away from giving advice how to rectify observed shortcomings. Our only obligation is to establish the condition of the subject property as observed at the time of inspection. We are simply not obligated to predict the future performance of building components, nor are we being paid to provide solutions how to correct or improve revealed defects and/or shortcomings.

The different resolutions posted how crawlspaces should ideally be vented or insulated gives reason to suggest that particularly home inspectors with little or no practical construction experience are well advised to keep their mouth shut to prevent complaints from consumers who have followed their advice - but where the suggested improvements have not solved the problem as predicted.

RUDOLF REUSSE - Home Inspector since 1976 - TORONTO