Bricks, Masonry, Asphalt and Concrete - How do I design a crawl space?

We're going to add a 6' x 8' extension to our 1 story house to expand a bathroom. The house has a full basement but there's no sense in putting a full basement under this little area, so a crawl space seems like the right choice.

I understand about footers being below the frost line and how the blocks are set, etc., but what other issues are there?

We don't want to lose heat through the floor which may even have full PEX radiant heating. The finished floor height is about 20'-22' above grade. Should there be a slab at grade level or just gravel? Should there be styrofoam insulation under the slab or gravel? Are there moisture issues in a crawl space?

All insight is really appreciated guys! :)

Your best option for a crawlapce under a bathroom would be a closed crawlspace.

In a closed crawlspace, there are no vents to the exterior. This can eliminate cold air penetration and pipe freezing in winter and condensation issues in the crawlspace when the vents are open in summer.

A closed crawlspace in your case could be used as long as the soil is covered with an approved vapor retarder like 6 mil polyethylene sheeting, the walls of the crawlspace are insulated, and the crawlspace air is either conditioned or opens to a space with conditioned air.

Polyethylene sheeting should extend up the walls to the sill plate and be sealed at all seams.

Insulation should meet minimum required by code for your area and extend from the top of the rim joists down along the wall to at least 2 feet below the frost line. If the floor of the crawlspace is higher than 2 feet above frost level, the insulation must continue in toward the center of the crawlspace floor on top of the floor at least 2 feet. Insulating the walls is a much more economically mean by which to keep the crawlspace warm and prevent pipe damage by freezing than any other method.

If you live where the International Residential Codes are in use, the crawlspace must be accessible either through an opening through the floor above at least 18x24 inches or through the wall of the crawlspace by an opening at least 16X24 inches. The minimum height of a crawlspace is 24" from the top of the finished floor to the bottom of the sill plate.

Crawlspaces that will contain usable space (storage) must have drain tiles and be damp or waterproofed form the exterior depending on soil conditions.

manhattan42 - "The minimum height of a crawlspace is 24" from the top of the finished floor to the bottom of the sill plate."


When you say top of finished floor, you must mean crawl space floor and not the finished floor of the bathroom right?

Existing construction is 2x10's on double sill plates so that height is 12.5" plus subfloor and finished floor. I don't think we even have quite 24" from top of the sill plates to the grade.

What happens if you don't have enough height - do you have to dig to full basement depth?

There is no way to have an access hole from the floor above so will access from a side wall compromise the "closed crawl space" concept?

Thank you!

Your local building code will often require certain distances from the crawl space soil surface to the joists. Here it is 36 inches or so. Information from the building permit folks might be a lot of help.

Syakoban:

Crawl space height is measured vertically from the top of the finished crawlspace floor to the bottom of the sill plate.

Under the IRC this height needs to be a minimum of 24" but could be more as chfite said.

One needs to excavate to achieve this minimum height if the height can not be achieved with the existing grade. This means the floor of the crawlspace will be lower than the surround exterior soil.

Under IRC, when the access is through the side of the foundation and any portion of the access opening is below grade, you must also provide an areaway in front of the access that is also at minimum 16x24 inches in area. The areway is just an excavated area that allows you easier access to the opening.

The bottom of the areaway must be below the threshold of the access opening to prevent flooding of the crawlspace.

Thanks a lot guys - this is very helpful!!

I'm starting to grasp this... to get a 24" or more clearance to the crawl space floor we will need to excavate some. My concern is that water will pool in the space since there is a lot of water flow on that side of our house during rain. So would it be helpful to pour a slab as the crawl space floor to minimize water infiltration or is that a bad idea or an expensive waste? Drainage tiles won't work because they would be below the level of existing drainage pipes.

You may want to rethink the crawl space. I have one now under my home and I regularly have issues with mice, and other creepies. Not to mention, cold floors, the possibility of frozen pipes, mold and lost living space compared to a basement. This is why I'm having it excavated and having a full underpinning job done to convert to a real basement. I know it costs more, but you'll be happier in the long run.

I've never lived in a house w/a crawl space so I appreciate the insight.

Anyone else have feelings on the pluses or minuses of a crawl space?

Thanks guys! :thumbup:

The whole concept of a "closed crawlspace" eliminates all those problems that Ben Garden mentioned.

-A closed crawlspace by definiton is properly insulated...

-A closed crawlspace eliminates cold floors by requiring the crawslpace to be a 'conditioned' (heated or cooled) space...

-A closed crawl space is properly treated for ground water vapor instrucsion by requiring 6 mil sheeting on walls and floor...therefore no mold growth because no condensation.

-A closed crawl space eliminates mold and moisture problems by having no vents to the outside into which warm moist summer air can enter and condense on cooler crawlspace surfaces.

A closed crawl space IS just like a basement because it is constructed just like a basement without the cost of a basement.

A concrete floor aka a 'rat slab' should be poured to keep out vermin and provide a better storage and walking/crawling area.

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Drainage tiles would be necessary around the crawlspace just as in a basement to keep the area dry if it is to be used for habitation or storage when the floor is below grade.

You could even avoid this step if you could keep the crawlspace floor above grade by rasing the bathroom addition up above existing floor level by stepping 'up' into it if it worked architecturally.

A closed crawlspace is a very wonderful way to accomplish what you wish economically.

Here's a few good links for closed crawlspaces:

http://www.crawlspaces.org/

WOW!!! I had no idea such a site existed. Thank you again for the info.

I haven't fully looked at the site but I think there is one big issue for me to overcome with a closed crawl space: conditioning. We have baseboard heat and A/C is in the attic. How do you condition the crawl space with those systems as opposed to forced air?

Thanks guys! :thumbup:

If your basement is already conditioned, and your crawlspace opens to the basement, then the crawlspace is also considered 'conditioned' and you need to do nothing further.

I suspect that this is your case and that you will use an existing basement window as the opening between the existing basement and new crawlspace through which your plumbing drains and plumbing supplies will be run.

For purposes of the building codes, a space is also considered conditioned if it has heating and cooling equipment in it.

If your furnance or boiler is in the basement..the basement is automatically considered 'conditioned' because the room is warmed by residual heat from the boiler/furnace and heat loss from the pipes/ducts. As long as the new crawlspace opens to the basement, then the closed crawlspace is also considered a conditioned space.

It does not have to 'also' be air conditioned...Just heated OR cooled. Not both.

manhattan42 - you've been truly helpful on this issue and I really appreciate it!!!

Yes that, is the best route. We will open to the basement and effectively treat the area as if it were a shallower basement area, in terms of sharing the same environment. I also realized that we can install a drain and tie that into the existing basement sump pump to aleviate possible water accumulation. I assume that the code is cool w/that. (???)

Thanks guys! ;)