Greetings Breaktimers. About 7 years ago, you all guided me through the design and construction of a second story addition that came out great. I am now embarking on the next phase and am hoping to once again tap into your wisdom. I hope you will indulge me again!
We are looking to add a 13 x 25 first floor addition to the back of our house…. for an expanded family room (13×18) with office nook (13×7) at the end. I am working with a capable contractor, and we are trying to figure out the best way to handle the foundation. House is in central CT.
Floor level will range from ~20 inches above grade at one end to ~5 feet above grade at the other. The addition is not adjacent to existing basement, so no matter how we do it, this will be unconditioned (ie. cold) space. Access and disruption from excavators and concrete trucks are also issues.
So, the options seem to be foundation walls with crawl space, or piers. If we do foundation walls, a good portion will have to be framed kneewalls because that is how the adjacent part of the house is built (where the tall foundation wall would tie in). There will be a deck across the back of the addition, so the appearance of the tall foundation is not a big issue. I can post a rough sketch if this is not making sense.
I should note that there will be no plumbing aside from baseboard water heat in the new space. These pipes would need to travel through the floor under a couple doorways, etc. but would mostly be above floor. We will have a gas fireplace in the addition for extra winter warmth.
I’ve been reading old posts, and the consensus seems to favor perimeter walls/crawl space over piers and post construction. We have no use for a full basement here.
Some of the reasons piers are still in the conversation:
1. A crawl space would still result in cold floors, limited access, potential critter intrusion, etc.
2. Access and disruption issues (excavators/concrete truck) would be avoided.
3. The underside of the floor would be mostly accessible if built on piers.
4. $$
So, I guess I have 2 main questions:
1. Why should we consider one approach over the other?
2. For both crawl space approach and piers approach, is there an accepted best practice for how to best compose (for moisture, insulation, air tightness, durability, etc).
Please let me know if you need any clarification. Thanks for any help!
Rick G
Replies
Foundation for Addition
Self-Bump
Rick
I'm hoping you get some feedback, but the question is tough to answer.
You know many of us would advise a full foundation, but you suggest you really want the alternative.
With hydronic heat, you pretty much would have problems unless you successfully detailed the install and/or used glycol in the system.
Open areas under a home is usually reserved for warmer climates or high water possibilities.
This will give you another bump up, best of luck.
Compromise
Every house is a compromise - There are no perfect solutions.
If you're interested in piers, check out Techno Metal Posts. There's a franchise in Naugatuck (I think - It's one of the Valley towns). Cheapest way I know of to install piers, and you end up knowing exactly what the bearing capacity is. Knowing about these, I doubt I'd ever build concrete piers again.
That said, I would always prefer a full basement or a crawlspace to a pier foundation in CT. You'll end up with a warmer addition, unless you go to extraordinary measures to insulate the floor (I'm thinking spray foam and some sort of rigid sheathing to keep out the critters). And keeping a crawlspace dry is actually easier than keeping a basement dry because crawlspaces are higher. If you go with a crawlspace, just build it like a short basement with a perimeter drain, vapor retarder, and slab. You can also insulate the walls from the outside when you're building, making for a much more energy efficient structure.
Basement vs Crawlspace
Thanks Andy.
So my next question is: what distinguishes a "full basement" from a "crawlspace"... especially if the crawlspace has vapor barrier and slab floor -- even if it is a so-called rat slab?
In my recent discussions with contractor, we are leaning toward the crawlspace with rough basement-like floor approach.
If we do that, I'm still curious about best practices for ventilation, insulation, location of vapor barrier etc.
Many thanks.
-Rick
Height
You should think of a crawlspace as a short basement. It's that simple. Drain it the same way as a basement, put crushed stone on grade and cover it with a poly vapor retarder, then pour a slab. Do the concrete work before framing so you can do a halfway decent finish on the slab - Nothing is worse than crawling on a really rough rat slab. Waterproof the outside of the foundation just like a basement.
Don't vent it - That just doesn't work in our climate. Vents let in humid air in the summer, which condenses and wets the crawl. In the winter, vents let in cold air that drives up your heating bills. And by putting in drainage, a vapor retarder, and a slab, you control ground moisture and eliminate the ostensible reason for venting a crawlspace. You wouldn't vent a basement, so why vent a crawlspace that's built like a short basement?
Basically, a "basement" is
Basically, a "basement" is anything over maybe 6 feet high. (Though under 7 feet it's probably more properly a "cellar" than a "basement".)
The other assumption is that a basement is fully enclosed, whereas a crawl may vary anywhere from simple lattice around a pillar foundation to a fully enclosed space.
If you go with a crawl space Google seal crawl space. Do not ventilate. Either way I would spray foam the floor to air seal (andbug seal) it then fiberglass below the foam, then cover with a sealed layer of plywood/osb.