Crawlspace foundation tips & (approximate) cost estimate?
Greetings, all. First post here.
Looking at building a small cottage (~600sqft) on my central Virginia property. Would like 24-30″ of “headroom” in a crawlspace foundation that I will have “semi-conditioned,” that is, it will be the return for conditioned air. It will also make maintenance of plumbing and electrical much easier, and possibly double as storage for lesser-used items like Christmas decorations, seasonal clothes, & etc.
The cottage should be as energy-efficient as reasonable so I anticipate SIP walls and roof.
The foundation will of course need some kind of vapor barrier as well as I expect some foam insulation under the slab.
I have plenty of room on my property so any removed dirt can just be moved ~200 yards rather than trucked off-site.
Questions:
1) Am I correct in assuming that I’ll want (starting at the earth) about 10-mil vapor barrier, then about four inches of EPS, then the slab would be poured over a rebar grid? How thick of a slab?
1a) I never see any “diagonal” rebar in pictures of concrete rebar grids. Wouldn’t a few of those make the whole thing more resistant to twisting in the event of an earthquake?
2) I have not installed any utilities yet. In that case, obviously I’ll want to put some pipe in before putting everything else down. Does it make sense to run about a 4″ PVC line out towards the future drainfield and maybe 2″ PVC towards the well and just cap them off underground for later use (if only as conduit). What would you all do in this situation? Any resources you can point me towards?
3) I plan to do most of the work on the cottage myself but the foundation is a bit beyond my skill level. Most online foundation estimates I’ve seen are $9-11k for a ~1700sqft home; I can use a calculator to extrapolate that down to about $4000 for 600sqft, but does it scale linearly like that? I plan to call some construction companies to ask their opinions but I’m at a conference this week so my free time during work hours is limited…
Thanks very much in advance for your thoughts here!
Jim
A picture of the site, for whatever it’s worth to you… (I have a bit more clearing to do before work begins)
Replies
Why pour a slab?
You're just pouring a "rat
You're just pouring a "rat slab". It doesn't need to be structural and even simple reinforcing wire may be overkill.
where is all this hair ...
dirt and stuff going to come from?
Just forget teh rat slab. Smooth the dirt out and place 6 mil , 10 if you like , down and forget about it. If your soil is rocky, put 10 yds of sand in there before you put your plastic down to provent it from get holds in it as you crawl around down there.
If you are anal a slab might be nice so you can lay wake at night think about how nice a crawlspace you have. LOL .. But.
Like I said, as a semi-conditioned space, I'll have the HVAC returns in the floor. I'm sure stuff will fall down there all the time...
Also I'm concerned that without concrete, the traffic will break through the plastic and probably break up the XPS insulation as well. I suppose I could put down a layer of bricks or something but that seems like more work than pouring a slab. Plywood?
Plywood?
no wood (or wood products) on the ground in the crawl-whether there's dirt, visqueen, stone, concrete or anything as a base.
When framing this building, DO NOT just broom off all the sawdust and small scraps over the side of the deck.
I'm guessing that's for termite control? Anyway, good pointer. I'll remember it!
Welcome Jim.
I have 3 slabs, all unreinforced, that suffered no damage from the Mineral epicenter 5.8 quake, about 40 miles from me. Why you would consider all that work and still not have a full basement is beyond me. I have yet to meet anybody with more space than they can use. Just a function of time before it's too small.
BTW, easier for your excavator if you leave those little trees. They pop out easily, with the root ball. Depending on WHERE in central Va, you will find large price discrepancies. I'm in Albemarle.
Good data points, thanks!
I don't need a full basement... This is just going to be a weekend place for now, in another 10 years I'll build a big house to retire in and the cottage will become the guest facility. The crawlspace will be primarily for utilities; I just figure I could use it a little bit for storage of infrequently-used items...
Also - my property is in Louisa, so not too far from you! Also closer to Mineral. lol
1. 24 to 30 inches of height in the crawl sounds like a drudge to me. I dislike going into my crawl that has nearly four feet. I'd never use yours for storage.
2. The crawl slab does not have to be structural. Forget any steel. Have the concrete mix guy add some fiber if it makes you feel better. My crawl has simple poly sheeting... three layers because its gotten torn up over the years and so it's easier to just add a new layer. I wish it had a light duty rat slab but adding it with a house already built is a bit of a bite.
3. Using the crawl as the return air plenum will result in a fully conditioned space, there won't be any "semi" about it. Put the air into ducts. There will still be residual heat loss enough to keep the crawl warm.
4. Design the utilities from the outset and install the appropriate conduit, pipe, etc. in the right locations. There is no upside to waiting to make those decisions. If you are uncertain about future expansion, then its OK to install "what if" conduit if you want.
5. No offense intended here whatsoever... You sound as if you don't have a lot of construction background. A visit to your local library to check out books on the matter will be well worth your time.
6. SIPs construction is new enough that you may not find anyone local to support this project or to work with you on it. A typical local contractor may well advise you to forget all that new fancy-schmancy stuff and go with tried-and-proven construction techniques. Be ready to walk away.
Sapwood, thanks for this detailed reply!
1. Fair enough. I don't figure on going down there often...
2. Gotcha.
3. I'll defer to you on "semi-" vs. "fully" conditioned. I figure I'll put a very small ground-source geothermal unit in, which rules out (I think) mini-split systems. But more research to follow on that subject as that part of the project is probably a ways off still.
4. Fair enough. I'm just trying to look at what I can accomplish this spring before the project probably gets abandoned for a year or two while I'm traveling for work. But I might be able to take an extra couple of days off to dig in the lines to the well and future drain field.
5. None taken! My construction experience is very much limited to sheds and remodeling. Nothing "big and habitable" from scratch. I've been taking some classes at the community college but unfortunately none of them have covered foundations so far and Barnes & Noble has been unhelpful.
6. My immediate next door neighbor has just finished a SIP home, so I think the precedent has been set there. Walk away?
Thanks again!
Conditioned crawl space
Here in Western WA unconditioned ventilated crawlspaces, with 6mil poly vapor barriers covering the ground inside them, and fiberglass insulation in the floor joists above them are far and away the most common type foundation for homes (and have been for many years). Such foundations usually consist of a 24" stemwall on top of 6-8" thick footings giving 30"+/- headroom in most cases.
All that to say, they are pretty darned common out here.
And it's a poor system thermally. Disasterous.
If you're at all serious about energy efficiency, go to Building Science Corporation's website and read up on conditioned crawlspaces. It will take a while. There's a lot to be said. And understood. And refuted by others.
But there's no "semi" to it. You either build a conditioned crawlspace, or you don't. There are distinct advantages, but it costs time and money, no question about it.
There are probably millions of homes in North America built the way I described earlier. There's nothing "wrong" with that system. But like I said, it preforms poorly thermally compared to the same house built over a conditioned crawlspace.
Anyone who could build an unconditioned crawlspace could also build a conditioned one. The techniques are not difficult or even tricky. But you have to commit to it from the start. You have to believe in it and understand the advantages and be willing to spend the time and money to vary from the norm. Plus, you might have to sell it to your local building department, they are not very common.
Jim, thanks for the suggestion on the BSC website. Will study it. I did some reading on the general subject of crawlspaces a few months ago, but that didn't really get into the nitty-gritty of actual construction. Just theory.
I'll do some reading at the other site and get back to it. I don't really get how it would be thermally disasterous if it's properly insulated, but I'll defer to you there until I have more knowledge on the subject.
Thanks again!
"I don't really get how it
"I don't really get how it would be thermally disasterous if it's properly insulated..."
"...properly..." being the key world here.
The quick answer is uncontrolled air and moisture movement through the crawlspace. Insulate the floor and walls, thermally isolate slab from stemwalls and footings, control air and moisture intrusion and you're well on your way to a warm, dry crawlspace.
If you can suspend disbelief long enough to consider what they write at BSC's website, I can't see how you won't benefit. At least you'll have considered the next evolutionary step in crawlspace type construction.
I hope soon to read about non continuous stemwalls. With the engineered lumber and hardware available today I'm not really sure why we have to pour all this concrete. But that's another topic.
Building practices evolve. But it's a slow march.
Follow-up question
So I'm reading about foundations at BSC.
Ref. this pic:
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I've seen in several places to have the ground slope away from the slab; but if the slab is for a crawlspace then hopefully it's at least partly below-grade since I don't want to step up 33"+ into the place when I'm done...
Since the footings need to go down (as much as) 30", can't the rat slab sit right on top of the footings and have the stem wall go up from there?
I realize this would challenge the drain solution; I might have to install a sump? Or...?
I'm sure I'm missing something, but any comments?
Thanks as always! :)
Quick follow-up to my above.
I shamelessly kluged together two different diagrams from BSC. (Center)
Is there anything wrong with this setup? I tried to make it about a 30" crawlspace. (Only roughly to scale.)
Thanks!
View Image
Yeah...
...the one in the center is closest to what I'd think you'd want, although I'd like to see more than 2" of insulation under the slab. Regional temperature ranges will help you decide how much to invest in insulation. Of course, you'll be able to add more to the walls whenever you decide to. But under the slab is pretty much a "now or never" situation and another layer (or 2) of EPS isn't going to break the budget.
To answer your grade question - You remember a couple weeks ago I mentioned we use 30" tall crawlspaces all the time out here? But we backfill the exterior to about 16" down from the top of the stemwall - leaving 12"+/- between finish grade and bottom of sidng.
The interior of the crawlspace VERY seldom gets backfilled. So in your center drawing there, if you could "click and drag" the exterior grade up to maybe 12" below the bevel siding covering your SIPS you'd have a pretty good representation of our normal detailing (gradewise).
But back to the rat slab/insulation thing - I don't know what your spans are, or how you plan to frame this, but if you have any pier pad footings inside your stemwall - like to bear a post and beam framing platform on for instance - don't forget to thermally isolate them from the slab too. Easy enough to do with a rat slab in a crawlspace, much tougher in a finished floor in a daylight basement.
You know, now that we're talking, I never could figure out why we don't put a vapor barrier under the footing instead of between the footing and stemwall. With all the freakin' rebar we use that capilary break they always call out is unrealistic. I think if I ever pour another of these things I am going to do just that, put a layer of visqueen under the footing...might even run one up the exterior face of the stemwall to just below finished grade - depending on how much time I have to allow the concrete to dry each way...just a thought.
Can't tell you how refreshing it is to have a reasoned discussion here instead of a vitriolic exchange. Thanks for your interest.
clever, isn't it......
Can't tell you how refreshing it is to have a reasoned discussion here instead of a vitriolic exchange. Thanks for your interest.
There is a positive to a difficult for many, forum software.
But we backfill the exterior to about 16" down from the top of the stemwall - leaving 12"+/- between finish grade and bottom of siding.
Well, that definitely makes sense, but then why does BSC have "Ground slopes away from wall" at a level below the slab? **confused**
But back to the rat slab/insulation thing - I don't know what your spans are, or how you plan to frame this, but if you have any pier pad footings inside your stemwall - like to bear a post and beam framing platform on for instance - don't forget to thermally isolate them from the slab too.
I'm just envisioning this for a 25x25 cottage. I still need to do some more research WRT the best way to frame up the floor. I think someone said I could use Parallam to span 25' easily, but I'm not certain I trust manufactured wood over that distance.
Here is a very basic, not-to-scale drawing I whipped up:
View Image
So the orange walls would be load-bearing. I could see maybe needing one pier in the middle where they meet, but I'm not sure about that. Thoughts on that? Am I worrying too much about a 25' span? Else with 2x12 joists I guess I'd need piers all the way down the middle of the structure, or another stem wall, eh?
You know, now that we're talking, I never could figure out why we don't put a vapor barrier under the footing instead of between the footing and stemwall. With all the freakin' rebar we use that capilary break they always call out is unrealistic.
So I was thinking about that. If you ran the under-slab vapor barrier under the footing and up to just below the surface along the outside of the stem wall, it seems to me that rain water would soak into the soil and get between the stem wall & vapor barrier. Then it would slowly seep to the bottom and work its way up to under the slab, with no way to escape once it was there.(Except up and in.)
On the other hand, with the VB under just the slab, you'd need a lot more rain to get underneath the whole structure before it could capillary up between the rigid foam and the inside of the stem wall.
The outside of the stem wall will be wet either way after a soaking rain, but if the VB is pressed up against it, there will be no way for it to dry.
I dunno, I'm not an expert, this is just my thinking about it!
I am also enjoying our exchange and I appreciate your knowledge.
"...why does BSC have "Ground
"...why does BSC have "Ground slopes away from wall" at a level below the slab? **confused**"
Well, that is one thing a little frustrating about their site. There are inconsistancies from one drawn detail to another; I've even seen drawings there that don't show any insulation under a basement slab. I long ago decided to overlook those inconsistancies and just absorb as much information as possible and try to develop my own beliefs based on that. To me the ideas represented there are far more important than the literal translation of the drawings.
"I think someone said I could use Parallam to span 25' easily..."
I don't know squat about parallams, but 25' spans for floor joists sounds like a lot. My first impulse would be a beam, centered, perpendicular to the joists. So that beam would maybe be a 4x12, with a 4x4 post centered, supporting 2x12 joists 16 or 19.2"o.c. Stout. Probably overbuilt, but solid. Of course that would require a single pier pad under that post. Around here that would be 24"x24"x12" thick if you use 4+1/2 sack redi mix.
Or, another option would be the same framing plan but using 4x8 beams, and two interior posts/pier pads. That might be better in your instance if those orange walls are in fact load bearing. That could provide the necessary direct load path for the point where those two walls intersect. Of course, I (or anyone else) would need far more information before saying this is the case. I'm just using examples to paint in broad strokes, here.
"If you ran the under-slab vapor barrier under the footing and up to just below the surface along the outside of the stem wall..."
Yeah, that is problematic. For a few reasons. I wasn't suggesting that. I was talking about what they term a "capillary break" in the first and second drawings in the three drawing panel you posted. See that lowest blue horizontal line between the top of the footing and bottom of the stemwall in the center drawing? That's the one I mean. Pretty much unworkable with all the vertical rebar we use out here to tie the footing to the stemwall. What they want there is something to stop water from wicking up through the footing into the stemwall, then causing moisture issues inside the foundation. I get it. But what I don't get (besides the rebar obstacle) is why not just put that same capillary break beneath the footing to keep it from absorbing moisture in the first place? It would be easy enough to do, that's for sure.
The other idea was to drape a second piece of plastic down the face of the stemwall...I guess after dampproofing it...is just that, an idea, an extension of the "keep concrete dry" thought. Probably wouldn't hold up well to abuse, but if you could protect it a little, I think it might help. But you're right, I'd definitely want an open joint at the bottom of that draped sheet so that gravity could pull any moisture that got behind that sheet down, instead of forcing it back under the footing and eventually the slab.
One other thing I read at BSC, that I don't know if you've given thought to, is about the placement of the vapor barrier in relation to the pea gravel and insulation. I think you definitely want the gravel below the vapor barrier and the insulation above it. I'll stop typing. If the advantages of that order are obvious I don't want to belabor it. If it's unclear why that order is best, let's talk about it.
Okay, so you're thinking more
Okay, so you're thinking more like this:
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(I added some yellow "spray foam" up above the sill plate so people aren't confused about fiberglass batt...)
That also kind of makes sense if the VB is also supposed to be keeping out any radon...?
The guy at BSC says Notice that the sheet polyethylene “vapor barrier” is located on the top of the rigid insulation between the rigid insulation and the bottom surface of the concrete – in direct contact with the concrete. Do not, and I repeat, do not locate the polyethylene under the insulation as it will keep the insulation wet. Trust me on this – or go back and read “BSI-003: Concrete Floor Problems.”
When I say "load-bearing," I mean part of the load of the loft, with the other part supported by the SIP walls on three sides...
If I were to go with a 4x12 beam down the middle of the floor (say, even with two piers) that means I'd have hypothetically 24'10½" to span, or 12'5¼" floor joists. I guess according to: http://www.engineeringtoolbox.com/floor-joists-span-d_1479.html that should be okay (or borderline on the heaviest case)... does that check?
Yeah, that's pretty close to what I'd do. A couple things though, I don't see any point in that jog on the inside face of the foundation wall. It looks like the very top of the foundation is what, maybe 8" thick? Then the bottom 4" of it is maybe 12"? So what's the point of that jog?
And now you got me wondering about the placement of that vapor barrier under the slab. I think it should go beneath the insulation so that it keeps your insulation dry. EPS will absorb water. We know that from all the waterlogged raft floats we've pulled out of lakes. If the vapor barrier is above the insulation, what's to keep it from absorbing water from the ground over time? Am I missing something obvious? Or is this one of those "Jim has such and such an experience and wrongly jumps to a conclusion about EPS insulation later in life based on that experience"?
Radon I don't know. We don't even think about it out here. Never once heard it mentioned except in magazines or online.
I don't know that engineering site you linked to, but I'll say with all confidance that spanning 12'5" with 2x12 joists 16, or even 19.2" o.c. is a solid floor. Drive your Miata right up on it if you want. Heck, if you have any reservations get you some I-joists. They are only about 30% more expensive than sawn dfir - might cost you a couple hundred bucks for your project. You'll still need the beam down the middle, but you'll be flat, and straight, and stable, and they are a lot easier to handle than sawn lumber, I love them.
Pay attention to how the vapor barrier is oriented so that it will not tend to "catch" water around the edges. You do not want to have an upward-facing lip that will catch water, nor a low dip that will hold water.
I don't see any point in that
I don't see any point in that jog on the inside face of the foundation wall.
Concur. Just an artifact of graphical kluging, I think.
Re: VB vs. XPS placement - I'll defer to BSC there, but I think they have more to read on that subject.
I don't think there is a HUGE radon problem in our area, but if you look on the maps it's usually shaded "moderate." Better safe than sorry, especially if I'm building a tightly-insulated structure with a conditioned crawlspace!
I don't know that engineering site you linked to, but I'll say with all confidence that spanning 12'5" with 2x12 joists 16, or even 19.2" o.c. is a solid floor.
Fair enough - I need to read more about flooring and why 19.2" vs 18"... but that's on me to do. In any case, I'm looking at the beams plus 34 x 13-foot 2x12 floor joists then. (Not counting the rim or sill plate.) Whee!
DanH: Pay attention to how the vapor barrier is oriented so that it will not tend to "catch" water around the edges. You do not want to have an upward-facing lip that will catch water, nor a low dip that will hold water.
Definitely! I still need to think about "solutions" there. Maybe a strip of aluminum flashing taped to the outside of the stem wall covering that lip or something? Anyway. More to ponder! Thanks.
Since the rat slab will be below ground level, how are you planning to drain the water under it? Can you drain to daylight, or do you need a sump?
If you can drain to daylight that's goodness -- some perfed drain pipe under the slab should take care of both water and radon. If you'll need a sump, make sure it can be enclosed to install a radon fan if need be (and plan how you'd route the radon vent up to the roof).
how can it be a disaster?
WEll disaster might be a bit much to say, but I have read that a conditioned crawlspace vs one with an insulated floor is about 20 to 25% moer efficient. Too me it is very simple, why do you want to have your house floating above a pile of 30 or 20 or 0 degree air? This is what you have with a vented crawlspace with an insulated floor. R value is just R value, you will loss heat through the floor. Not to mention that if you use FG there is no way those bats will be tight to the sides of the floor joists.
Placing foam against teh stem walls is insulation and a air barrier, no worry about FG pulling away from floor joists. Secondly, any heat loss from heat duct is not really lost, it just warms teh crawlspace and keep it dry. This strategy even works without insulation on the ground. Teh ground acts as a heat sink and you do not loss very much heat down into the ground (heat rises)
There is much more to this, but you can save 20 to 25 % of your heating bill with a conditioned space.
Hm. There wouldn't be a point
Hm. There wouldn't be a point to insulating a ventilated crawlspace I don't think.... would there? I agree, conditioned/unvented is the way to go.
I didn't mean to imply fiberglass anywhere, I was just using that as a generic "insulation" indicator since it was in the BSC diagrams. Sorry if I confused anyone... Maybe spray foam would make more sense.
In a vented crawlspace FG between the joists is very common. I suppose you could spray foam it, but again why. If you put ridgid foam on the stem wall you have less sq ft of foam and thus cheaper, and you can see all your mechanicals between the joists, and can more easily make alterations there of.
crawlspace doesn't have a conc. slab
Hi there - Structual PE (licensed in VA, MD & DC)
...there is no need to pour any concrete slab if you pan on having a crawlspace. A crawlspace is build with commodity lumbar whereas a concrete slab would be used if you where pouring slab-on-grade. Just note that in VA, the frost depth is 30", so your footings need to be 30" below grade.
Sure, but going for energy efficiency and cleanliness, I'm going for something along the lines of figure 1 here:
http://www.buildingscience.com/documents/insights/bsi-059-slab-happy
The 30" info is a useful data point, thank you. I'd like to think whatever contractors I hire to do the job would know that, but I'm glad I have that knowledge as well so I can double-check them!
You need to find the frost depth for your particular location.
30" is pretty safe for most of Virginia, but it is quite variable (e.g., 10" in VA Beach, 18" in Charlotteville, 24" in Blacksburg). Your local building official will be able to give you specific information.
Moisture Barrier Under Footing
There's a product for forming and water proofing your footings http://www.fab-form.com/fastfoot/fastfootOverview.php
I've used it on 3 projects, the most recent for a 26x32 shop (see photos). I'll be using it for a my house soon,
I think you're on the right track in seeking a conditioned and clean crawl space. I'm living in a rental house right now with a vented crawl space and supposedly insulated (fiberglass batt) floor. The vents are closed and sort of air sealed, but the floor is still cold, especially with this freezing cold weather in the Shenandoah Valley.
Keep in mind heat doesn't flow from left to right or down to up, it flows from hot to cold. So if your cottage is well insulated and the heat finds that it is easier to find cold through your floor, that's where significant heat loss will occur. A big plus for conditioned crawls. Best wishes on your project!
Cool, I'll take a look at
Cool, I'll take a look at that. I guess with a shop you're less concerned with pouring a rat slab? Or is that just something you did subsequent to these pics?
Anyway, thanks for the link!
Shop slab
Yes, it is a shop so I prepped and poured a 5" slab after building CMU stem walls. The footings are at least 30" below grade (frost depth here). I glued 2" XPS to exterior of stem walls and under the slab in case I decide to put heat in the shop. I did put 6mil plastic under the foam and #4 rebar on a 2'x2' grid on top. To date the slab is dry as a bone.
Very nice! Do you mind me asking how much that ran you?
Nice
Jim, To protect the XPS foam, I applied a foam compatible peel n stick. At grade I'm attaching used strips of vinyl siding for additional protection from the gravel. There's a faux stone wainscot that comes down to the gravel at grade.
Jaime, Cost? Well, 2" XPS runs about $30 plus per sheet, but I purchased recycled stuff from Insulation Depot for about $11.50 per sheet including shipping. http://www.insulationdepot.com/ Picked it up at a warehouse in Waynesboro, west of Charlottesville. The peel n stick should be available at a concrete accessories store for about $130 a roll. The Fast foot fabric is about $1/LF.
Thanks! But I meant for the whole foundation. ;)
...and did you DIY or hire a contractor?