I will be building an ICF house next year.   My plan now calls for a crawlspace but I am wondering if I should forgo it and use a slab on grade.  The original reason for having a crawlspace was to have the HVAC ducts undeneath the 1st floor.  We prefer air vents in the floor - not the ceiling.  But now that I have decided to insulate the underside of the roof and make the attic a conditioned space it seems to make sense to use that space for my HVAC ducting since I won't have to be so concerned about duct leakage.

Is there any good reason to keep the crawlspace if I can accept the HVAC vents in the ceiling instead of the floor?  Isn't building on a slab cheaper than having a crawlspace anyway?  If I keep the crawlspace I will probably try to incorporate a concrete deck of some kind on at least the 1st floor.  I haven't decided about using a concrete deck for the second floor.

My house will be in south central Tennessee on a gentle sloping lot.  It will be 1.5 stories with no basement.  I will be using a closed loop direct goethermal HVAC unit.

I hope this is enough information for someone to offer some guidance.  Thanks in advance for any light you can shed on this decision process.

JKC 

First off, your choices are slab, crawl, and basement. I'm not sure how much slope you have, so the crawl may offset increased slab costs to handle the slope? However, here in the Northeast, basements are normal, and if we have a crawl space planned, in most cases it goes to a full basement. The caveat is what is the norm for your area, basement? crawl? slab? I would assume crawl is considered the same as slab in most cases as there is no increase in square footage?

Look closely at a full basement if you have to pay to do a crawl, you're only increasing the digging and more wall concrete(or ICFs).

As a sidenote, for your ducts, if you do place them in the attic, you can still bring the ducts down close to the floor in the interior walls and use sidewall registers. Yes, it takes a little planning, but the square ducts, say 3 1/2" x 12" are everydsay sized ductwork, and it's inexpensive.

Just some ideas: As a builder of stick frame homes from 1981 to 1998 I became very aware that a home in the northern climates needed forced air at the floor, customers with finished walkouts eventually complained of the colder lower 2' ft. Since being involved with ICF's in 98' I have found with ICF and a under slab insulation that the heat is just fine in the ceiling of a basement home. If your torn between slab on foundation, and crawl, the slab would get my vote. I live in Northern MI and I wouldn't be caught dead on a crawl. If you have a space above grade dedicated to the mechanical you will never ever go in the crawl so why have it. The footing cost and the stem wall will wash with either method. The floor of a wood crawl will cost twice as much as the slab on foundation (sof) and the slab of a (sof) can be colored and stamped, or saw cut and stained for an additional cost much less than floor coverings. The crawl floor also has to be insulated. As far as mechanical go you still can run your duct work under ground with the slab home, just make sure you use a under slab insulation. Crawl spaces create potential water problems that you will not have with the (sof). Make sure that you run extra pvc sleeves to your mechanical room from the areas of exterior electrical, telephone, etc, for future lines like high speed and anything else that comes into the new wave future. Speaking of new wave future look for my upcoming post on basements.

Mr. Eggert:  I cannot use a basement because the rocks are too close to the surface and it would cost a fortune to blast them out.  So my decision is to have a crawlspace or build on a slab.  The slope is gentle enough that I can dig the house into the hill a bit and have a modest 4’ stem wall to fill on the downhill side to make my slab level.

 

I am intrigued by your idea of putting the registers near the floor and use small ducting inside the walls.  It would be no big deal to put a duct inside an interior 4” wall cavity (as long as I remembered it was there and not try to hang a picture on the wall above the register.  Ha!)  Are these sorts of ducts readily available or are we talking a lot of special manufacturing here?  A 3.5” x 14.5” rectangular duct would be equivalent to an 8” round duct in area, so I would have to see if my ventilation needs can be met with that size duct.  Do you think there would be an uncomfortable draft with air coming out of the side wall instead of vertically?  I know I’ve seen registers in the wall, but I’ve not had any experience with them.

 

A question for Walltech:  How can I have ducting under a slab?  The ducts would have to be at least 50square inches or so in size and they would be running to every room.  Does this make sense?  Maybe I just don’t understand what you are suggesting.  I’ve never seen registers in the floor of a slab home.  Am I missing something?

 

Thanks for the ideas.

 

JKC

If you are worried about heating consider radiant heat in the floor or even a Legalett air heated slab. For more information look at this web site. www.arit.com

We do radiant heat even here in the South in slabs.

Jay
The HVAC requirements of each room dictates the size of the duct, however, interior wall ductwork is common enough where assuming your walls are typical 2x4, the 3 1/2" ducts are available any size you need, and if you need a large amount of BTUs, you can use a duct 3.5" x 22", fitting between 24" oc framing.....or you could use two smaller feeds for the same room.

Don't worry about a picture nail in the duct, you fell and hear it before you blow thru it!

I would use a dual heat pump with the leagellett "frost protected shallow footing" system Tom suggested.
This may eliminate the rock blasting, as no frost walls are needed with FPSF.
Or
you could do as a client of ours did here in Michigan with Her 1 and 1/2 story ICF.

She bermed the North and even the South walls . Grouped most of Her windows on the South for solar gain.
She then added solar hot water tubes on her roof, witch are the primary heat source and a simple tankless heater for back up.
We won't know how well this will work because she dosent have all of the required panels installed this year.
However when completed the solar can provide enough heat( per. energy audit).

Eldon Howe

Tom:  I have thought about radiant heat as you suggested and I like the idea.  I wonder why more folks here in the South don’t consider it.  Since I will also have a wood stove for supplemental heat, I think I would like to have radiant heat under the 1^st floor, be it a slab or hanging concrete deck.  I guess it is possible to use a geothermal system for radiant heat in the Winter and forced A/C in the Summer.  Or is that a waste of an expensive HVAC system?

 

I think I will probably end up with a slab on grade with radiant heating tubes in the slab (and good insulation underneath).  I will need to rethink the purchase of an expensive geo system unless I can find one that is optimized for my situation.

 

I looked at the ARIT website.  Other than eliminating the possibility of water leakage, what advantage does the air system have over the water system?  It seems that the denser water would be more efficient than the air system.  Couldn’t any geo system provide the water heat needed for a water system?  I will do some more thinking on Legalett.

 

Tom:  I still like the idea about the ducts inside the walls.  Here’s my concern; I’ve always seen registers along the exterior wall.  If my registers end up being all along the interior walls, does that mean I am giving up some benefit that the exterior walls would have given me?  Are exterior wall locations for registers inherently better than along interior walls?

 

Eldon:  I am too chicken to venture into solar (and maybe too cheap as well).  Although, the Lord knows the Sun sure does shine a lot where I will build.  Cooling is my primary cost consideration since I will have lots of free wood for my stove to heat with.  Unfortunately, my house plan has lots of windows on the South side that will have to be shaded or else I will have WAY too much solar gain in the summertime. 

 

Thanks everyone,

JKC

Jay, the Legalett system indicated above runs around 10.00 per sq. ft of floor space, and is generally a choice of ranch homes only. They haven't perfected 1.5 or 2 story homes as of yet and will require an accessory heat source for the upper level. Their are a lot of other options available for heat including the ones posted above from those who sell it. As far as the solar gain goes the idea is to build your overhangs large enough to shade the glass from the high summer sun, and take advantage of the suns low winter angle. As far as the ducts go you can convert to 2"x 12" at the lower exterior wall and chase it threw the foam to the upper floor and exchange back to the conventional 4" x 10" i register in the floor. Use a 2.5" thick foam ICF to combat this issue.
As far as AC goes I would like to here more comments from ICF guys in your area whether or not you actually need A.C. in a ICF house with the appropriate overhangs, and a radiant shield between your trusses or rafters. We have to remember this home is in Tennessee, and it maybe taking advantage of a slab on grade/foundation for supplemental cooling.

I'd go with slab. If you want vents near the floor, use the stud bays. Even a 2x4 16" o.c. stud bay can handle a lot of air, and standard sized duct work is readily available at the large hardware stores.

I'm in the duct work stage right now in a "simple" shop/garage and have some advice. Space saving duct work requires a lot more forethought than the old "run it through the attic or crawl space" HVAC ducting. You will want to try to reserve stud bays for this purpose. No switch boxes or receptacles in the stud bay. No wires crossing the stud bay (although technically allowed by code, some inspectors say it isn't and, more importantly, it's a PAIN to install a stud bay duct and work around a wire). HVAC is traditionally one of the last trades on site and they install around prior work. That can be a lot more complex if you don't have a crawl space. Think about placing an HVAC wall just like you would a wet wall for plumbing. An HVAC wall could have a little extra space for several ducts, no wiring, and ready access to the attic or floor joists. Since ICF walls and concrete slab limit the placement of duct work, the directional layout of joists, rafters, trusses, etc. can become much more important.

Mark Fleming

A water to water geo system does supply the water to heat with. A water to air unit supplies the heat and air as a regular heat pump would. It would also give you hot water with the addition of a desuperheater as long as you were in the a/c mode. You still need duct work with the water to air system to distribute the cool/hot air into the dwelling.
The Legalett system is used on multi story but my understanding is it only heats the slab. I believe it was designed to allow the construction of slabs without deep frost footers in the frigid climates of the world thus reducing the costs involved.

Walltech:  I understand the eaves need to help out on the solar gain issue, but I haven’t worked out how an ICF block handles a roof panel yet and that will dictate how I solve that problem.  ThermaSave panels are manufactured close to home and I like their panel, but there have been posts here that they are not too customer friendly.  I will still probably contact them and see what kind of overhang I can get on top of my (still undetermined) ICF wall.  Are 4’ overhangs still fashionable?  TVA used to push this design but I don’t see many of them anymore.  Don’t know why.

 

With my HVAC unit in the conditioned and sealed attic space I don’t think I will have a problem cooling the half storey on that level or the 1^st floor below.  I may even go with a few 6” interior walls on the 1^st floor to help with the ducts in the walls.

 

Mark:  Yes, I think you are right.  A slab is the way to go.  I will probably have radiant heat in the slab and use the HVAC unit for heat upstairs and A/C for both floors.  I want one geo unit to do both jobs (if I go with water for radiant instead of air, as proposed above).

 

To solve the bay conflicts I could tape up the stud bays that I need for the ducts before the electrician gets to the house.  Is it possible to get a layout of the ductwork from the HVAC contractor early in the game?  If I could I think I can solve the problem you mentioned.  I know my architect will produce a layout of all of the ceiling joists before construction begins.  Couldn’t I give that to the HVAC guy and help him block out the stud bays he needs?

 

How does ductwork in the bays go through the top plate of the stud wall?  Wouldn’t cutting a huge gap in the top plate weaken the wall a lot?  All of my registers will have to be on interior walls.

JKC

Tom:  To make sure I understand what is available, please let me state what I think and I ask you to either verify this or tell me I am completely wrong.

I THINK:  One GeoThermal unit can provide water for my radiant heat system in the slab PLUS provide hot water for my house PLUS provide hot and cold conditioned air to ventilate the house with.  OR do I need something separate (like a hot water heater) to provide hot water for the radiant heat and house water use in addition to a GeoThermal unit?

The reason I have to ask is that earlier post have suggested that GeoThermal is a waste of good money when I already have an ICF house.  Others have suggested that I forgo Geo altogether and just get something to supply hot water for both ther radiat heat and the house use, and then get a regular A/C unit for cooling the house.

What do you think?

JKC

Unless there are serious scheduling conflicts(even with) the HVAC guys s/b the 1st in. They simply can't work around the work of the others(plumbers & electricians) as easily. In fact, I would guess that a lot of time & $$$ would be wasted, by letting another trade start 1st(assuming forced air). And, I've never seen it done any other way.

We can only guess if geothermal is the right choice for you. I would suggest that you talk to Richard Badger at www.HVAC-consult.com 219-548-7270.
We us Richard to properly size HVAC ducts and all. For a fee he will provide a energy audit to the penny, no guess work.

at our latitude here in Michigan a 2' overhang in most cases will block most of the unwanted summer sun. I refer to bible of solar design, The Solar Energy Book By Edward Mazria. Use the expanded professional edition.

Geothermal Domestic water heating will require probably more cash than it is worth.
Two heat pumps are more efficient than one in the long term.

Compare the square footage of the Icf walls to sqft of the roof , windows, floor , crawlspace an you will see that icf or any wall system is not the cure all.

Simple hard wired electric radiators will get you through the building codes on your 1/2 level. With a radiant floor water or air your second level will only be 2 degrees cooler than your radiant floor level . Another option is to install the radiant tubes on the second level and allow the heat from that floor to heat both levels and at no additional cost. This is where a concrete floor will have a great advantage both in storm protection and efficiency. Concrete floors will bring your heat pump COP up from standard 3-4 to 5-6 depending on the thickness.

Eldon Howe

Eldon: I had forgotten about the Sun's angle determining the amount of overhang. My calculations show that at 35 degrees North latitude (South Tennessee), the highest angle of the Sun at noon on the 1st day of summer will be 78.5 degrees and the lowest angle at noon on the 1st day of winter will be 31.5 degrees. So I guess I need to have my roof panels overhang enough to shade the windows so that all of the winter Sun can get in so I will use the 31.5 degree angle to figure my overhang based upon the height of the windows on the south side.

Thanks for the reminder.

JKC

Jay,

Eldon has a test house in MI.  He can give you more factual info than I can. 

Jay, cutting the top plate does weaken the wall. I'm looking at doing it in nonbearing walls, but I still don't like it. Better to have an HVAC wall, or enter the duct into the stud bay in a closet or hidden in an upper cabinet, etc. I've got a spot where I can build a false wall in the back of a closet and duct three rooms.

Code is likely to require that the opening through the plate be a metal duct. No fair just drilling 4 2" holes into the stud bay and putting a register in the drywall (although it sure is tempting.) It might pass code if the holes entered into a metal duct above, but I don't want to try to defend that with the inspector. My false wall idea may also need some kind of a fire stop, depending on how my inspector views the occupancy of a shop/garage. I like the idea of fire stops and metal ducts regardless of what the inspector wants.

Find out if your architect uses Chief Architect software or a similar program that can show sun angles in great detail. It's what I used and the time-lapse sun angles are really facinating. Figuring the effect of sun angles is much more complex than simply where the sun is on June 21 at high noon. Chief Architect allows you to see the sun at any time on any day during the year. It shows you not just the roof overhang, but also the effect of your thick ICF walls (which can have a significant shading effect if the windows aren't too tall). The wall thickness can also have a great effect when the sun is south and low in the sky (not so much if the windows are wide). Watching the shadow angles change shows you how much the sun sneaks around during the year. It can also help with landscaping. You can tell where you need to plant that shade tree before you even break ground.

One of the biggest mistakes is not figuring out where the sun is going to be at 5 pm in July-August. Some of my neighbors have window walls on the west and they cook in the afternoons in late summer. The sun roasts them from 5 until 9, so the noon angle isn't that important. One actually installed sliding barn doors on a track so that he could shut off that whole side of the house. Desparate times call for desparate measures. Others have custom automated Levilor blinds to fit odd shaped windows high on the gable wall. Even with them, one neighbor can't get the heat pump/AC to keep it below 90 most summer days.

We've gone quite a distance from talking crawlspaces. If you're really thinking radiant in a slab, look into HRV ductwork that can transfer that heat to other areas in the structure. My house design (hopefully starting next summer), uses the HRV to suck the downstairs radiant heat into the upstairs bedrooms, which have no independent source of heat. One of the reasons I'm putting an HRV in my present 1,000 sf shop/garage project is so that I can experiment with the system as a trial run for the house. The smallest unit I could find (70cfm) is still more than I need. My main concern right now is noise. One of my goals is a silent house.

Mark

Alright guys. All of you have been a great help. Although we crept a little bit away from the initial question of whether a crawl space was really needed, all of the information passed has helped me.

I have made these preliminary decisions based upon my research and your assistance:
1. I don't need a crawl space, therefore I will build on a slab.
2. My HVAC equipment will go in the sealed and non-ventilated attic space.
3. Radiant heat will be used on the 1st floor (although I haven't decided between air or water) and a separate A/C system will be located in the attic to serve both floors. Supplemental heat for the 2nd floor will be accomplished by ducting hot air around the wood stove to the 2nd floor.

So in my situation there is no real need for a crawlspace. Thanks a lot. I will be askihng for your opinions in the future, too
JKC

Weakening the wall by cutting the top plate is eliminated by securing the top plate on each side of the duct run to the ceiling joists!

Simple solution.