Replies

1. 
   kyeser | Feb 18, 2015 07:24pm | #1

   I'd check out this NAHB publication.

   It addresses unheated structures, detached garages, etc..

   I've built a couple detached garages with stamped plans from an engineer following these guide lines.

   http://www.toolbase.biz/PDF/DesignGuides/revisedFPSFguide.pdf

2. GBA Editor
   Martin Holladay | Feb 19, 2015 04:54am | #2

   Moses,
   The idea that a frost-protected shallow foundation depends on heat loss from the building is a misunderstanding. Frost-protected shallow foundations can be used for unheated buildings like garages. I wouldn't worry if I were you.

   For more information, see Frost-Protected Shallow Foundations.

3. 
   charlie_sullivan | Feb 19, 2015 08:55am | #3

   In your climate, the guidelines don't require anything too drastic for a heated building. Upgrading to the requirements for an unheated building shouldn't be very expensive or difficult, and might help your energy performance long-term anyway. However, those guidelines require foam under the footing. That doesn't cost much and is a good idea even in a heated building, but sometimes it is a challenge to get builders and inspectors comfortable with it.

   If you don't go with that, Martin might be right that you'd be fine anyway, but to be sure, you could lay down some extra foam board over the slab, particularly over the footings, just temporarily for the winter to keep the ground warmth in better. Or hay, or sawdust, covered with a tarp.

   (The way it works in an unheated building is that the warmth from ground, which might be 50 F in your climate, conducts up through the ground. Putting foam over the ground holds that heat in and keeps the ground from freezing.)

4. 
   mosesdraper | Feb 22, 2015 01:43pm | #4

   Thanks for the feedback.

   One thing I forgot to mention which might effect whether it's a viable option to leave it over winter, is that the foundation is a stem wall only, not a full slab, as we'll be doing a finished earthen floor eventually. So it would just be a stem wall with gravel fill on the inside for the moment.

   It seems that the main difference in the unheated guidelines is the 6" layer of gravel underneath everything, given that I'm planning on using R12 Roxul comfortboard as both vertical and horizontal insulation, which exceeds the minimum value in our climate zone.

   Does the stem wall information change any recommendations on leaving it out for the winter?

   Thanks

5. GBA Editor
   Martin Holladay | Feb 22, 2015 05:30pm | #5

   Moses,
   So it's not a frost-protected shallow foundation after all (unless I'm misunderstanding). It's just a traditional perimeter stemwall foundation on footings -- right?

   If that's the case, you need to protect the footing on both side of the stemwall by piling up as much hay as you can afford to buy, and hope for deep snow.

6. 
   user-4524083 | Feb 22, 2015 09:11pm | #6

   6. No. Your plan is unlikely to work as I am understanding it. It sounds like you're just building a 8-12"wide foundation wall that only goes down maybe 12-24" below grade, then 6" of "Gravel" which means different things depending on your location. "Gravel "in coastal southern Maine (Freeport / Brunswick) is sand and small stones, sometimes up to 4". I would prefer 1"( easier to rake) or2" ( once placed and sitting through a few months of snow or rain is less likely to move) "CRUSHED ROCK" . Mechanical tamping is advised anyway. But, more importantly, you will need insulation under the stem wall or its footing if that is a separate pour. You could probably,as in very probably, get away with just wing insulation which could be placed fairly shallowly (you'll need wider - 4' would be foolproof I'm guessing).And then nothing under the bottom of the wall. But If the wall is not very deep,I'd put foam under the bottom of the wall or the footing.You could dig the wing insulation up carefully and reuse under the earthen floor, and keep the outer one in place. The idea is that you are not allowing the frost to get into the soil around the wall and especially around the bottom of the wall, or the bottom of the crushed rock. Depending on the soil, filter fabric is a pretty good idea to prevent mud and dirt from infiltrating your crushed rock. The "rule of thumb" which is probably overkill, says to go out as far (horizontally) with the insulation as the potential frost would go deep vertically.In your case, you would need to go out 4' inside and out.In 2002, I built a insulated slab for a garage that overwintered.We removed 12" of top soil, replaced it with 2"foam plus 10" crushed stone. 6" slab with 12" wide and 12" deep edges sloping up to the slab itself - One pour. I used 1/2" rebar every 2 feet to reinforce the slab with fiberglass pieces in the concrete, with 2 levels in the thickened edges.The foam went only 2' horizontally all around beyond the slab which I thought I could get away with due to the 10" crushed rock, an extra large reservoir for rain water that might infiltrate under the slab. I studied the then current literature including the NAHB site,which is a good resource.This thickened slab ended up with a 24" high by 8" wide stone- backed- by -concrete wall to get all the wood 24"off the ground( and to look good- it was in the Fine Homebuilding book on garages.) It's been 12 years without trouble of any kind. OR - you could put the hay( every year for the life of the building) and hope that there's lots of snow (every year for the life of the building) I wish you the very best in your venture. Read - don't listen to a builders unless they seem really smart. No offense intended to builders, but many of them in this area don't understand any way to build that isn't what they usually do and what they know works.

7. Expert Member
   MALCOLM TAYLOR | Feb 22, 2015 09:30pm | #7

   I'd take Kevin's advice.

8. 
   mosesdraper | Feb 23, 2015 01:30pm | #8

   The foundation is designed from the Revised FPSF guide, to be 14" below grade. There is a 12" wide footer, 8" deep, below that. The horizontal and wing insulation are R12 roxul, which far exees the requirements. The guide had examples of stem wall FPSF and didn't mention any modifications for this, so I was just assuming that the only difference for mine was that I would have a 3" finished earthen floor instead of a concrete slab.

   It sounds though like it's not a good idea to leave it unfinished, so perhaps I'll either work to get the roof and walls on and/or consider pouring a thin concrete slab, so that insulation can go in now and the finished earthen floor can go on in the future over the concrete.

   Thanks for the help

9. 
   user-4524083 | Feb 24, 2015 12:17am | #9

   Moses - Sounds like a good plan. I've never used Roxul below grade, though I've heard about it on this site. I'd consider placing filter fabric above and below the Roxul - to keep dirt from infiltrating the fibers - but I don't know if that is necessary or not. Best of luck with your building.