Replies

1. GBA Editor
   Martin Holladay | Jul 08, 2013 05:37am | #1

   Shane,
   There are a couple of issues here.

   You are building an addition on piers. You want to install wood skirting around the perimeter of your pier foundation.

   This type of wood skirting is designed to keep out cats and racoons. Skirting is usually somewhat temporary. Eventually the part of the skirting that touches the ground begins to rot. Frost heaves can buckle the skirting.

   If you want to build a crawl space with insulated walls, I recommend that you build your crawl space walls out of poured concrete. The concrete walls should extend from concrete footings that are poured below the frost line. This type of wall can be insulated.

   If you want to build a house on piers, the insulation goes between the floor joists (or, better yet, as a continuous layer directly under the floor joists).

   The second question your raise concerns whether it is possible to install spray foam directly on the ground. The answer is (kind of): yes, you can. However, usually the spray foam installer sprays the foam on a layer of crushed stone that will support a basement slab. After the foam is sprayed, the concrete crew installs a slab to protect the foam. You need a full basement for this type of installation, however, because the spray foam contractor has to be able to stand up.

2. 
   user-1002337 | Jul 08, 2013 12:11pm | #2

   Thanks Martin.

   Yes that is what I was origanlly going to do is have mineral wool insulation in the floor joist and then 4" continuos layer of polyiso rigid foam beneath the joist. I got the other idea when I had talked to a spray foamer about spray foaming instead of what I was going to do. Would it be better to just spray foam the entire floor istead of the mineral wool and rigidfoam? I was also going to use pressure treated wood for the floor joist As well as the skirting. Do you see any issues with using pressure treat for the joist or is it not nessaray? Would I need any cross ventilation below this foor? II was thinking of just a access door to inspect on one side. I have no access from the basement right now. I would have had to cut one out to turn in into a crawl space. And should I stool have a vapor barrir on the ground for this type of design?

   I have already poured these piers and was going to go with this design. Now I'm wondering if I should do like a grade beam to have a concrete wall hoping to not have to re dig down below grade again. Could I design this method with he walls insulated and floor sealed? What do you think of the grade beam design having the wall just sitting on the grad like a slab? That's what I got on my existing addition that was here when I moved in. Its a grade beam wih piers foundation. Theres insulation on the walls but not much and a vapor barrier under some gravel on the floor. I can reach from outside and touch the insulation that extends below the ground.

3. GBA Editor
   Martin Holladay | Jul 08, 2013 02:16pm | #3

   Shane,
   You wrote, “I have no access from the basement right now. I would have had to cut one out to turn in into a crawl space.”

   You'd have to do more than that to turn the area under your house on piers into a crawl space. If you cut an access hole from your basement, you'll just have a window to look out at the area under the new room. You won't have a crawl space. For a crawl space, you need poured concrete perimeter walls.

   It's usually a good idea to ventilate the area under a house on piers. It's also a good idea to install polyethylene on the dirt; the poly can be secured with bricks or stones.

   You can insulate with spray foam between the floor joists if you want, but the other plan -- mineral wool and continuous polyiso under the floor joists -- would be better. Unlike the spray foam, the polyiso would interrupt thermal bridging.

4. 
   user-1002337 | Jul 08, 2013 04:26pm | #4

   Ok thanks Martin. In terms of building material. Would it be better to use pressure treated wood for the floor joist or is just regular spruce just fine. I know pressure treat is saturated in water when treated so I was worried that it would not dry out before I need to insulate the floor ad potentially trapping it. I was thinking the pressure treat would be better for resisting any mold. Or with what I had planned to do and seal everything should I not have to worry about using pressure treat over untreated wood? Thanks!

5. GBA Editor
   Martin Holladay | Jul 09, 2013 04:54am | #5

   Shane,
   The floor joists of a room on a pier foundation do not have to be pressure-treated. If your floor assembly is so damp that you need pressure-treated lumber, something is seriously wrong with your assembly.

   The floor framing of a building on a pier foundation should stay dry. To be sure this happens, you need good details. The dirt floor under the building needs a polyethylene ground cover. And you need adequate clearance between the dirt and the bottom of the floor joists. (I recommend at least 3 feet of clearance.) If you follow these principles, the floor joists don't need to be pressure-treated.

6. 
   user-1002337 | Jul 09, 2013 08:51am | #6

   Thanks Martin for the great advice. Unfourtunitly Im only going to have about 20" under this space and that's not including the 4" of rigid foam. This is due to being a addition and matching my existing grading with the house. I was also going to add 2" of pea gravel on top of the dirt for extra drainage. Do you see a need for any gravel or is dirt usually just fine?

   Thanks Martin

7. GBA Editor
   Martin Holladay | Jul 09, 2013 09:30am | #7

   Shane,
   If the clearance under your addition is only 20 inches, that isn't ideal. (It's not fun to work in such a tight space.) But the tight clearance isn't necessarily a disaster, as long as your site is dry and well drained. The grade at the perimeter of your house should slope away from the house in all directions.

   Putting stones on top of soil does not improve drainage. What improves drainage is making sure that the grade slopes in the right direction -- away from the house.

8. 
   user-1002337 | Jul 10, 2013 01:23am | #8

   Sounds good! Yeah I was just thinking that by having a little gravel there would be some separation between the vapor barrier and the dirt so If water did some how find it's way in there then the vapor barrier wouldn't be in direct contact with it. Especially if the was a little dip in the grading that was missed then the water wouldn't touch the poly and have time to drain into the dirt.

9. 
   user-1002337 | Aug 23, 2013 05:24pm | #9

   Hi Martin,

   In regards to how I was going to insulate the floor joists. Instead of the Roxul mineral wool between the joist. Would it be ok to use 10" of the polyiso rigid foam between the 2x10 joist and another 4" below to stop thethermal bridging? This would give me allot more R value then the mineral wool. Or would this create some issues in regards to the joists/floor?

   Thanks!

10. GBA Editor
    Martin Holladay | Aug 24, 2013 04:05am | #10

    Shane,
    Insulation contractors don't cut rigid foam into thin strips and insert it between floor joists -- the so-called "cut-and-cobble" method -- because the work is so fussy and time-consuming. Ten inches is a lot of foam to install this way.

    If you want foam between your joists, use spray foam.

    If you insist on taking the cut-and-cobble approach -- I don't recommend it -- then every single piece of rigid foam needs to be installed with attention to airtightness. You will have to seal the perimeter of each rectangle of foam with caulk or canned spray foam.

11. 
    user-1002337 | Aug 24, 2013 10:12am | #11

    Ok thanks. I will stick with the roxul and 4" of polyiso below.