Air-Sealed on Piers – open to the elements?
Tiziano
| Posted in General Questions on
I am about to begin a project that will have an almost identical flooring system as described in the 2019 Fine Homebuilding article Air-Sealed on Piers (https://www.finehomebuilding.com/project-guides/insulation/air-sealed-and-on-piers). The only difference is that the structure is not attached to another.
I’m in Minnesota. The issue is that I’m doing this project solo, during off-hours and as such the floor may well sit in the elements for quite awhile before the entire structure is enclosed. Since the floor as built dries to the inside, and the inside will be outside for a time, should I worry about it in regards to rain and humidity?
If so, I was thinking that I could cover the subflooring with a heavy tarp, overlaying the sides by quite a bit, then attach the plates. This would keep the floor system dry while I’m erecting the rest of the structure. Finally, once the structure is roofed, sheathed, and sealed, I would simply cut the tarp away from the floor (although it would remain under the plates).
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Tiziano,
The Achilles heel of assemblies that have to be insulated prior to the roof going on is just that: their vulnerability to damage when it rains. Unfortunately tarps, penetrated by fasteners for the wall plates and the plumbing rough-ins that need to be done, while at the same time being used as a work platform as the framing progresses up, are no guarantee that moisture won't get down into the floor system. I've done what you are suggesting on small projects with pond liner, but even it was not entirely waterproof, and is expensive.
if your floor is going to be exposed for more than a brief period in inclement weather I would choose another floor assembly that you can insulate from below.
Thanks Malcolm. That confirmed my fears.
My other thought was to skip the tarp, but just after adding the plates, temporarily attach Zip roofing sheets to the sub-flooring (including Zip Tape on the joints between the sheets, on the nail holes, and the plates-to-sheets joints). Then, once I'm fully sheathed-in, remove the temp sheets. More rabbit trails I suppose.
No plumbing. It's a 12'x16' workshop/studio, shed-style roof.
Tiziano,
If it's that small a structure, I'd buy a pond liner for the floor but also tent with a tarpas Bill suggests. A 15 ft 20 ft liner will set you back just over $100.
I really like a lot of these floor assemblies once they are finished, but here in the wet PNW have struggled with exactly the same issues. If moisture does get into the floor it can be a big problem. Last summer I built a 14 ft x 24 ft recording studio using this floor system, and really sweated the weather until we got the roof on.
https://www.southmountain.com/wp-content/uploads/2019/02/19-Small-Home-Harmony-Article-PDF.pdf
Sweet!
You're going to have some weather exposure regardless of what you do. I think most jobsites probably will actually. The best you can do is to try to keep bulk water (fancy way to say "rain") off of the structure as much as possible as you work. I would do the following:
TENT the floor with poly or a tarp. Ideally use one of the breathable tarps. Don't put the tarp/poly right on the floor, since it will tend to stay damp that way and mold. Ask me how I know... Anyway, strap the tarp/poly at the edges as best you can with bungies or pull string, then use some sawhorse or whatever you have handy to prop it up in a few spots to keep it up off the floor as much as you can. Try to tent things in such a way that water will run off the edges and not pool up anywhere. The circulating air space under the tented tarp/poly will massively help to keep the wood from holding moisture, limiting the issues with mold and delamination.
Try to get the exterior walls framed as quick as possible. Get trusses on as quickly as possible after that. Once you have a rough framed roof assembly, even without any sheathing, you can get a tarp up there to keep rain off of the structure again. That's the goal: get some kind of rain shedding "roof like thing" up as quickly as you possible can to protect the innards of the structure from rain.
Don't worry about humidity. There is nothing you can do about it. Keep rain out as much as you can and that's about as good as you can do. I entirely agree with Malcolm regarding choice of insulating materials too: if you can't insulate after you're dried in, or at least close enough to that that you're pretty much rain-proof in the areas to be insulated, then I'd pick a different means of insulating things.
Bill
Nice tent idea Bill.
Good advice from others. My wife and I added a second-story to my first house with the help of family and friends while we lived there. Kept a huge blue tarp over the roof until the second floor roof was felted. Getting it dried in took months. You spend a lot of time tenting and moving tarps to work, but there's little alternative. Make it easy to cover and uncover and make sure any tarp or other covering is held down well. Heavy rain often comes with wind.
This is great guys. Thanks.
Going with a giant tarp, throughout the build process until the building is sheathed.
Not sure why I overcomplicated it.
When you tie that tarp down, try to spread the load between a number of the eyelets. That goes especially at the corners, but also periodically along the sides -- especially in the middle. I've had wind get under edges, then cause a surprising amount of flapping, and the eyelets will sometimes tear out. The more you can distribute the load, the less likely you'll lose an eyelet.
BTW, once one eyelet lets go, it's common for others to go too, like a zipper. Before you know it, your "giant tarp" is airborn...
Bill
Good advice Bill. Thanks.