Forgoing 2x4 or 2x6 floor joists

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Adam Kartzke

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Apr 30, 2016, 10:11:10 AM4/30/16
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has anyone skipped out on using floor joists under the subfloor? if the trailer is already framed with steel cross members at 16"oc whats the point? i would rather take back the 3.5" or 5.5" of headroom. Anyone done this? have pictures? horror stories? or am I just a flipping genius ;)

Dee Williams

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Apr 30, 2016, 10:37:35 AM4/30/16
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You are a genius and the floor joist that we suggest are less structural than a mechanism for insulating the floor.  In most conventional new houses, 2x6 studs and rafters are required for the same reason.  In a tiny house, you can get away with 2x4 walls and rafters for structural reasons, and the space still stays pretty warm.  But in the floor, due to thermal bridging with the trailer, you need MORE insulation... in fact, as much as you can cram in. 

If you can build the floor box inside the trailer space (not above it) to include at least 4 or 5 inches of insulation, you'll be in good shape.

That's my two cents worth.

Hope all is well!  Cheers on your house!  Dee

On Sat, Apr 30, 2016 at 7:11 AM, Adam Kartzke <adamk...@gmail.com> wrote:
has anyone skipped out on using floor joists under the subfloor? if the trailer is already framed with steel cross members at 16"oc whats the point? i would rather take back the 3.5" or 5.5" of headroom. Anyone done this? have pictures? horror stories? or am I just a flipping genius ;)

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Lina Menard

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May 10, 2016, 3:41:47 PM5/10/16
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Hi Adam,

I skipped out on the wooden floor joists under my subfloor, too. If I had steel studs 16" OC, I'd DEFINITELY skip the wood floor joists. The floor joists are important if you want to use fiberglass batt insulation or any other insulation that doesn't have good compressive strength. Basically I just filled that space with rigid foam insulation instead. 

You can read about how I did this for my first tiny house, The Lucky Penny, here: http://thisisthelittlelife.com/2014/05/17/floorbox-lessons-learned/ and here: http://thisisthelittlelife.com/2014/05/21/floorbox-continued/. 

And you can read about how we did it last week for our new tiny house here: http://thisisthelittlelife.com/2016/05/03/t42-undercarriage-floor-insulation/ and here: http://thisisthelittlelife.com/2016/05/05/t42-subfloor-bottom-plates/. After doing it a second time, I still think it makes sense to do it this way, but I'd probably go with 1-1/8" subfloor next time around to make sure it was super rigid. There's a little give with the 3/4" subfloor in our house, which isn't a big deal since we're little and we figure the flooring will still go down just fine (and probably help, too, as it did for my last house), but it would be nicer to have a super flat surface for most other folks. 

On Sat, Apr 30, 2016 at 7:11 AM Adam Kartzke <adamk...@gmail.com> wrote:
has anyone skipped out on using floor joists under the subfloor? if the trailer is already framed with steel cross members at 16"oc whats the point? i would rather take back the 3.5" or 5.5" of headroom. Anyone done this? have pictures? horror stories? or am I just a flipping genius ;)

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Brett MacPherson

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Jun 23, 2016, 3:39:33 PM6/23/16
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I'm looking at the Tumbleweed trailers and thinking the same thing. I don't think they use floor joists on their Low Wider trailer. I've seen pictures and it looks like they just put plywood down on top of the frame and then insulate between the steel. That seems like a big thermal bridge though? I'd imagine that you'd be wanting some thick socks and slippers if you're planning to use it year round in a cold climate like mine.

There are insulated panels that are used for concrete floors (SIP panels?) I'm not sure they're strong enough to use alone or if they need to be placed on top of plywood. By the time you do plywood-insulated panel-plywood you're probably better off just doing full floor framing. I'm sure the panels aren't cheap and you're probably almost as thick.

Dee Williams

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Jun 23, 2016, 4:54:58 PM6/23/16
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Take a look at the tiny house trailers at Iron Eagle Trailers.  They're designed to minimize thermal bridging and maximize space inside the house!  


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Brittany Yunker

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Jun 25, 2016, 5:47:24 PM6/25/16
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Tumbleweed recommends the use of a thin foam flashing tape applied on top of the trailer joists to minimize thermal bridging, so you wouldn't have the subfloor in direct contact with the metal trailer.  Full disclosure - I work for Tumbleweed & believe we offer a quality product.  My colleagues have a Tumbleweed trailer & have travelled 24,000 miles with their house & it's still going strong.  :)  There is a trailer manufacturer for Tumbleweed in Sutherlin, OR.

But, I also echo Dee's recommendation to consider Iron Eagle Trailers as another quality-built option.

Cheers!
-Brittany

Brittany Yunker

The Bayside Bungalow
~A tiny house vacation rental on Puget Sound~
www.baysidebungalow.com
Or book directly on Airbnb

Olympia, WA
bbyu...@gmail.com
360.556.9719

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Brett MacPherson

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Jul 7, 2016, 10:16:55 AM7/7/16
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Hi Brittany, thanks for your input! Do your colleagues have a Tumbleweed trailer that doesn't use a fully framed floor? I'm just curious if the floor is noticeably colder because of it. The foam tape seems a like a pretty simple thing to implement.


On Saturday, June 25, 2016 at 6:47:24 PM UTC-3, Brittany Yunker wrote:
Tumbleweed recommends the use of a thin foam flashing tape applied on top of the trailer joists to minimize thermal bridging, so you wouldn't have the subfloor in direct contact with the metal trailer.  Full disclosure - I work for Tumbleweed & believe we offer a quality product.  My colleagues have a Tumbleweed trailer & have travelled 24,000 miles with their house & it's still going strong.  :)  There is a trailer manufacturer for Tumbleweed in Sutherlin, OR.

But, I also echo Dee's recommendation to consider Iron Eagle Trailers as another quality-built option.

Cheers!
-Brittany

Brittany Yunker

The Bayside Bungalow
~A tiny house vacation rental on Puget Sound~
www.baysidebungalow.com
Or book directly on Airbnb

Olympia, WA
bbyu...@gmail.com
360.556.9719

On Thu, Jun 23, 2016 at 1:54 PM, Dee Williams <pad.dee...@gmail.com> wrote:
Take a look at the tiny house trailers at Iron Eagle Trailers.  They're designed to minimize thermal bridging and maximize space inside the house!  

On Thu, Jun 23, 2016 at 12:39 PM, Brett MacPherson <terb...@gmail.com> wrote:
I'm looking at the Tumbleweed trailers and thinking the same thing. I don't think they use floor joists on their Low Wider trailer. I've seen pictures and it looks like they just put plywood down on top of the frame and then insulate between the steel. That seems like a big thermal bridge though? I'd imagine that you'd be wanting some thick socks and slippers if you're planning to use it year round in a cold climate like mine.

There are insulated panels that are used for concrete floors (SIP panels?) I'm not sure they're strong enough to use alone or if they need to be placed on top of plywood. By the time you do plywood-insulated panel-plywood you're probably better off just doing full floor framing. I'm sure the panels aren't cheap and you're probably almost as thick.

On Saturday, April 30, 2016 at 11:11:10 AM UTC-3, Adam Kartzke wrote:
has anyone skipped out on using floor joists under the subfloor? if the trailer is already framed with steel cross members at 16"oc whats the point? i would rather take back the 3.5" or 5.5" of headroom. Anyone done this? have pictures? horror stories? or am I just a flipping genius ;)

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Derin Williams

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Jul 7, 2016, 12:18:37 PM7/7/16
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I wonder how much the foam tape ( Aka sill sealer) has an effect on minimizing thermal breaks? It's mainly used in standard construction to minimize air infiltration at the sill plate or wrap plumbing penetrations coming up through a slab on grade foundations. Anyone know the R-value of the foam tape?

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Adam Kartzke

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Jul 7, 2016, 12:25:30 PM7/7/16
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R-rating is probably next to nothing. 1/2" styrofoam insulation board with foil on both sides is what I'm using. It's only rated at a 5. But it is for sure enough to eliminate bridging. If you are looking for Better insulating I'd highly recommend spraying the underside of the trailer and subflooring with closed cell foam after construction is compete

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