Beating Thermal Bridging between Trailer Mainframe and Tiny House in natural mold-preventative build

We are looking to design and build a mold-preventative tiny house on wheels roughly following some of the design/materials in laid out in this build:

https://www.mychemicalfreehouse.net/2020/12/passive-house-tiny-house-detailed-mould.html

We've been learning a lot about failed tiny house builds where thermal bridging isn't avoided and condensation builds around bolts and trailer studs inside of the house.

If we build a subfloor in this way on top of the trailer, wouldn't the steel bolts running through the 2x6 sub floor down to the trailer crossmembers still cause a good deal of thermal bridging? Any ideas on this or how to beat it?

Thanks!

Emily Gonzalez wrote:If we build a subfloor in this way on top of the trailer, wouldn't the steel bolts running through the 2x6 sub floor down to the trailer crossmembers still cause a good deal of thermal bridging? Any ideas on this or how to beat it?

Welcome to the forum, Emily!

Could you tell me why you feel the bolts are causing thermal bridging?

What the heck is thermal bridging?

Anne Miller wrote:

What the heck is thermal bridging?

Thermal bridging is the issue you get where something solid (or just more conductive), like a bolt, or the framing in a traditional framed wall, passes through your insulation.  An an example, you have a wall insulated with 6 inches of insulation between your studs.  6 inches of insulation gets you right around R-20.  Problem is, the studs have the insulation value of wood, which in pine is about 1.3 per inch.  So your R-20 wall isn't nearly R-20 anymore.  It's R-20 mixed with less than R-8 at every stud.  There are ways to mitigate the issue, but they complicate building and so are rarely done.  A bolt would be far, far worse than wood.  A carbon steel bolt 6 inches long has an R-rating of .018, so for all intents and purposes, 0.

This sounds like a necessary evil...

So does making a base and then building on top of the base solve this problem?

Anne Miller wrote:This sounds like a necessary evil...

So does making a base and then building on top of the base solve this problem?

It isn't really so much a necessary evil as it is just somewhat harder and more time consuming to build around it.  There are a number of ways to overcome it to varying degrees, but yes, you're one the right track with it.  You just have to come up with ways to minimize the materials that cause the bridging.  A "Mooney Wall" is a way to minimize thermal bridging in wall construction.  There is an example of how to do it here BuildItSolar Mooney Wall article  The same kind of idea can work on a floor too.  The main thing to do is minimize the amount of material that directly touches both the trailer base and the floor structure of the house.  I would probably do it with a series of blocks that held my floor off the trailer platform with insulation covering everywhere between the blocks.  It's hard to say without being able to see the actual setup.

Thermal bridging is a pretty big issue when it comes to making really efficiently insulated houses.  It can make a 30-40% difference in your true R value.  

Would something like this work between the base and the floor?

https://permies.com/t/80/177485/Homegrown-Mushroom-Mycelium-Insulation-Panels#1722301

In Ecovative's prototype mycelium-insulated tiny-house trailer build, they attempted to completely circumvent thermal bridging by growing the mycelium insulation in place, achieving structural stability and insulation with the at the same time.  Testing and reports have fallen off after the first few years of that project.  I am attempting to continue this work.

30-second synopsis starts here, discussing thermal bridge mitigation specifically:


We began testing grown-in-place mycelium insulation here:
https://permies.com/t/181810/permaculture-projects/Mycelium-Core-Door

Hi Emily,
Welcome to Permies!

Specific to your question... Could you build the subfloor (joists and all) with minimal contact with the frame, and then on top of the subfloor, before you put down the flooring, could you add a couple inches of Roxul Board (that is the mineral wool answer to those 4x8 sheets of pink and blue foam, which you could also use if you don't mind using that type of material). This would reduce a lot of the thermal bridging in the floor at least.

Thank you everyone for your replies!

In the article I posted, I think they mitigated some of the potential condensation/thermal bridging by applying the Rosoco waterproofing pitch to the underside of the marine grade plywood and taping all parts of contact with the trailer frame with the thermal tape. Then they place the joists over this, the insulation board, and tongue and groove pine.

My biggest concern is just the areas where the bolts go through the subfloor joists into the trailer cross members. As soon as the PNW freezing air hits the cross members, cold air is going to move through the joist wood to the warmer region, condensate and possibly start to mold. No matter how it's built, we couldn't avoid bolting down into the trailer frame itself. Is it possible to seal the top and bottom of the bolts somehow so that cold air couldn't move through as easily?

Hello Emily, I do not know if you will receive this message. I see that your question was posted several years ago. I am currently in the same position of question and mold related health issue that you were in. I am on the East Coast and finding it very  difficult to find a builder who understands thermal bridging where the trailer and wood framing are concerned. Did you solve it to your satisfaction? How is the state of your home a few years later?

Maybe as you build it, slather on some borax solution onto all the spots where the bolts will go, after drilling the holes.  Borax and boric acid will kill and prevent mold, and are relatively non-toxic and inexpensive.  Just dissolve either or both into boiling water to saturation and paint on.  After it dries and crystallizes, you can wipe the excess off and paint over it.  This is good mold prevention for any wood in general, except that exposed to continuous water which will dissolve it away.