Stilt Home Barndominium
For many challenging building sites (those with grade change, in flood zones or close to oceans or seas) stilt homes are a viable and practical design solution for barndominiums.
Reader DAVID in EMINENCE writes:
“We are planning to build in southern Missouri a 30′ x 36′ x10′ post frame home on a rocky slope terrain. We want it on stilts. It would be 3′ on one end and 7′ on the other end approximately. We are planning to put reflective bubble wrap on the floor joists with the subfloor on top then place down rock wool and another subfloor on top. We have 99% humidity most of the year (10 months for sure), lots of rain. We do not want a crawl space; we know the horrors of the crawl space. We may enclose the high end using a simple temporary enclosure to dry it out as needed. We are going to use a mini split heating system and composting toilets. No worries about placement of the utilities and pests. Would this be a sound construction system?
We would like to know your viewpoint on this since you are the wise guru.”
Mike the Pole Barn Guru writes:
Thank you for your kind words. Stilt houses are very easily done using post frame. I have a post frame combination garage/studio apartment/office at our home near Spokane, Washington on 14 feet of grade change and built it as a stilt building. Has been great for going on 30 years and would have been the only practical way to build on this site (for extended reading on stilt houses: https://www.hansenpolebuildings.com/2017/09/stilt-houses/).
Not sure why you are considering two layers of subfloor. I would be inclined to use either steel or an exterior rated sheathing product on the underside of my joists (with a Weather Resistant Barrier between). Rock wool is a good choice for insulation between joists as it is not affected by moisture. Place a vapor barrier on top of joists and then your subflooring. A radiant reflective barrier (bubble wrap) can be used as a vapor barrier, but will not provide any benefits you wouldn’t get from well-sealed visqueen – and would be far more expensive.
Condensation Even With Radiant Barrier Installed Under Roof Steel
It seems every winter I get a few messages similar to this, So far, this winter, I have gotten two, both from newly constructed post frame buildings and from the same area of the United States (which is known for high humidity).
Reader SAM in GREENBANK writes:
“Hi there. I’ve almost completed the building, and It’s been getting cold lately. I noticed that every time we have a frost I have condensation drips all over inside the barn. I was under the impression that the foil backed bubble wrap was supposed to prevent this. Is there something I can do to stop it? Is this normal? I’m having to keep all my tools under cover even though they are inside. Not ideal.”
Mike the Pole Barn Guru Responds:
The radiant reflective barrier does keep warmer moist air from contacting the underside of the colder roof steel and condensing. If it has been below freezing inside your building, and fairly humid, water vapor will freeze to the underside of the barrier as well as the roof framing members. Most often the excess humidity is a product of a relatively freshly poured concrete floor (tremendous amounts of water vapor are expelled from a concrete slab as it cures), a concrete floor which does not have a well sealed vapor barrier beneath it, or (in buildings without a concrete floor) the ground under your building will not freeze and when the ground outside starts to freeze the excess ground moisture rises inside of the building (think of a cork being removed from a bottle).
Possible solutions (may have to be used in combination): heat the building to just above freezing, open the doors to allow excess moisture to escape (especially in cases with a fairly fresh pour or no slab), if no vapor barrier under a slab – seal the surface of the slab to prevent moisture from coming through. Basically it takes a reduction of humidity inside of your building.
In cases where eave and ridge ventilation was not provided for initially, adding gable vents might help to alleviate some of the interior humidity. When I was a builder, we erected a boat storage building over (we found out later) what had been a pit where asphalt waste from old roads had been deposited. Water sat in between the chunks of asphalt – we had basically built over an underground lake!
The only solution was for the owner to install power vents to pull the humidity out of the structure!