Moisture management for tiny house subfloor?
My husband and I are just about to start building a tiny house on a trailer in Nashville, TN, and I’d like to make sure our subfloor does a good job of protecting against moisture, as well as drying out when necessary. Any advice would be greatly appreciated!
Our current setup, from bottom to top, is:
– Steel trailer frame
– Metal flashing
– 2×4 framing filled with Roxul, anchored to steel flanges on the trailer using bolts
– 3/4″ Advantec with taped seams, which will connect to ZIP system sheathing as our air barrier
– Cork floating floor
I am wondering about a few changes to address potential moisture problems:
– Adding a layer of XPS just above the metal flashing, and below the 2×4 framing, to increase insulation and discourage water from moving up into the framing. I’ve run the numbers, and 25 psi compressive strength should more than handle the loads from the exterior walls. Does anyone see potential issues with adding XPS here?
– Adding a vapor barrier somewhere, perhaps directly above the XPS, as you would with underneath a concrete slab? Or will that cause negative consequences?
– Using a gasket or something else underneath the framing members to act as a capillary break (instead of a vapor barrier?).
I would really, really appreciate your advice! Thanks in advance!
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If you are insulating between your 2x4 floor joists with mineral wool batts, then you need an air barrier under the 2x4s as well as over the 2x4s. The top air barrier is your AdvanTech subfloor. But you need something underneath the 2x4s, too.
What is the "metal flashing" you are talking about? Does that mean steel panels that totally enclose the bottom of the mineral-wool-insulated cavities? Or just strips of metal above the steel trailer frame?
Depending on what you mean by "metal flashing," you probably need a layer of OSB or plywood under your mineral wool -- unless you want to switch to a different approach to insulation.
If you want to use rigid foam insulation for your floor assembly, the easiest approach would be to leave your 2x4 joist bays uninsulated. After the AdvanTech subfloor is installed, you could install one or more layers of rigid foam above the AdvanTech, followed by a second layer of OSB or plywood subflooring.
That may be a more elaborate approach than you want to use, but it's the best way to go.
Thanks so much for your advice! I wish I would have known more about these green building principles *before* I bought our custom tiny house trailer.
Unfortunately, given our trailer setup, I can't think of any way we could utilize the AdvanTech-foam-OSB sandwich, as great as that sounds! We are about an inch and a half shy of our 13'6" maximum height (per DMV regulations), so we don't have the vertical space to add three or four inches of foam on top of the 2x4 subfloor framing. I know that some tiny house builders are skipping the 2x4 subfloor and building directly on the trailer, but we have been counting on the subfloor framing to allow us to cantilever over the sides of the trailer by a couple inches on each side. I guess I'm not willing to sacrifice another four inches on the width of our already-tiny house. :-)
The metal flashing will be strips of metal over the top of the trailer frame. I know it can't serve as our exterior air barrier, since we need to keep the flashing open in at least a few spots to allow for drainage if ever needed.
Would it work to use 1" XPS, with taped seams, as our exterior air barrier? This would go directly above the metal flashing and below the 2x4 subfloor framing. I can tape between the XPS and our ZIP system wall sheathing to connect the air barrier there.
Also, just confirming: We are using Roxul in our wall stud bays, with ZIP R-6 sheathing on the exterior as our air barrier. We do *not* need an interior air barrier there, correct? (Based on your blog, "One Air Barrier or Two?", I assumed that one continuous barrier on the exterior was sufficient.)
For our roof, based the GBA details for a thick roof, we are using 2x4 rafters filled with Roxul, topped with ZIP sheathing, then 2.5" of polyiso with taped seams, then 1/2" ply, then peel-and-stick membrane, then a metal roof. We will use applied overhangs per the details in the article, "High-Performance Homes on a Budget." Again, we do *not* need an air barrier on the interior of this assembly, correct?
Thanks again for all your help! I am sure that what I've learned from this website will add years to the life of our tiny house.
Your trailer home needs to be able to handle the vibrations of road travel, I assume, so I don't recommend that you install a continuous layer of horizontal XPS between your steel trailer frame and your 2x4 floor joists. The foam will interfere with your ability to make a strong structural connection between your 2x4 floor joists and the steel frame.
One way to build this assembly is to sheathe the underside of the 2x4 floor joists with OSB or plywood before you install the floor assembly on the steel frame of the trailer. (Build the floor on a concrete slab near the trailer, upside down, and flip it over onto the steel trailer after the OSB or plywood is attached to the 2x4s.)
Another way to solve your problem is to forget about the mineral wool, and to simply insulate your joist bays by installing spray polyurethane foam against the underside of the subfloor.
Concerning your question about an interior air barrier for your walls: It never hurts to pay attention to airtightness with every layer. But in your case it hardly matters. Your house is so tiny that it will be easy to heat, no matter how you build it.
Last year I helped a young relative build a small trailer home. His interests tilted towards natural materials and ecological purity end of the spectrum, rather than energy efficiency. As a result he ignored all advice on air sealing, and finished the interior with wood boards insulated with sheep's wool. As fast as you throw heat at the house it disappears. Small as it is, it turned out to be a very uncomfortable place to spend time in when it's cold.
Thanks for the story.
So, Michaela, you heard Malcolm: Don't ignore air sealing!
Thank you again, Malcolm and Martin! After all I've read on this site, I wouldn't dream of ignoring air sealing. :-)
Martin, based on your advice, I'll go with a layer of taped OSB at the bottom of the floor assembly to form an air barrier.
Now, one last question: Should I include a vapor barrier anywhere in this assembly? Most instructions I've seen for cork flooring call for a vapor barrier just beneath the flooring. I guess I'm concerned about limiting the subfloor's ability to dry inward if needed, since the bottom of this assembly is coated in metal flashing, which I assume won't be conducive to a lot of drying. What is your opinion?
No, you don't need a vapor barrier anywhere in this assembly. OSB is a smart vapor retarder. In any case, don't worry.
Hi again, Martin,
My husband and I just started building, and I realized that despite all my planning, I overlooked a serious breach in the bottom air barrier (OSB) for our subfloor: We have 50 carriage bolts running through the subfloor framing and the OSB beneath, then attaching down through our steel trailer beams.
Here is the best on-the-fly solution I can think of to maintain the air barrier:
- Lay foam sill seal between the OSB and the steel trailer beams, with small holes for the bolts to run through. Perhaps run a bead of caulk around each hole to ensure a good seal near the bolt penetrations. This would seal between the OSB and the trailer on the top side.
- Then, beneath the trailer beams, I am wondering if it would work to coat the bolt/washers/nut protrusions with duct mastic. It seems that is flexible, weather-resistant, and air-sealing, so I wonder if that would work in this abnormal setup. This would seal between the bolts and the trailer on the bottom side, preventing any air from leaking up through the bolt hole.
What do you think, Martin? (Anyone else, please chime in if you'd like, too.) I appreciate your advice!
Your plan sounds fine. You can also install a short piece of housewrap tape to cover the bolt head if you want.
During the framing process, our tiny house was exposed to a LOT of rain downpour while we hurried to get dried in. Unfortunately, we had standing water on our subfloor many times, although we tried to swab the decks soon (as in, a day or two) after each rain.
The subfloor is 3/4" plywood, with the seams taped with ZIP tape. We saw some bubbling and delamination here and there, but the tape stayed on well, and we hoped the rain didn't get into our subfloor bays. The water pooled most heavily on one end of the house, which is my area of most concern.
Now that we've been dried in for a little more than a month, I've been noticing that the sheathing feels slightly damp near where it meets the kick plates of our walls. Today we pulled back the ZIP tape on one section of the exterior of the house, where the wall sheathing meets the ZIP panels at the bottom side of our subfloor assembly. To our dismay, it was quite damp beneath the tape, and the edges of the ZIP panels are expanding and darkening.
Then, from the inside, we drilled 1" holes in a couple places near where the sheathing looked particularly damp, and it does indeed feel moist when we stick our fingers down into the Roxul.
I'm trying not to panic--but I do covet your advice!
Because the bottom of our subfloor is sealed with taped ZIP panels, it seems that any water that did get in during the framing phase would have a hard time finding its way out of our subfloor. I want to make sure that we don't leave too much moisture trapped in there, but I also don't want to rip up our whole subfloor unnecessarily to let everything dry out.
Can anyone offer advice? What would you do in my situation?
I am planning to buy a moisture meter in the morning and test a couple of the floor joists. What else could I do to evaluate? And if the subfloor does need to be dried out, how should I go about that? I suppose I could remove either the 3/4" plywood on the top of the assembly, or the ZIP panels on the bottom of the assembly. Would one be preferable?
Thank you so much in advance!
PS For reference, my husband and I used the following assembly for our subfloor, from the bottom up:
-Steel trailer frame -- with lag bolts drilled through to attach house framing
- ZIP sheathing with the green side facing downward, seams sealed with ZIP tape and expanding foam tape, and a sill plate gasket where the sheathing meets the trailer
- 2x4 subfloor framing, bays filled with Roxul
- 3/4" plywood subfloor sheathing--glued, screwed, and taped
In retrospect, I'm sure that you realize that your assembly sequence was flawed. You never want to install any batt-type insulation, or seal up any framed assemblies from both sides, while a building is still exposed to rain. This type of building should have been built in a large garage or barn, or you should have come up with a different construction sequence for the floor.
Drying out the floor assembly won't be easy. Ideally, it would be great if you could park your tiny house inside a large, conditioned garage for a couple of months. But I doubt if that's possible.
If I were you, I would drill a series of holes in the subfloor (the top side) as well as holes in the sheathing installed on the bottom side of your floor joists (if this sheathing is accessible), and I would install a heater in the tiny house and keep the house warm for several weeks. A moisture meter will help you monitor the progress as the floor assembly (hopefully) dries out.
Thanks again for your invaluable help. Yes, this is a lesson I won't ever forget!
After reading your post on Friday and then checking some test areas with a moisture meter, we decided to rip out the top sheathing on the subfloor, remove the insulation, and get some fans in there. Now that I know we have a real problem, I'd rather address it head on and make sure we eradicate the moisture.
With much appreciation,
This is a helpful read - thanks....
I am also about to embark on this same journey, yet I have decided to use EPS directly in the trailer frame. The aluminium sheeting will be screwed to the steel trailer from underneath, the 3" EPS foam will then be placed inside the frame, with 3/4" TnG ply ontop. I propose to put sill gasket ontop of the metal frame using PL construction adhesive and then using self-tapping screws to screw and glue the ply ontop of the sill gasket.
My question here is, should I seal the gaps around the EPS with expanding spray foam, or a flexible caulking, or anything at all? I have been told at a tiny house workshop that I should leave it alone, but make sure there's at least an 1/8" gap between the top of the EPS and the ply. I have also seen people add a clear poly sheeting between the ply and trailer, but to me this seems to restrict the flow of breathability. Living in the rainforest of BC's west coast, many construction workers have said that the buildings that are allowed to breath the most, will be less susceptible to rot. Building that have used modern plastics in their build seem to rot quicker in this 100% humidity location.
Based on your 'build order' suggestion above, would you also suggest that I do the gasket and ply ontop first, then build the house, then add in the insulation underneath, followed by the aluminium sheeting?
Would love to know your thoughts on this. Thanks so much.
If you insulate between the steel members of your trailer with EPS, but you fail to install a continuous layer of rigid foam above the steel frame, then the insulating value of your EPS will be almost entirely wasted. The thermal bridging through the large pieces of your steel frame will suck far more heat from your house than you realize, and the EPS between the pieces of steel won't do much.
It would be far better to make a site-built SIP for your floor (plywood / rigid foam / plywood) so that there is a continuous layer of rigid foam separating the conditioned interior from the steel frame. However, you may not want to do this because of height limitations. But it's the best approach from a thermal perspective.
Q. "Should I seal the gaps around the EPS with expanding spray foam, or a flexible caulking, or anything at all?"
A. It hardly matters, since the heat will escape from your floor assembly due to the thermal bridging through the steel frame. But in general, you would want to seal the edges of the EPS with canned spray foam -- especially if we were talking about a wood frame. With a steel frame, however, the thermal bridging problem dominates the performance of the assembly.
Q. "I have been told at a tiny house workshop that I should leave it alone, but make sure there's at least an 1/8 inch gap between the top of the EPS and the ply."
A. The people who told you that didn't know what they were talking about. There is no justification to leaving a gap between the rigid foam insulation and the plywood.
Q. "I have also seen people add a clear poly sheeting between the ply and trailer, but to me this seems to restrict the flow of breathability. Living in the rainforest of BC's west coast, many construction workers have said that the buildings that are allowed to breathe the most, will be less susceptible to rot. Building that have used modern plastics in their build seem to rot quicker in this 100% humidity location."
A. You are mixing up a variety of concepts here. First of all, you don't need polyethylene in this location, because your plywood subfloor is already an air barrier (if properly installed) as well as a vapor retarder. Second, the "breathability" issue is a red hearing. "Breathability" has no scientific meaning, but it usually refers to vapor permeance. For a discussion of this issue, see the section with the heading called "What do I need to know about vapor drive and vapor permeance?" in this article: How to Design a Wall. (It's the section of the article that begins, “Many green builders think that a wall has to ‘breathe.’”)
Q. "Based on your 'build order' suggestion above, would you also suggest that I do the gasket and ply on top first, then build the house, then add in the insulation underneath, followed by the aluminum sheeting?"
A. If you want to keep your floor assembly dry, it makes sense to get the roof on your building as fast as possible, and then to install the floor insulation later -- as long as you have enough access to the underside of your trailer to complete the work you envision. That said, I still advise you to go with a better insulated floor assembly -- the "site-built SIP" solution.
Thank you so much Martin for your time in answering my questions.
So the sill gasket (thin piece of foam on the steel beams) will do nothing?
I will examine the plans to see if I can afford the additional height of 4 1/2" of floor assembly. If I can, and I add this ontop of the framing, would it be of any benefit at all to add EPS in-between the steel as well?
This way I would get the 3" EPS in the trailer and an additional 3.5" in the subfloor. Or would it be safe to eliminate that additional cost and just insulate the wooden framed floor?
Perhaps install the EPS as originally stated, but maybe add ontop of that a smaller sip? Would a 1" EPS plus 1/2" ply either side be a sufficient addition to assist in the thermal bridging issue?
Another alternative perhaps, might be to drop in some notched 2x6's into the 3" trailer frame so that the bottom side of the 2x6 is flush with the underside of the trailer beams and causing a raised height of 2.5" above the top of the trailer beam... this would give me an opportunity to slide some EPS between the steel beams and the ply and also offer up a better nailing surface underneath to attach some ply (marine paint coated)?
Do you feel this would be a viable option?
Thanks so much.
Q. "So the sill gasket (thin piece of foam on the steel beams) will do nothing?"
A. Foam sill seal is 0.1875 inch thick and has an R-value of less than R-1. In most of the U.S., the minimum code requirement for floor insulation varies from R-19 to R-38 -- so the answer to your question is, "Sill seal will do almost nothing."
Q. "I will examine the plans to see if I can afford the additional height of 4 1/2" of floor assembly. If I can, and I add this on top of the framing, would it be of any benefit at all to add EPS in-between the steel as well?"
A. Not really. Don't bother.
Q. "Or would it be safe to eliminate that additional cost and just insulate the wooden framed floor?"
A. If you are planning to install wood floor joists, of course it is possible to insulate between the wood floor joists. Although you'll have thermal bridging through the wood joists, that thermal bridging won't be as bad as the thermal bridging through the steel frame.
Q. "Perhaps install the EPS as originally stated, but maybe add on top of that a smaller SIP?"
A. I would forget about the idea of installing EPS between the pieces of steel if I were you.
Q. "Would a 1 inch EPS plus 1/2 inch plywood on either side be a sufficient addition to assist in the thermal bridging issue?"
A. You'll get a little bit more than R-4 with that approach. It's certainly better than nothing, but nowhere near code minimum requirements.
Q. "Another alternative perhaps, might be to drop in some notched 2x6's into the 3" trailer frame so that the bottom side of the 2x6 is flush with the underside of the trailer beams and causing a raised height of 2.5" above the top of the trailer beam... this would give me an opportunity to slide some EPS between the steel beams and the ply and also offer up a better nailing surface underneath to attach some ply (marine paint coated)?"
A. That's possible. But it won't be anywhere near as good as a continuous layer of horizontal rigid foam above the steel frame (a site-built SIP).
Thanks again for your time and advice. I will aim towards building myself a sip with 3" EPS to place on top of the trailer frame and will look for resources on how best to do this - thanks again. I will definitely mention your site as I build out my own site in regards to my build. Thank you.
I'm late to this party but I have the same trailer as Alan and have been researching ways to use the steel members as the base of my subfloor so I'm glad I found this discussion. Judging from Martin's response, it doesn't look like installing insulation between the steel joists is the way to go. I'm curious how this is different from houses that are framed directly onto a concrete slab. Both steel and concrete have a significant impact on thermal bridging. If I'm not willing to sacrifice height by making my own sip panels, it sounds like it would be best to just leave the bays empty between the steel joists and put the plywood decking directly on top of the steel floor. I'm in Texas, so I won't be dealing with the cold weather too much, but it would be nice to not lose a lot of my cool AC air through the flooring. Or will it not even matter that much considering the size of the structure?
Q. "It doesn't look like installing insulation between the steel joists is the way to go. I'm curious how this is different from houses that are framed directly onto a concrete slab."
A. Several ways, including these:
(1) In a cold climate, a concrete slab needs to be insulated. So does the floor assembly of a tiny house. For more information on insulating a concrete slab, see "Determining Sub-Slab Rigid Foam Thickness."
(2) Steel is more conductive than concrete. Medium density concrete has a thermal conductivity of about 0.6 W/(m K), while steel has a thermal conductivity of 50 W/(m K).
(3) A tiny house has outdoor air under the floor assembly, while a concrete slab doesn't. Outdoor air in winter is usually colder than the soil under a concrete slab.
Thank you, Martin!
One more question - What do you think about closed-cell foam between the joists?
Closed cell foam between the steel joists would be a waste of expensive insulation that, depending on what specific foam you mean, might also be produced with a blowing agent with high global warming potential.
But some cheap insulation, such as fiberglass, thrown in, will help a little, and won't cost most, out of pocket or environmentally.
But if you can afford even 1.5" of increased floor thickness, you could make a SIP with 3/4" of polyiso insulation and accomplish more than filling the joist bays with foam or fiberglass.
Another option would be to put strips 1" of compacfoam, which is like super-high-density EPS, between the joists and the subfloor, and fill the bays with fiberglass. That would get you better performance than the sandwich with 3/4" of polyiso, I think.
Edit: rats, compacfoam seems no longer available in the US:
I'm also in the same boat with considering insulating between the trailer frame crossmembers of a Tiny House trailer. Having read the comments so far, it occurs to me that I only really need to be mindful of space for the area that's under the loft. So what I think I'll do now is either frame out an insulatable subfloor for the rest of the area or do a thick DIY SIP that's been mentioned, and for the loft area(s), I can either do the unfortunate between-the-crossmembers rigid insulation with a small SIP above, or something similar. That way most of the trailer has good insulation, and I still have the head height.
One question comes to mind though. Have the above commenters, and Martin considered the option of putting skirting on the tiny house. As little as 1" of rigid taped on, or possibly the rigid and bales of hay behind. Wouldn't that help mitigate the thermal bridging under the trailer in any scenario?
You wrote, "I only really need to be mindful of space for the area that's under the loft."
This sentence confused me. After I scratched my head a few times, I think I understand what you meant. Perhaps you meant to write, "Most of my tiny house has a high ceiling, so I can add adequate floor insulation under most of my house. However, part of my tiny house has a low ceiling -- the area with a loft above. In the part of my tiny house with a low ceiling, I can't figure out how to insulate the floor."
Is that what you meant?
If you are planning on two different floor levels, with a step up and down between the two floor levels, that would be a mistake. A single riser like that is a documented trip hazard.
Q. "Has Martin considered the option of putting skirting on the tiny house?"
A. You could do that -- it's the standard "mobile home solution" -- if you want. Needless to say, this only makes sense for a tiny house that stays in the same location permanently (rather than being towed from one friend's house to another friend's house).
From a thermal performance perspective, a skirt -- even an insulated skirt -- will perform worse than a well-insulated floor assembly.
If you go that route, make sure that you have a plan to keep your plumbing pipes from freezing. For more information on skirts, see this article: "Crawl Spaces vs. Skirts."
Martin, you puzzled correctly!
Hmm yes, it would be a tripping hazard. I figured I'd thought of the perfect solution, but alas. It sounds like it's probably best to just have a lower area (a tripping hazard for the head, so to speak) under the loft to have to duck under (though my loft is partially a bump out, so it's only really a foot of the trailer length where you could stand and have to duck your head), and in exchange be able to do a better insulation job and then not have to worry about the diminishing returns task of insulating between the crossmembers of the trailer.
I'm planning for the tiny house to stay put for 3 years, though I haven't given consideration as to whether to leave skirting on through the summer as well. As you've mentioned and I've read elsewhere, the vapor barrier on the ground would be essential and probably wouldn't hurt even if I don't skirt and just leave it open.
And yes, I've been giving thought to the water pipes. Some combination of heat tape and insulation for the exposed section between the ground and bottom of the tiny house and probably putting some sort of insulation along the water pipe below ground where it rises from its horizontal run.
My wife and I bought and sited a tiny house on the back corner of our property in New Mexico to live in while renovating our 1920’s cottage. We don’t really have any intention to ever move it, especially now that we’ve build a porch to wrap around the S and E facing sides. Our reasoning for buying a tiny house on wheels rather than having a small house site built include timeframe, financing, finding a builder, etc., etc. Our tiny house was delivered from Wisconsin within 60 days of placing our order.
My suggestion for you deals with the water line. I highly suggest you look at the Merrill Any Temp Hydrant, designed for mobile homes and RVs. Read the description, installation and water line tab information. The hydrant and water line combo takes care of freezing concerns above and below ground. The folks who answered my technical questions were extremely helpful and pleasant.
Oh, and regarding building a floor directly over the metal frame, it’s no bueno. Even in NM and with a 3/4 ton Fujitsu mini split, our cold floors were unpleasant during the couple of cold winter months.
Aaron, that sounds like a great product. Ideal except for the price, but good to have that in mind as a reference point. The self-regulating heat cable is a big bonus. And reading the parts you mentioned has led me to think more about what's needed there. Thanks for that.
Martin, I read on another thread of a similar question about insulating between the frame of a trailer...
... where you said "Two choices: (1) Stop worrying about a minor thermal bridge, and allow the plywood to be indirect contact with some of the steel framing or[...]"
I assume the difference there (the questioner had a photo attached in the other thread), was that his trailer frame had a few metal stringers running the length of the trailer, whereas what I and the original poster of this thread seem to have is cross-members every 16" running across the trailer, and therefore much more metal to be causing thermal bridging.
Another commenter on the other thread claims the main concern here should be with condensation forming between the metal and a plywood floor. Will sill gasket and a moisture barrier (like a 6 mil poly sheet) between the metal and plywood sort that out? I think I've seen elsewhere on the site you said a moisture barrier underneath was unnecessary, but that context wasn't regarding condensation. Unfortunately (or fortunately, actually), I've been reading a lot of articles on here lately and forget where I would've seen that.
> Martin, based on your advice, I'll go with a layer of taped OSB at the bottom of the floor assembly to form an air barrier.
After reading this, I'd like to try this approach too. Is there a limit on the thickness of the plywood for it to be a reasonable air barrier? Since I'm using 3/4" on top of the joists, the sole purpose for the plywood (subsituted for OSB above) on the bottom of the joists is to form an air barrier. Could it be as thin as 1/4"?
Also, if I put a layer of polycro and heavy duty floor protection over the whole assembly to keep the rain off while building the walls and roof, do you think it would would be sufficient to ward off a couple of summer showers? Want to try and learn from Michaela's experience, but don't have access to an indoor space for construction.
Check around for pond liners. Depending the size of your subfloor they would be about $100 at a big box store.
I am new to this site. I’m trying to plan out my subfloor and am trying to piece together the best conclusion to draw from this thread. I am repurposing an old rv frame and had planned on doing metal sheathing on the frame under the floor framing. However, has it been determined all that is needed is a layer of zip sheathing followed by the framing, insulation and OSB on the top? I live in wintery Montana so I want to get this right the first time. Thank you so much
Also any advice on the best way to box out the wheels is appreciated.
The steel pan is an excellent vapor barrier, this means your assembly will always have a cold side vapor barrier. The only way to make that type of assembly work is with spray foam.
What I would do is install 2x2 strapping over the pan than set your floor joists perpendicular to the strapping. Insulate the whole assembly closed cell spray foam. The cross strapping bellow helps reduce the thermal bridging from the floor joists and it increases the overall assembly R value by a fair bit. Similar idea around the wheel wells.
The important part is you always want at least some foam between any of the steel structure and the interior of the place. This prevents the wintertime condensation and mold problems.
Thank you for your reply. So you would recommend From the bottom up:
1 Steel pan
2 2x2 strapping
3 Floor joists perpendicular to strapping
4 then fill entire void between steel and top of joists with foam and then either osb or plywood on top for subfloor?
Nothing between the 2x2 strapping and floor joists correct? No Zip Sheathing?
I would use plywood instead of OSB for a bit more robustness. Akos pretty much already covered the insulation issues.
I would make SURE that the steel frame has a good multi-coat paint job to protect against rust. That frame is your foundation, and if it fails, so does your house. Make sure to take all possible steps to protect the frame. I have found the Rustoleum Stops Rust products do a pretty good job here, but you have to prep for them and you have to use the primer before the enamel for best results.