Where to place vapor barrier?

[BlackNBlue]

I'm doing a lot of research on how to layer my materials, and getting conflicting information, much of which makes assumptions that aren't true in my case. I'd like to hear from people who have experience with similar circumstances (materials and location).

My materials:
 * Aluminum-bodied stepvan. Built fairly well, not quite airtight but nowhere near as airy as corrugated metal RV siding. It won't rust, so if I design a system that gets it wet, not a problem per se.
 * Fiber/paper-faced (not foil-faced) 4" polyisocyanurate boards. This stuff is apparently much less effective when wet, unlike XPS.
 * Some kind of heavy plastic sheeting.
 * Adequate ventilation: floor vents, opening skylights, and probably extractor fans.
 * Heat will initially be catalytic propane, to be mostly replaced by a woodstove in the not-too-distant future. I'm also looking into air-air heat exchangers.

Intended climates:
 * Priority one is very cold/dry (Rocky Mountains in the winter).
 * Priority two is cold/wet (PNW in the winter).
 * Priority three is hot/dry (central California or Montana in the summer).
 * Hot/wet will be rarely encountered; I'm willing to sacrifice suitability for this climate to optimize for the other three.

Do I place sheeting between the insulation and the alu skin? Then won't moisture from the inside condense on the sheeting and soak the insulation?

Or do I place it between interior plywood and the insulation? Then won't any moisture that does get to the insulation be trapped forever? Would drain holes help? I suspect not; that it has to breathe to get dry. An air space between the alu and the polyiso is not a good option because it takes up even more space, but I'll consider it if it works really well.

:huh:

 

[BlackNBlue]

Or Tyvek?

Or should I not bother with one at all, given that the alu skin is basically already a vapor barrier about as tight as I can make any other, and the way to dry a wet interior is to heat it, during which I'll be comfortably warm (and relatively dry) anyway?

 

[BradKW]

Or Tyvek?

Or should I not bother with one at all, given that the alu skin is basically already a vapor barrier about as tight as I can make any other, and the way to dry a wet interior is to heat it, during which I'll be comfortably warm (and relatively dry) anyway?

I've been in construction for a very long time, and one thing I've always noticed is that old, late-1800's farmhouses always had solid, dry wood in the walls whenever you open them up. They also didn't have any attempt at vapor barriers...heating those drafty old homes is always a challenge.

Compare that to newer homes with vapor wraps and you find all sorts of moisture problems in the walls. I'm not saying that vapor barriers are bad per-se, but perhaps as you've seen in researching them, climate, insulation and ventilation all are key factors in determining how and where to place the barrier. In an aluminum walled van that will visit a variety of climates, I'd be very tempted to just skip a barrier/wrap.

 

[BlackNBlue]

Yes, those old farmhouses were very well ventilated, whether one liked it or not! Back when the human population was more reasonable, wood was in greater abundance, and the occupants of such buildings were typically numerous enough that there was a constant supply of firewood, this worked ok. We've got quite a different scenario here.

Research (ongoing) so far points to using no additional barriers. In fact, it's revealed a potential new problem: the plywood with which I plan to panel the interior is also a vapor barrier, depending on how tight I build it. So unless I switch exterior materials (impossible), interior (switch to what?), or add ventilation holes to either (neither sounds appealing), I've got two vapor barriers to deal with. Oy vey...

I just started ripping out the ceiling panels (sheet alu) to discover about 1" of yellow fiberglass batting stuffed up there, in between alu C-channel roof braces. Useless due to the massive thermal bridges caused by the C-channels, but there appears to be no condensation problem up there even though it's pouring rain. On the other hand, the space is about as airtight as a jungle gym at the moment...

 

[Suanne]

I'm just learning about insulation myself and had been wondering about vapor barriers. So, the answer to this question may be obvious, but not to me. Since polyiso is a closed cell foam, why wouldn't it also be considered a vapor barrier?

 

[BradKW]

PolyIso, plywood, and in fact many common building materials are indeed vapor barriers of a sort. Part of the problem is that we struggle with names and terms. We have vapor retarders, we have vapor barriers, we have vapor permeable we have vapor impermeable, etc.

A true contiguous vapor-proof membrane would also be airtight, since water in vapor form is carried by air...and that is not really realistic in most any building scenario. So we're left with the goal of preventing moisture in AND if it gets in, letting it out so things dry. Not so simple.

In a vehicle, I think its more important to understand moisture Drive than it is to worry about vapor barriers. No matter how tight you cut your plywood or ISO, you really can't come close to creating an airtight cavity. If you provide paths for hot air to vent, you will also control moisture...this is seen in residential housing in ridge venting. You can still heat the house, and allow control of moisture drive for the most part.

 

[Optimistic Paranoid]

Tyvek is not a vapor barrier.  It goes on the OUTSIDE of buildings to serve as a wind barrier and it is also waterproof against rain, but, like Gore-Tex, it is allows water VAPOR to pass through.

 

[BlackNBlue]

Brad, you just introduced me to the term "moisture drive", and now I'm back to attempting to crash Google's servers.

At the moment (subject to much change as I read and digest more), the true optimal solution would be to have a well-ventilated air gap between the exterior walls and the insulation. That would involve drilling a few holes and perhaps installing additional vents. I'm hoping to find a solution that's nearly as effective yet involving less work and less space sacrifice.

I'm just learning about insulation myself and had been wondering about vapor barriers. So, the answer to this question may be obvious, but not to me. Since polyiso is a closed cell foam, why wouldn't it also be considered a vapor barrier?

I'm a bit confused myself, but it seems that polyiso (when not foil-faced) is considered semi-permeable, and therefore a vapor retarder not a barrier. Unlike other rigid foams, it can absorb water - according to one source, 5% by volume that results in a 40% loss of thermal resistance. It is however enough of an air barrier that it won't let hot air escape.

 

[Suanne]

Thanks Brad. That makes sense to me.

If I may ask another question, hopefully in line with the BlackNBlue's OP, how do you provide paths for hot air to vent. Are you talking about ceiling vents/fans? Since there's no way to make polyiso or plywood airtight, does that mean you don't need to create vents in the upper (interior) walls to drive moisture and hot air to vent out from between the steel/aluminum of the vehicle and the polyiso?

Would the answer be different for BlackNBlue's location in dry/cold Colorado, vs. humid/moderate PNW?

 

[BradKW]

So here's the thing. Misunderstanding and misuse of vapor barriers in all their forms has caused an incredible amount of damage to homes throughout the country. Some of it is due to really smart people over-thinking and engineering solutions that fail real world applications, and some of it is due to well meaning people applying an square peg to a round hole. Then there are good solutions that fail simply because of minor details like utility penetrations. There is an abundance of scientific studies done on all sides of this.

I've not yet built an RV, but when I do my plan is to simply provide for adequate ventilation with roof and floor venting, and not try to seal things up so tight that condensation that does occur on the inside walls will find its way back out when the sun heats the metal again. Cooking and propane use generate a lot of water vapor, but even just breathing puts out something like a gallon of water per day.

I've learned much of what I know about these things through diagnosing failures and problems that take some sleuthing to trace back to moisture drive. I don't design systems and have lost trust in many that claim to know what they're talking about, so I don't want to give specific "answers" when I myself will just be winging it based on the situation and what "feels right"...

 

[66788]

PolyIso, plywood, and in fact many common building materials are indeed vapor barriers of a sort. Part of the problem is that we struggle with names and terms. We have vapor retarders, we have vapor barriers, we have vapor permeable we have vapor impermeable, etc.

A true contiguous vapor-proof membrane would also be airtight, since water in vapor form is carried by air...and that is not really realistic in most any building scenario. So we're left with the goal of preventing moisture in AND if it gets in, letting it out so things dry. Not so simple.

In a vehicle, I think its more important to understand moisture Drive than it is to worry about vapor barriers. No matter how tight you cut your plywood or ISO, you really can't come close to creating an airtight cavity. If you provide paths for hot air to vent, you will also control moisture...this is seen in residential housing in ridge venting. You can still heat the house, and allow control of moisture drive for the most part.

Here's an interesting short link to Vapor Drive.

 

[BlackNBlue]

So here's the thing. Misunderstanding and misuse of vapor barriers in all their forms has caused an incredible amount of damage to homes throughout the country. Some of it is due to really smart people over-thinking and engineering solutions that fail real world applications, and some of it is due to well meaning people applying an square peg to a round hole. Then there are good solutions that fail simply because of minor details like utility penetrations. There is an abundance of scientific studies done on all sides of this.

Well, my worst-case scenario is the heavy insulation losing 40% of its effectiveness because it gets wet. Oh, and mold, I suppose. Structural problems are not plausible.

I've not yet built an RV, but when I do my plan is to simply provide for adequate ventilation with roof and floor venting, and not try to seal things up so tight that condensation that does occur on the inside walls will find its way back out when the sun heats the metal again.

What would you do if the metal's already sealed pretty tight? "Unseal" it (that is, add vents or holes)?

I've learned much of what I know about these things through diagnosing failures and problems that take some sleuthing to trace back to moisture drive. I don't design systems and have lost trust in many that claim to know what they're talking about, so I don't want to give specific "answers" when I myself will just be winging it based on the situation and what "feels right"...

You're in Zone 1 it seems, and quite experienced (I'm not at all). Do you have any experience with Zones 4 and up? If so, how would you wing it given my scenario?

 

[akrvbob]

I lived in Alaska for 6 years in a box van. Thirty below zero temps were normal for me.

If it were me, I wouldn't bother with a vapor barrier, use that money for more polyiso.
Bob

 

[flying kurbmaster]

was there any mold in the ceiling when you took it apart? in my opinion what all this boils down to is to prevent that, if you are intending to install a wood burner or a vented propane heater then you will likely be able to dry out any water from your insulation, at least that is my guess, if moist air can get in then dry air should be able to as well.????if you wanted to create an air gap you could glue strips or 1/4 inch ply to the alluminium then glue your insulation to that drill 1 or 2 inch holes through your pannelling and polysio top and bottom with a grill on them or something to finish them and allow air to circulate but you would lose some insulating value doing this as the cold hot alluminium would warm or cool the air as it went through which may even cause more condensation.????? just thinking out loud.

 

[BlackNBlue]

Zero mold, just some gooey yellow residue. I'm worried a little about mold but mostly about compromising R-value, which I just recently found out is more of an issue with polyiso than XPS.

You have the same vehicle, more or less. Tell me - what climate do you live in, what do you have for insulation, and how's it worked for you?

more polyiso.

More than 4"!? I thought I was already overkilling it...

 

[BradKW]

At a quick glance, I think your concerns related to ISO water absorption might fall under the "overthinking" category for your application.

I see some studies done with XPS, foil faced, etc, and all the testing is basically 2-24 hour complete submersions, or 3-4 day exposures to simulated wall cavity leaks. I'd suggest doing your own test: break off a piece of your ISO and float it in a salad bowl in your kitchen. Let me know when it starts to sink

Also keep in mind that by all accounts, even the most rudimentary insulation in a van makes it so just a Little Buddy heater actually overheats the area pretty fast...the wood stove setups I've watched seem to basically burn a pine cone every now and then...

 

[BlackNBlue]

overthinking

*sigh* I'm accused of that a lot, in general. I bet people who raised concerns about interior polyethylene barriers in the 70's and 80's were also accused of overthinking.

The saving graces are mostly that 1. it'll still work pretty well running on pinecones, as you point out, and 2. having to retrofit several years down the road won't be the end of the world. It's my first such project and I'm mentally quite prepared for the reality of doing it better a second time. Similar things happened with my first major car build, and six years and 50k miles later, I don't really have regrets, just lessons to apply the next time I do such a project.

 

[flying kurbmaster]

Zero mold, just some gooey yellow residue. I'm worried a little about mold but mostly about compromising R-value, which I just recently found out is more of an issue with polyiso than XPS.

You have the same vehicle, more or less. Tell me - what climate do you live in, what do you have for insulation, and how's it worked for you?

I have a similar vehicle, I left the roof as is, that is why I wondered about the mold in yours, it is good to hear there is no mold in yours, when I cut my chimney I had a look between and it looked good. I installed 3/4 inch foam insulation, shinny foil like stuff on one side towards the inside, plastic on the other, glued to the walls with foam board adhesive, 3/16 Luan glued to that. the center brackets, cabinets and other furniture work as a mechanical fastener holding it in. I have lived in it as low as mid 20,s with a small electric heater on low, or with the vented propane heater, 12000 btu, Dickenson also on low. I stayed warm, not sure how much colder I could go. I want to install a bit more insulation in the roof I was thinking another 1/2 inch of similar stuff and a sheet of panelling, I don't have as much height as you do mine is only 6 feet 1 inch high, I insulated the floor with 2 inch blue foam from the underside between the beams, I am not sure that is a great idea as the floor and the ceiling for that matter are connected to the sides and the allumnium conducts the cold, I lived in it in temparatures in the nineties with high humidity and have never felt too warm, I don't have a roof vent, but do have a cross draft with the windows, and those little vents in the upper back as well I can open the back doors . I have sealed the rear doors and have a insulated bulkhead separating the cab from the back, my space is only 10'x6'4"x 6'1" so a bit smaller then yours, I think your idea to put a wood stove in is a good one, especially if you are wanting to spend winters in it. I installed clip together laminate flooring with a foam base over the alluminium, I may install another layer of flooring over it someday some clip together real wood this time. I would like to remove a section of my panelling and have a look to see what the foam looks like after 6 years. I am curious to see if any mold or moisture is in there, the van feels dry and does not smell of mould. My roof has always stayed dry except very close to  both sides at times I have felt condensation on the alluminium  where the roof meets the walls, when no heat is on, on cold nights. If the heat is on, it is dry. if you have 4 inches of insulation you will be very toasty, sealing the rear door will be you weak point do you have a roll up or barn doors.

 

[BlackNBlue]

6 years? Should be plenty of time for any issues to present themselves. So if I got this straight... your walls, outside to inside, go: alu, plastic, foam with foil pointing inside, luan, all glued and also held in by cabinets and such? Meanwhile, your roof is unmodified from how it came from Grumman: thick alu sheet, alu c-channels with fiberglass between them, thin alu sheet?

It sounds like you do have significant thermal bridging with the roof and the roof corners. That said, if yours is like mine, the roof is not quite as bad as it may seem; there is a layer of some kind of caulk/sealer between the roof skin and the braces. However, the under-floor insulation sounds useless.

Mine has dual 19" barn doors, on which I intend to replace the weatherstripping, insulate to full wall-thickness, and use to access storage.

 

[flying kurbmaster]

yes you are right that is how my van is set up and you may be right about the under floor insulation maybe it keeps some noise out while driving? It saddens me to think like that so I choose not to as it took a lot of work getting it all in under there, I also have lockers under the van on both sides made of plywood and covered in allluminium and over the lockers I bent another piece of alluminium to run the full length(aerodynamics) maybe with all that there is some insulation value. How will you insulate your doors to wall thickness and still be able to open them perhaps on an angle off the first opener, I have the same doors with windows in them 4 windows originally, changed the lower windows for lockers, accessable from outside. I made them out of alluminium the insulated them and covered them with luan on the inside.Actually they are about the same thickness as yours will be when insualted so as long as you are a couple of inches back from the edge you should be able to open he door. They are wide enough to store things like paint cans, oil, etc... I made the floor go right into the doors which are insualted and covered in luan as the rest of the van, then I made a L moudding out of wood that goes from the luan on the side of the doors to the doors, I am trying to eliminate any traces of alluminium inside for a thermal break.