Ventilation, Condensation & Woodburning Stoves

[HoFFdOg]

Hey guys.

An issue which has been creeping ever more into my mind as i continue on my epic housetruck build is that of adequate ventilation!

I have approached my build to make it as a toasty as possible & have done a good job of sealing right up. Tho now im wondering whether this could giv me a few problems down the line.....

Condensation/Ventilation in Wall cavity: First off im a little concerned about condensation accumulating/rotting behind the ply over the years (ive just seen caravan monsters post)! The ply was screwed straight onto the ally frame & i noticed when using an electric heater inside, slight damp would appear in strips exactly where any metal framework was behind. I want to know whether this is as bad as it will get?

The insulation ive used behind the ply is 25mm Celotex siliconed to the sheet metal and then rockwall in the remaining gap. Im hoping the celotex will act as a moisture barrier ensuring i dont get problems behind the ply. However a few sections of wall which are now inaccessable behind my shower & kitchen worktops only had rockwall between the ply & sheet metal. Will i find my fully tiled shower room collapsing because of wetrot in the ply wall in years to come?

To combat this will it help if i fit lourvred/hit&miss vents into my ply walls, venting into the cavity? Or am i just wasting my time


Woodburning stoves & Ventilation: My other concern about ventilation is the fact i have a wopping great wood burning stove. Ive been told this could be dangerous (carbon menoxide build up) & affect the effeciency of the stove if i dont have constant ventilated air coming in. Is this correct?

Obviously on a cold winters night im going to be compromising accumulated heat for fresh ventilated air. Would you guys consider this an essential trade-off? My housetruck has two side windows and three large skylights, would opening any of these slightly suffice or would a permenently fixed vent near the burner be better?

Ive been looking at roof mounted marine vents with a solar powered fan inside which can circulate around 25 cubic meters of air an hour. Would this solve my concerns regarding both venting the woodburner & condensation accumulating behind my ply?
http://www.force4.co.uk/7057/Force-4-So ... -300S.html

I eagerly await the tsunami of wisdom ive come to expect from members of this forum.

Thanks.

Dan

[Sharkey]

Well, Dan, put on your water wings, here comes the flood.

These are two subjects on which I have a lot of experience. I am being forced out of my Housetruck after 35 years due to the effects of condensation.

The damp you were seeing was the result of the wall surface over the inside framing being colder than the interior air, resulting in condensation on the cold face of the interior. The only effective method of preventing this is to have double wall insulation that covers the framing, separating it from the interior thermally.

In a "normal" house using modern construction, there is a vapor barrier on the interior side of the framed walls, the purpose of which is to prevent moisture from the living space from migrating to the interior of the wall. On the other side of the insulation, a permeable surface is fitted, one that will allow moisture to penetrate. The idea is that you don't allow any moisture to enter the wall from the inside, and any that does is allowed to exit the building on the exterior. So far, so good.

The type and thickness of the insulation plays a big part in the overall picture. The purpose of insulation, of course, is to prevent heat loss, but it serves another duty as well. Since completely preventing moist air from entering the walls is basically impossible, it's important to make sure that any that does is not allowed to condense on a cold surface. This means that the dew point of the temperature/humidity of the environment is contained inside the insulation itself, where there is a limited amount of (or no) moisture available to condense. This works best in non-permeable insulations such as the Celotex that you are using, although it is also effective in fiberglass insulation, assuming that the moisture has a way to migrate fully out of the material eventually.

My problem is that with steel walls, the moisture, once inside the walls is trapped, never being allowed to exit. Vapor barrier is only so effective at keeping moisture from seeping into any space.

The bottom line on all of this for a truck or bus is that since it is such a small space, and usually has thinner walls with less carefully installed vapor barriers and insulation, condensation is more likely to be a problem. Lessening the amount of water in the interior air is your best bet. I run an electric dehumidifier several times a week during the wettest parts of the season, pulling over a gallon of water out of the air and furnishings over the course of a few hours. I also try to ventilate the living space more fully when cooking things that produce a lot of steam. Since I don't have a shower, or dry bathing towels inside, I don't have moisture from that, but I could see it being a major contributing factor.

Most "normal" homes interiors are too dry in the winter, and users of wood stoves frequently put on pots of water to try and increase the interior humidity. We have the reverse problem, and the solution is not nearly as simple.

As for wood stove safety, carbon monoxide poisoning won't be an issue of you don't have leaks in your stove or stove pipes. If your house is seals so thoroughly that the stove could suck the pressure down to dangerously low oxygen levels, you need to open it up a little! The fire would probably go out from lack of draw way before you would notice anything yourself.

Providing proper air intake for the stove need not compromise your comfort. My Housetruck is so leaky from the original back doors and crappy entry door that I don't need to ventilate, but installing a vent for the purpose of introducing combustion air is a good idea.

The whole point of a combustion air vent for the stove is to allow it to draw the air it needs from the exterior of the living space without having to expose you to the cold draught that it produces. As such, what you want it a vent to the outside that can be closed partly or fully and that is as close to the stove's air intake as possible. That way, you don't have cold air moving through your living space on it's way to the stove.

When my friend Woodley installed the stove in his step van, he cut a 2" hole in the floor next to the stove, then had me fabricate a sliding shutter for the top of a projecting stand pipe, which was exactly next to the stove's air intake. With no other paths to the exterior, all of the air the stove needed was drawn up the pipe and made a straight line for the stove. No more air than the stove needed came in, so there were no draughts. When the stove wasn't in use the pipe could be closed off.

Had enough? Keep treading, don't sink!

[southpier]

Mr Sharkey,

could you please explain why you're being forced out of your 'truck because of condensation problems?

i've seen trailers with delaminated plywood because of moisture, either infiltration or condensation, but in reality probably a symbiotic relationship between the two.

thanx!

[longjohn]

Mr Sharkey
would a vent such as used for hvac with sliding shutoff ( a register, i think they are called) ducted with some type of filter work for this ? i know that they come in all sorts of sizes and finishes, plus a handy metal worker could make something real cool

[graydawg]

In a previous factory made RV I had a leak from the roof that went into a wall and had mold growing in the wall, I went through a bout of lung problems and moved out of the rv and have had no problems since and do not want to go through that again, I have a 64 4106 bus presently and would like to put a small woodburning stove in it with a outside air intake for the stove, when it was cold in LA, 30 degrees or so I noticed condensation on the roof of the bus inside, would expanding foam (great stuff) eliminate this problem or would I need to do something different to the bus or would the stove correct this problem itself, by drying out the air, I was using portable electric heaters at the time. THANKS IN ADVANCE FOR THE INFO GRAYDAWG

[rlaggren]

Search the web for words like/including ... insulation, envelope, weatherstripping, mold, siding.... And spend a couple weeks sifting through the results.

The Canadians have done a LOT of "building science" work on this problem. Unfortunately that's because they had to - it's not a simple fix, ie. It usually involves carefully planning the very deepest guts and structure of your habitation. One of the hardest parts in the RV scenario might be the "draining" of your walls, like Sharky mentioned. The chances of an RV getting water inside its walls is much greater than that of a stationary house (small interior space, easier to get leaks) and you want it to get out without letting in more water and/or insects and vermin. Different ways to do this, and mostly it depends on where you start and how much work/expense you will undertake. So you got a little thought and research ahead of you.

They do sell a vapor retarding paint (notice that they no longer make any claims for vapor PROOF) which would probably be a fairly easy to include in any plan. High levels of ventilation also greatly reduce the moisture but in deep cold or torid heat that's much less attractive. Air conditioners will help a lot (they are effectively dehumidifiers) provided the condensate drains are working right.

I live on a boat in a mild climate and so far haven't had too much trouble. One note: I use a skinny cheap foam pad between the sheets and the mattress. This prevents moisture from migrating into the mattress and provided I leave the linens turned back during the day to air out I have no problems with mustiness in the ten years I've been onboard.

Best luck.

Rufus

[Jones'n4chrome]

Rufus,

Welcome to Sharkeys forum.
Thank you for the info. Do you also have a Housetruck or bus?

Chuck

[rlaggren]

Chuck,

Thank you. No I don't own a land yacht. We city folks don't have the discretionary space to do it part time or at least not easily. No place to park/store/repair - especially repair.

I happened here on other business and decided to stick around a little. I passed the site on to a guy who restores gypsy caravans though not sure word got through.

Understanding and fixing the functional side of habitations is my job and my hobby. Folks sharing their stories sort of re-up my interest so I appreciate all the great contributions here.

Cheers, Rufus

[yoeddynz]

Our Bedford TK housetruck in England was all aluminium- tig welded frame work with 3mm thick alloy sheeting on outside. In the bedroom over the cab during winter we would get a small amount of mould from the condensation dripping onto the ceiling cladding from the cold alloy roof.
This still happened even though we had put plastic sheeting between the cladding and the roof to allow water to run off to the walls and down through drains. There was insulation there too but not a lot- maybe 20mm polystyrene. I guess the damp was what was sitting between the cold plastic sheet and the pine. See here..

Adding to the problem was that the bedroom never got the same heat as the the rest of the truck due to the small wall off the ceiling stopping the heat travelling through to the bedroom. We sold the truck last year - but had we kept it and brought it over to NZ (were not allowed to- long story- NZ's silly import laws) we would have fitted a fan to pull the heat through.

I had also added vents through the floor to the back of the stove where it had its main air intakes. This worked well. This truck was built to very high standards as it was a BBC broadcast unit and hence built with tax payers money So it was very well sealed.

We had a carbon monoxide alarm- it was needed! With a truck as well sealed as this bedford with two living in it on a cold night , stove pumping away it was easy to get the alarm to start beeping its first warning! A window opened would sort it out after a while. And you could tell the air was a bit 'stuffy' as we would be all drowsy! But that could be the wine....

I will always recommend one of these alarms! could be a life saver- worth a few dollars!

Are there any links to this housetruck build of yours Dan?

alex

[Stealth Camper]

I live most of the time in a commercial RV and moisture is always a concern. The sleeping area is so cramped that there is the 'feel' of excess CO2 some mornings. Stuffy.

Have been thinking about this subject a lot and I think the "final" solution will be to have an outside air with a heat recovery system, similar to what is used in commercial buildings, only on smaller scale. Need plenty of fresh make-up air, but don't want to spend stupid amounts to heat/cool the thing. It's a dilemma.

Added note; several of the windows in my RV have little weep holes at the bottom. About 1/16" x 1/2". And the vent hood over stove leaks like a sieve. Can get cold (0 degrees F) from time to time, so it can be an adventure living with the drafts.

[rlaggren]

Giving a moments more thought to this, I wonder if a "rain wall" design would work on buses which are fully stripped to start with. I don't know any other way to deal with the totally impervious metal skin on buses which prevents water from escaping or evaporating once it gets behind the metal skin. The concept is to have most of the water shed by a water proof outer shell (outer skin on bus). Beneath that there is an air gap of 3/4" in houses but probably 1/2" would work if it could be maintained throughout. The next layer is also designed to be water proof but it must also be permeable so that it doesn't capture and confine moisture inside the insulation cavity in the wall; common materials are tar paper (works very well, is the most permeable) or Tivek housewrap. The bottom of this vent space _must_ drain easily and top must be well vented to allow circulation so moisture will be removed. Behind the tar-paper/tyvek is a "standard" wall of some type. The spacers used in wood frame homes to locate and secure the outer skin are "1x2" straps installed vertically so they don't prevent drainage; in a bus, with the space premium, 1/2" strips might work provided they were space closely enough to ensure that outer and inner drain planes never touched. But I'm not sure if this space saving idea is practical or a good idea or even matters much.

The thing the makes me think of this design is that a bus skin is metal and totally impermeable and _that_ is a serious problem for a wall. But it _has_ to be that way to stay dry in on-road conditions. So a bus needs that second ventilated permeable drainage plane behind the metal skin to allow it's insulated walls to breath and get rid of water and moisture.

The problem with the "rain wall" is the existing (and required) framing to support the metal outer skin (and the roof). This structure is "wet" because by design the inside surface of the outer skin is "wet"; if this structural member extends inside the inner permeable tar-paper/tyvek wall, then that drainage plane is broken and each structural member (ie. "studs") can carry water into the insulation cavity and this won't work. I'm not familiar with bus framing so I don't know quite how to keep that "wet" structure from getting water into the wall. (I am assuming here that the framing is greater than 3/4" so we can't just build the inner tar-paper/tyvek drainage plan _inside_ the structure - that would surely be the best drainage solution but you just kiss off the vent space between the outer skin and the inner drainage plan. I don't know the framing size to guess how many inches that might be...

Another solution might be to remove the metal outer skin entirely and use the existing structure to hold the insulation cavity and the permeable drainage plane. Then add 1/2"-3/4" vertical strapping over that and re-install the original metal skin on that strapping to give the required vent/drain space between behind the outer metal skin. This would need good fabrication technique but might not be that hard. The "interesting" part would be at the roof where we need a water repellent full length vent and the ends where we just added a couple inches to the linear surface and the old panel won't quite meet to cover. But over all that way seems doable and most likely to get the desired results.

In theory behind the tar paper or tyvek, the wall should remain dry. On the inner wall surface there has to be a vapor membrane to keep the water we breath out from getting into the wall.

But: Water may still get into the insulated cavity and although it _should_ evaporate out through the tar paper/tyvek outer skin that may not be quick enough. The insulated cavity should be able to drain, maybe out onto the tar paper/tyvek drainage plane. Haven't got that one figured out yet. <g> The cavity above windows also needs to drain.

Isn't it something, trying to outsmart a drop of water?

Rufus

[rlaggren]

And here's a link to some dirty detail fix-it tips fr RVs

http://www.phrannie.org/moisture.html