Re: Questions regarding vapour barriers
- Thank you Ron, that seems sensible. The vapour barrier under the floor insulation looks like the best of both worlds to me. For the walls I wasn't sure if you put tyvek and a poly barrier on the inside. There certainly is a lot of different opinions out there online, thanks for the help.
> There is controversy regarding vapor barrier that I am sure you can find if you google around. The common practice is that it goes on the heated side of the structure to prevent moisture from entering the wall and creating a mold/rot problem. There is some argument about putting the vabor barrier on the exterior where the weather can create a moisture problem and allow the wall to be conditioned by the interior heat. There are some folks that think it is best to put in it inside the wall with furring strips or foam insulation before the drywall t0o keep the electrical from puncturing it. Personally, I would just go with a vapor barrier on the inside and Tyvek on the outside - Tyvek allows the wall to breathe. For the floor I would probably put it at the bottom, nearest the road (similar to dealing with a crawl space).
> Ron Czecholinski
I slept a couple of winters at 9000 feet in the CO Rockies in an unheated up-and-over pickup camper. I couldn't afford to heat it in the winter, it had almost no insulation. The first winter I slept in the over-the-cab part until I discovered a puddle of my own condensed "sweat" under the mattress. It was a hassle dealing with it. Moisture will condense on the first relatively impervious surface that is below "dew point". The second winter I made my bed where the table makes into a bed so I could at least lift the mattress daily and let the moisture sublimate out. I was already familiar with doing that since most mornings I would wake to a halo of hoarfrost around my pillow and sleeping bag When I got "richer" I could add a little heat to the camper, but found that it drove the moisture and condensation point to the inside of the exterior walls. The camper had the typical aluminum sheeting on the exterior.
What I am getting at is that each situation is different and changes with your environment. I'm not sure exactly what the specs are on the Kingspan insulation you are installing, but from a quick search on the web it appears to have a very low perm rate, meaning it does not allow vapor to pass very quickly. It appears that it will already act as a vapor barrier. If this is true and you put an additional vapor barrier on the underside of the floor, you may be locking in any moisture that will find its way in through cracks in caulking, etc. This could be disastrous if you are using wooden joists. From the trailers that I have torn apart, the first thing to go is the lower "rim joist" where the condensation dripping down from the inside of the aluminum exterior wall has been locked into the wall cavity and is soaking the lowest portions of the wood structure. Putting Tyvek under an aluminum exterior wall does no good, the moisture may get through the Tyvek but it will still condense on the inside of the aluminum wall. (assuming a cold mtn. climate)
The first question you need to answer is what environment is your structure going to be located in. Will the moisture want to travel from outside-in (warm moist south) or inside-out (cold dry north) or a combo.
--- In email@example.com, Justin Peer <justin.peer@...> wrote:
> Hi all,
> so once I've sourced a new welder and fixed the trailer, the floor's
> got to go down. The 2x4 frame will have 1mm aluminium sheets (4' x 8')
> on the underside, infilled with 3 layers of 25mm kingspan, sealed
> around the edges with expanding foam. the top surface will be sheets
> of 11mm osb2, ultimately followed with a finish flooring. I'm unsure
> whether to put a polythene vapour barrier down under the osb sheets.
> The advantage is that it would prevent any damp coming up into the
> floor in the future. The downside is that it prevents me fixing the
> floor sheeting with both construction adhesive AND screws, which is
> what I'd originally planned.
> If I put the aluminium sheeting on the underside with glue and screws,
> and then sealed the foam boards with expanding foam followed by glue
> and screwing the osb, do I need the vapour barrier? If I do, then I
> guess it's just screwing down the osb sheets. Alternatively, what
> about putting a poly vapour barrier between the frame and the
> aluminium sheets, effectively below the insulation, are there any
> advantages/disadvantages between the two methods?
> When I do the walls, I'll be putting homewrap/tyvek type vapour
> barrier on the outside of the walls. Again, the walls will be infilled
> with 3 layers of 25mm kingspan board, sealed at the edges with
> expanding foam with tongue and groove planking on the inside. I'm not
> planning on a poly vapour barrier on the inside so that the walls can
> breathe. Is this the normal way of doing things?
> Many thanks,