Where best to put the vapour barrier for my tiny house?

[mr_magicfingers]

Hi all,

as some of you may know I'm building a tiny, off-grid house on a trailer.
I'm looking at where would be the best place to put a vapour barrier, if in fact I need one,
for the floor and walls and would appreciate any advice you can offer.

The 2x4 framing for the floor 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. It also means that any damp would condense on the barrier
below the flooring, potentially causing damp/rot in the osb.

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,

Justin.

http://jaystinyhouse.com

[EccentricAnomaly]

When I do the walls, I'll be putting homewrap/tyvek type vapour
barrier on the outside of the walls.

Homewrap/tyvek is a breathable membrane; it's a bit confusing to call it a vapour barrier, I think, as it's specifically supposed to transmit water vapour (but not liquid water, of course).

I'll be intrigued to hear any informed responses here as I'm really unsure how plastic insulation is best treated with respect to water.  

An aluminium skin on the outside is, from the classic building science point of view, a bit a disaster (unless there's a cavity behind it)  but for a road vehicle it seems reasonable.  It makes me wonder if there would be any sense in making it detachable for when in "house mode" and attachable for when in "trailer mode" - i.e., as a temporary protective cover used when moving only but removed the rest of the time to let the floor breath.  Used like that aluminium would be excessive, maybe some sacrificial ply would do.

[dhaslam]

The aluminium itself should keep moisture out.   But whether the kingspan etc would absorb moisture from the room or not is a bit more difficult.  The OSB might need a small air space over the insulation.  This is where the upper vapour barrier would go in a normal floor but in your case the floor should be completely dry underneath if not touching the ground. 

[Justme]

Get a copy of Builddesk U 3.4 (free trial for 30 days).

DOWNLOAD HERE
(I just put rubbish in the details)

Input your build & it will tell you.

I did mine & I dont need a vapour barrier in the walls but I do in the floor but I am using diff insulation to you but still 2 x 4 studs & innner & outer wood skins with air spaces & breathable membranes.

Justme

[mr_magicfingers]

Thanks for the replies, sadly I'm running a Mac so can't use the neat software. Might see if I can get over to a friends and try it out at the weekend.

I wonder if just screwing the aluminium sheets on the underside will give sufficient vapour transmission around the edges if I don't seal them down. if the insulation sheets are sealed around the edge with expanding foam as I'm planning bwtween the framing, that should be ok with just a poly vapour barrier under the osb flooring to prevent moisture going down into the floor. There'll be another wood floor over the osb so I guess it should be fine that way.

I'll be doing the walls the same way except with tyvek and shiplap on the outside instead of the aluminium which is under the house to protect everything from road damage and rodents.

I'm thinking that I might get some closed cell foam and trim 1.5" strips of it to go on the inside of the 2x4's to prevent cold bridging as the walls are quite thin. I'd have liked to use 2x6 but with a house that's only 8x16' I needed every square inch I could manage inside. It'll be heated with a small woodburning stove and use solar hot water and pv panels for electric.

Cheers,

Justin.

[KLD]

Hm, quite a challenge. As EA put it, an unventilated airtight outer shield is a bit of a disaster. There will be very few days in a year when the inside of your cabin is cooler then the outside. Since the water vapour gradient correlates with the temperature gradient, you'd want to make the shell all around more vapour tight on the inside than on the outside.

In that light, I'd be tempted to invent something that turns the aluminium shield into a ventilated cover. Maybe parallel battens, the ends of which are ventilated?  And then I'd look for a way of putting a vapour barrier onto the warm side of the insulation. Does the finished floor have to be stuck down (maybe a silly question for a movable trailer, but let's put our brainstorming hat on for the time being:-)? If not, you could put one of these combined vapour barrier and floor underlay thingies down. The layers would then be: Aluminium sheet, ventilated space, frame with Kingspan in between, OSB, underlay, finish floor.

On the walls: Do you need 100mm studwork? If you could reduce that to 75mm and then add a continuous layer of insulation, or -- to facilitate fixing the T&G cladding -- you could put 25mm cross battens on and put insulation in between? If you do that on the inside, then the vapour barrier could probably go between this 25mm layer and the outside 75mm layer. Makes it easier to fix, with fewer penetrations.

What's the roof going to be made of?

Klaus

[mr_magicfingers]

I wonder if drilling some small holes every so often in the alu would be enough ventilation for vapour flow and ventilation.

the studwork is all done, the walls are framed, they just need erecting and cladding with osb. I can't really afford the extra thickness involved with additional battens and insulation. I did think of putting a layer of the aluminised bubble wrap all over the inside of the walls instead of a poly vapour barrier which would act as both prevention of cold bridging, additional insulation and a vapour barrier if I taped the sections of it together although it would add a couple of hundred quid to the cost.

J.

[billi]

sadly I'm running a Mac

    first time in live i heard that phrase

I know not much help here , but i would use 2 layers on the floor area , if you think your alu floor can seal  then one is fine , but upto now ( i only have a short builders history) i have not managed to get our building on stilts airtight

I would consider one layer (wherever that goes between) of the membrane and overlap on the outside wall behind the cladding  , so under the rafters / insulation


In other words if you donot get the floor airtight in a building above the ground you are fu....whatever

Its possible to seal from inside but its hard to belive how much force wind has under a building above ground

Billi

[EccentricAnomaly]

I wonder if the aluminium underneath is overkill, really.

Old style glider trailers traditionally had a wooden top or an aluminium-skinned steel frame top, either of which would be with a wooden (ply) floor.   Generally speaking it wasn't the floor which went except usually at the back end of the trailer where it sat on the ground.

(Current ones tend to be fibreglass on top with an aluminium floor.  They're also designed to sit level with the back off the ground.)

I had a trailer which was built in, I think, 1978.  When it was nearly 20 years old I replaced the back two bays (8 feet) mostly because the steel frame had gone.  We reused most of the aluminium and only added some more because of a redesign to make it more convenient to use.  I replaced the back 8x4 sheet of exterior ply in the floor because the old one was pretty rough and it would have been silly to put it back but the rest of the floor was in perfectly good condition and there was no thought of touching it.

Glider trailers don't get moved all that often - maybe a dozen or so times a year but when they do they tend to take a bit of beating - bouncing off the ground going through dips in field gates and so on or going fairly long distances (took mine from the south of England to the Alps once and to northern France and Scotland a few times).  I think that'll probably top the treatment the average tiny home is likely to receive.

I'd suggest that ply on the bottom would be fine if you keep it off the ground and don't let anything grow up underneath it.  Just plan to replace it every 30 years or so.

[Brandon]

I can not help with specifics, but the ROT is that the inner layer needs to be 5 times LESS permeable than the outer barrier.  Your Kingspan walls if done well (and taped across the front of the studs, in order to use the foil face of the insulation -and the tape to keep it continuous- should act as a vapour barrier).

Could you put a layer of 25mm insulation over the floor construction, and then lay the osb, if taped it would give you a vapour layer and air tightness, as well as mitigating the thermal bridging of the floor joists...

just a thought

edit: correcting a blatant error...

[EccentricAnomaly]

I can not help with specifics, but the ROT is that the outer layer needs to be 5 times LESS permeable than the inner barrier.

Does "ROT" = "rule of thumb"?

Generally you want the outer layer (strictly, the layer on the cold side) to be MORE permeable, not less.

[Brandon]

yes on the ROT, and that is what I meant, still learning this damn language...  less impermeable?

or less permeable on the innner

I will edit it now

[mr_magicfingers]

thanks for all the really helpful replies. I think I'm going to go with putting the poly barrier in the inside, it seems to be the best practice. Now I just need to get the damn trailer welding finished. Pulled the wheel hubs off to grease the bearings today and found one hub didn't even have brake shoes inside. Bloody cowboys some people.

Cheers,

J.

[Justme]

With an internal poly barrier wont you have huge condensation issues unless you supper ventilate the air space as the breathable layer wont be able to "breath" your moisture away?