Insulation, Vapor Barriers & Moisture

[teknomad]

I was thinking of installing 'house wrap' vapor barrier around the entire interior, before putting insulation in. The goal is to help make the interior air-tight, prevent drafts, etc.

I was thinking the layers should be:
body metal > Plastic Wrap > insulation > wood > floors/wallls?


Im concerned with moisture buildup between the metal and plastic. Or really, anywhere in the bus. What is the best order to put these materials in?

This will be in Canada, so there is moisture and lots of temperature variations.

[Bert06840]

I think your analysis is correct. Depending on air temperature and relative humidity, the dew point will move deeper and shallower “into” the bus. I would not put a vapor barrier outside the insulation.

I am going to seal my insulation on the inside, but also will accept a certain amount of loss/draft/leakage. It will cost you a bit of energy, but unless you have an active air replacement system with positive or negative pressure, you are going to need it anyway.

So. I would say, drop the tyvek, accept a few extra percent of energy, breathe healthily and don’t rot your steel from inside out.

[Bert06840]

I’ve built a house in Scandinavia, where this is a real issue. (Canada? Meh. Too easy!). So yes. I take the heat loss not without consideration.

And my house HAS active air replenishment, once every 45 minutes. Luckily with energy recovery! Possible, but not sensible, for a bus.

[SIMJEDI]

Using something sprayable like Lizard Skin would be a better option as it can seal far better. Pricey though and you will need good air exchange rates.

[Rucker]

Originally Posted by teknomad

I was thinking of installing 'house wrap' vapor barrier around the entire interior, before putting insulation in. The goal is to help make the interior air-tight, prevent drafts, etc.

I was thinking the layers should be:
body metal > Plastic Wrap > insulation > wood > floors/wallls?


Im concerned with moisture buildup between the metal and plastic. Or really, anywhere in the bus. What is the best order to put these materials in?

This will be in Canada, so there is moisture and lots of temperature variations.

To be clear, house wrap like Tyvek is infiltration barrier, not vapor barrier. It slows leaks, which is a good thing in itself. It's installed on the exterior of houses to reduce air infiltration through cracks.

Vapor barrier = plastic sheeting. In residential construction this goes behind the sheet rock and prevents the warm humid air on the inside from coming into contact with the exterior of the building envelope where it could reach dew point and condense.

The vapor barrier needs to prevent the inside air from reaching dew point, so it needs to be on the inside of whatever insulation or thermal bridging you have. On a bus, there will always be some wood or metal almost in direct contact with the air inside of the bus, sadly.

So it is somewhat a balancing act. The idea is to minimize the number of places on the inside of the bus that go below dew point, by a) minimizing those cold points of conduction on the interior and b) keeping the inside temp high enough and air circulation great enough that stagnant air has a harder time cooling. Air circulation is important because it helps evaporate any condensate.

Tools in the tool box: vapor barriers; insulation; heaters; fans; other thermal bridging products to reduce heat/cold transfer. I'd add dehumidifier to the list if it wasn't so energy-intensive.