Plastic or tar paper - opinions are divided...

[tallmilan]

Borrowing the thread a little because I'm facing the same dilemma as I'm about to purchase the materials before I can continue with my construction. I'm building an apartment in an old barn/stable with parts from the middle of the 1800s in the framework, but we're only talking about logs as the frame, so to speak, with regular planks externally. I'm now building inside this, and I'm letting the old structure remain and it's partially built-in. The original idea is to use ecofiber which can transport some moisture out and a diffusion-open construction with Halotex RS10 as the underlayment roof without an air gap (under a metal roof) and Halotex D50 as a vapor barrier on the inside. The difficult part is sealing where the logs at certain places penetrate the partition walls from the outer wall. They can't exactly be temporarily cut off to fit a vapor barrier in between...

- Is it enough to seal around these as best as possible and bring it all inward a bit?
- Should one use this diffusion-open construction in this case or not?
- Is ecofiber the best alternative with its hygroscopic properties?
- Which vapor barriers are the most cost-effective including all accessories like seam tape etc.?
- Same question as above, but with the underlayment roof?
- Using exterior gypsum on the furthest side of the walls as wind protection. How is this in combination with a vapor barrier? I want to achieve a resistance difference of about 5-10 times between the vapor barrier and the wind barrier/underlay...

These were such good questions, so I think you should create your own thread with them. It's not entirely uncommon for someone to want to insulate their old outhouse, etc., and they usually have quite sturdy frameworks.

 

[monsterbyggare]

A silly question perhaps, but some use both windproofing foil on the exterior wall and then also exterior gypsum board or asphalt board. Isn't the exterior gypsum board sufficient, as in my opinion it is adequately sealed compared to the old porous asphalt board?

Which is correct?

 

[Mikael_L]

I think one! windbreak should be enough.

Whether it's asphalt felt, asfaboard, wind fabric, or exterior gypsum board is a matter of personal opinions and technical arguments. (Gypsum and asfaboard, for example, give the wall more stability).
But several different layers seem pointless.

Don't confuse this with any vapor barrier on the inside of the wall, it has a different purpose.

 

[monsterbyggare]

But isn't it overkill to have both wind paper or wind protection foil and then add exterior gypsum board? Providing stability to the entire house structure is one of the reasons I'm choosing it. But having both is still a bit much, isn't it? The exterior gypsum board is sufficiently windproof, isn't it?

 

[Styx]

But having both is a bit much, isn't it? The outdoor gypsum board is well enough windproof?

Yes.

 

[Mikael_L]

Yes ...

And that's probably what I was trying to say ...

 

[monsterbyggare]

"Bang in the construction" then Mikael You need to phrase it better...Knew the answer already but still wanted to ask

 

[F.Wester]

Those are two entirely different functions within building physics that are important to understand when discussing one and the other.

Plastic/foil/vapor-tight/UV-stabilized: used to prevent warm and humid indoor air from moving out into exterior walls/attic joists with hygroscopic (water-absorbing) material. Especially insulation. If the insulation becomes moist, the air content in it decreases, reducing its insulating effect. The zero-point in the wall/floor (where warm indoor air is cooled down to an average of zero degrees as cold outdoor air cools from the outside) then shifts towards the inside wall/ceiling, allowing for further encroachment of the zero-point as new and moist air condenses closer to the room side. It is therefore important to 'brake' the moisture to prevent the insulating effect from deteriorating to the point where it effectively becomes null, leading to mold and rot.

A newer material instead of plastic foil exists. It's called a vapor barrier. It is not diffusion-tight. (I have no experience with this material.)

Combined with a well-functioning and properly sized indoor ventilation (mechanical supply and exhaust with possible heat exchanger), a vapor barrier may manage to keep the rest of the moisture at bay? This would theoretically mean that the moisture follows the path of least resistance and moves out with the air rather than trying to fight its way through the wall and end up in the insulation?

Windproof sheeting: used on the exterior side of an exterior wall and atop the insulation against cold air. It is not diffusion-tight (and shouldn’t be). Its task is to stop air movement within the insulation layer. It’s not the insulating material itself that insulates, but the still air within it, as air is a poor heat conductor. But if the enclosed air is disturbed by free air from outside, the warm air in the insulating material is stirred and replaced with new cold air. The wall then becomes colder and more energy is needed to heat up the new cold air in the insulation, which is what we want to avoid. Otherwise, you might as well skip the insulation.
__________________
Byggaren

Hello
Trying to revive an old thread.
We are renovating a turn-of-the-century house that is plastered on the outside, we are insulating the walls with wood fiber. I've installed windproof sheeting on the inside of the exterior walls, mainly because I think it will prevent drafts through the plank wall and cool down the insulation. Now I'm wondering if I am ruining something/creating new problems in the plank wall with moisture, or if traditional windproof sheeting lets moisture through? I suspect that behind the plaster there is an old windproof sheeting that the reed mat is nailed to the plank wall.
Am I doing something right or completely wrong / Fredrik

 

[Mikael_L]

So you're insulating on the inside of the exterior walls?

In that case, you're moving the dew point inward, which can be problematic. But it's not something that can be said with certainty.

Working constructions work as long as you don't change the construction, the use of the building, or the climate.
If an existing construction is functional but borderline, it can take very little to change that. For example, if a new family moves in that showers more and for longer, or dries clothes indoors instead of outdoors.
Many have added some insulation in their cold attic, only to find a bunch of mold years later when it became colder.

The only thing known for sure is that all indoor moisture must be expelled. So the way to avoid problems is to prevent moisture from forming, ensure it exits as desired (mainly with ventilation), or engage in various ways to patch up the problem but not remove the cause, i.e., fiddle with plastic/paper/vapor barriers/ecofiber and whatever you believe in.