Rookie question: Tricky construction - How should I think about vapor barrier against the foundation?

[seerikand]

Hi,
Rookie question;

We have an extension in our house from the 1930s that was done sometime in the 1970s.

I recently tore down the old panel and tretex as well as previous plastic (which, by the way, was just overlapped and not sealed at the joints), and everything looked surprisingly fresh.

The base is a cast foundation that is difficult to access, asfaford, joists with insulation, chipboard, and stone slabs/marble on top.

Now I have improved the wall insulation, plastic sheeted and sealed seams, and put battens according to standard practice on walls and ceilings. However, the construction is done a bit differently so I can't access the sill that the vertical studs rest on. It is kind of under the chipboard. See picture.

Now to the question; How should I think when sealing the plastic to the floor? I am now worried that when I do it "right," I'll end up with moisture coming in from underneath and that I've made it too tight since I assume the moist outdoor air entering through the vents in the foundation has nowhere to go... 😟? As mentioned, I can't reach and affix/seal the plastic all the way from below/underneath.

I've considered filling the space with expanding foam as best as I can and then sealing from above. That is what you see in the picture. When I stick a measuring tape into the hole, I reach down to the joists, the insulation, and finally the asfaboard.

Difficult to explain but I gratefully accept answers, questions, and tips!

 

[Cheesen]

In a 30s house, one should preferably avoid plastic. It is not constructed to have any vapor barrier.

 

[Rabbithole]

Moisture travels from warm to cold, so you can attach the plastic towards the inner floor.

If there are gaps, seal them with a backer rod or foam sealant to minimize drafts/cold bridges.

Moisture in the foundation is primarily a problem of rising ground moisture; if it's a crawl space, you can lay plastic on the ground to reduce the amount of incoming moisture.

And finally, it doesn't matter when the house was built, as long as you understand the physics and execution.

 

[Nissens]

Moisture migrates from warm to cold, so you can attach the plastic towards the inner floor.

Agree

If there are gaps, seal them with a sealing strip or joint foam to minimize draft/cold bridge.

Avoid joint foam, it risks loosening/cracking over time as it's not flexible.

Moisture in the foundation is primarily an issue of incoming ground moisture; if it's a crawl space, you can lay plastic against the ground to reduce the amount of incoming moisture.

Agree

And lastly, it doesn't matter when the house was built, as long as you have an understanding of the physics and execution.

Agree

 

[Rabbithole]

Agreed

Avoid expansion foam, it risks detaching/cracking over time because it is not flexible.

Agreed

Agreed

Expansion foam is probably better if there are slightly larger cavities inside behind and such, but I don't think it matters much.

 

[Nissens]

Foam sealant is probably better if there are slightly larger cavities inside behind and such, but I don't think it matters much.

If it doesn't matter if it leaks and causes drafts, then no, it doesn't matter.