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What to do about the vapor barrier?

I am currently using a formerly unheated basement room as home office. The room is not properly insulated and rather hard to keep warm. I am thinking about installing a nicer floor and insulating it to make the climate more pleasant. The floor is currently bare concrete and I assume there is no vapor barrier under the slab. About 50% of the room is below the ground. We live in the Arctic, so the temperature ranges from about -35C to 25C, and it is mostly considerably colder on the outside than inside. I don't have much experience and am on a very tight budget.

The initial idea was to lay out PE foil as vapor barrier, build a wooden grid (60cm wide) to fill with glass or rock wool and to install wooden planks on top to walk on.

I am very unsure about the materials and vapor barrier thoug.

According to previous replies, I would end up with trapped water under my foil without channels to escape if the foil was placed directly on the concrete. As I can not build a pit, that makes me consider to ditch the vapor barrier altogether, to allow some ventilation, but that seems to eventually soak my materials.

I am not primarily concerned about the materials as in bent wooden floor planks after a few years. I am mainly concerned about mold, I don’t want to make the room a few degrees warmer but trade it for smell or an unhealthy environment.

I have attached an artistic masterpiece (sorry!) of what I mean and hope that someone might have a pointer what you would do.

I am still lost and would appreciate any help, I am also open to different materials!

current options

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  • Have you considered using a vapor-impermeable insulation? (Such as rigid foam with the seams taped) – ThreePhaseEel Feb 25 at 12:39
  • @ThreePhaseEel Would you recommend XPS Styrofoam in the wooden grid without any additional products or layers? I favored the idea of rock wool because the potential for ventilation in the more loose material seemed reassuring for me concerning my mold fears. – VionaB Feb 25 at 12:50
  • If you're going to lay wood directly on the concrete like that, using pressure treated lumber (or whatever they call wood that has been chemically treated to prevent rotting in your country) will significantly increase its lifespan. It still won't last forever, but you'll very likely get more than 20 years out of it - people use PT lumber for building decks outside that can last 20 years. There are concerns about using PT indoors, but I believe that was with older chemicals, not the newer ones used now. – FreeMan Feb 26 at 16:55
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The best place is for the vapor barrier to be is UNDER the concrete slab. This prevents moisture from the ground from seeping through the slab into the living space.

Assuming that's not the case, then your next best choice is to apply a moisture channeling product on top of the slab. You don't want to use poly film here as that will cause moisture to condense between the slab and the poly with nowhere to go.

There are any number of commercial moisture channeling products but these provide both a vapor barrier and a system of channels for the trapped moisture to drain, most often to your sump pit.

Even though your floor is "dry" now, that doesn't mean there is no moisture coming through it. Concrete is porous and allows water to penetrate. The reason it seems dry is that the moisture is evaporating into the air. Try taping a sheet of poly to a small area of the floor for a few days and you should see moisture trapped under it.

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  • Thanks for your answer! As the house is from 49 I am assuming there is no vapor barrier under the concrete, there is no sump pit either. Maybe my dreams of a relatively easy fix are quite naive, but I hoped to be able to find some material combination that I can just put on top of my existing floor. Do you see any idea with which I can avoid to break the floor? – VionaB Feb 25 at 13:05
  • You don't need a vapor barrier @VionaB, use permeable insulation and allow the concrete to breathe. – GdD Feb 25 at 15:19
  • That's the purpose of the channeling product. If you let it "breath" it will breath into whatever you place above it. Often that causes damage to things like wood, flooring adhesive, carpet, etc. – jwh20 Feb 25 at 15:23
  • If there's no sump the OP will need to have the basement graded and a sump installed. That's a lot of work and cost. @VionaB, it's a judgement call, if you don't do a sump you may get damp problems which reduce the life of the materials, or they may last long enough for your purposes. – GdD Feb 25 at 15:50
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I had a similar problem in my basement (but our climate is mild not arctic, if it even matters)

First inspect the floor to see if you have a water problem (through cracks or excessive water under the concrete) or whether the dampness is just from humidity due to porosity.

If you are lucky, you can recognize water ingress by the mineral deposits which show as white lines. If you have a water problem, you need to ventilate (or drain) between your subfloor and the concrete.

If it's just humidity you can block it off with XPS without a vapour barrier. Use shiplap 2in/2.5in thick boards, and tape the seems. XPS give you better insulation than EPS, and it is a 90% VP. This should last 25+ years. Lay a ply-wood subfloor over it, and then your decorative flooring.

You mention being on a budget. I applied a thermal underlayment with VP, which is much cheaper than XPS, and it got rid of humid smells in our basement.

Certainly, underlayment does not provide the same thermal insulation as XPS, but for the basement to feel warmer you really need more than just floor insulation:

  • adequate heat supply
  • proper heat retention (doors/windows, stairway drafts, floor/wall insulation)
  • floor level cold-air return to keep warm air at seating level, and
  • flooring with low heat-capacity (MDF & area rug, not Vinyl or tile etc..) to feel warm to the feet.

As for condensation under the VP, as I work on our floors over the years, I have had no problems so far.

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Although it is ideal to have one, and @jwh20 is right on with how you would do that, you don't have to have a barrier. You just need to be aware that even permeable materials will trap some moisture, and that moisture will degrade those materials over time. How quickly that happens depends on the moisture level and the materials used, so you would need to go into that with your eyes open. You could build a floor and it could rot out in 5 years, or it could last for 20.

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  • Thanks for the input! Because you write that the longevity depends on the materials used - Do you think XPS/EPS would be a better choice than rock/glass wool if theres no foil to protect from the moist? – VionaB Feb 26 at 18:23
  • They are all synthetic, I wouldn't expect those to be problematic, it's the wood and carpet that's the problem. – GdD Feb 26 at 18:27

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