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dvg 2017-09-22 01:47 PM

Basement vapor barrier - poly sheet enough?
I am looking at the costs to finish my basement.

I am sort of shocked at the cost of polystyrene foam. My understanding is that the main purpose of the polystyrene is to serve as a vapor barrier- keeping moisture from the wet concrete out of the house. Im wondering if this could just be done with a poly sheet instead, as is done above grade in a house.

If so, I don't really see the purpose of the polystyrene. It is expensive and does not look easy to work with. If I wanted better insulation I could just use 2x6 studs and get almost the same boon to the R rating at a lower cost.

If anyones curious, I have arrived at estimates of 3.71/sq ft for the floor (DMX,OSB, rolled rubber for gym), 3.85/sq ft for the ceiling (Drop ceiling), 0.65/sq ft for uninsulated walls, and, either 1.45 or 2.95 for insulated walls, depending on the answer to this question.

dvg 2017-09-22 02:25 PM

1 Attachment(s)
forgot to attach the picture, but I actually think I understand now - while the polyethylene vapor barrier would be sufficient for keeping moisture in the concrete from entering the house, the poly would be very cold in the winter, and condensate would form on it from the warm humid air inside. So I guess the polystyrenes R9 brings up the temperature enough so that that is not an issue.

gerapau 2017-09-22 08:30 PM

The vapour barrier always goes on the warm side of the insulation. So it would not go directly on the concrete walls.

dvg 2017-09-22 08:40 PM

Most experts recommend vapor barrier on the outside below grade, a lot of moisture comes in from the concrete, it's a different situation. There are some good articles on basement vapor barriers, our climate it's recommended to do either spray foam or r7.5+ polystyrene on the cold side

xdarrylx 2017-09-22 10:04 PM

I used rigid panels and tucked taped the seams and spray foamed the top and bottom plates so I did not need a vapour barrier as I would have had 2 warm zones. If you don't go the rigid panel route, a vapour barrier should be used as stated above.

dvg 2017-09-22 10:06 PM

Yeah I understand now why polystyrene is preferred, I'll be doing the same

xdarrylx 2017-09-23 12:24 AM

And the rigid panels aren't very difficult to work with at all, honestly.

They cut easily with a utility knife and when used with PL Foam (300), it bonds well to concrete walls - just be sure to get rid of the raised edges from the poured concrete forms.

Also, I got all my stuff at Rona when they held a weekend 15% off sale - which is almost every weekend, so the savings was quite a bit if you're smart about it.

Good luck! ;)

BartBandy 2017-09-25 10:08 AM

This is where I like closed cell spray foam. I like doing everything myself, but I'm glad I got spray foamed by a contractor. Closed cell creates a vapour barrier at 2" thickness and it's a lot less work. I used rigid board in a cold cellar and can foamed the seams, and it took forever. Of course, the formwork was obviously old and just taking a cold chisel to the joints was a pain.

dvg 2017-09-25 11:28 AM

Do you really have to chisel down all the form lines when using the polystyrene boards? It does seem like it may be worthwhile to do sprayfoam instead. Sucks to pay almost 1k for the service but I suppose it would be a lot less work for me.

xdarrylx 2017-09-25 05:33 PM

Sprayfoam will no doubt be superior if done properly but it'll cost significantly more than rigid foam panels. For my 600sqft basement the quotes were around $4k-$5k for 3-4" thickness (~R20-R24 value). You certainly save a lot of time, but at a sizable cost. Comes down to timeline, budget and personal preference. :)

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