My basement has insulation floor to ceiling

Robes888

I'm wondering if I can begin framing by placing the perimeter walls directly against the plastic (vapor barrier)? Can I push the framing into it or leave a gap?

jklingel

A little more info on exactly what you have and want to do is needed. The vapor barrier in the basement is a bad idea. Read on buildingscience.com; it leaves the wall with no direction to dry.

Robes888

Why would the builder then install insulation in this manner?

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Robes888

Also, I've have watched and read various DIY's about framing... Some say to leave 3/4" gap on top plate of wall incase concrete "swells" while others just attach top plate directly to floor joists above (this is typically what I've done in past with OLDER homes not a new home)

jklingel

I suppose "habit". I am not implying incompetence, as there may be a reason for it. However, I'd suggest you read about this and reason; SOME water vapor is going to get into the wall, if not liquid water in trace amounts from the outside, and where is it going to go w/ a vb in the way? I would never have a vb in a basement. It apparently works in some places, and can be a disaster in others. Visqueen is an old practice, that is even being shied away from above grade. People report all the time of mold in basement walls w/ vb's. Do some reading on buildingscience.com and greenbuildingadvisor.com and make up your own mind. I think you'll see two ideas: fiberglass, though ubiquitous, is not real good insulation (another habit; it is readily available and cheap), and vb's in basements CAN be a bad idea. I just like to caution people about both of them. After you read, talk w/ the contractor and see what he/she thinks. On the gaps between insulation and walls, read about convective loops generated thereby, and how you lose energy. Spaces are not a good idea for that reason.

mae-ling

ON exterior walls I go tight whereas on interior walls leave a 1/2" to 3/4" gap. I have had to Fix houses where there was no Gap left and the floor heaved.

The gap can be at the top or bottom depending on the ceiling material. For a drop ceiling just make the wall a little short and nail it in. For a drywall ceiling (actually always the best IMHO) use a double bottom plate with the gap between the plates. Screw the drywall to the top one of the bottom plates, and nail baseboard to the lower plate. Creates a slip joint.

As for insulation. Do the suggested reading.

MecGen

That is such a good idea, is this common practice?

mae-ling

Not common in my experience. Most put them in tight or leave a gap at the top. The double bottom takes a little more material and time.

Gary in WA

Isn't the gap for expansive soils as in Colorado or Texas? When the slab actually moves from the soil moving, not slab "swelling".

Gary

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mae-ling

I am in Canada, here we use it. I have seen floor heaves in all 4 western provinces.

Gary in WA

Interesting, thank you.
I can see where that will be a problem for those following BSC's guidelines to air-seal the drywall to keep moisture from the concrete wall. Then again, the air barrier with a separate vapor barrier suggested for those locations are in conflict with them also. More to keep in mind when answering questions....location, location, etc.

Gary

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