Insulation question

[DrPizza]

I'm on the last leg of my remodeling project, getting ready to finish some walls. 8x16 concrete blocks for the first 20 inches, with an 8x2 sill plate, and a 2x4 wall. 2x4's are flush with the outside edge; OSB and vinyl siding on the exterior of the wall. The 2x4's, 16" on center, are set back a few inches from the front edge of the sill plate. In the other room that I did, I made a shelf that went around part of the room; turned out nice enough. But, I don't want a shelf in that room, which will be a bedroom/bathroom.

Plan: frame another wall, flush to the inside edge of the sill plate/block wall, then drywall (greenboard) down to the floor.

Question: the existing insulation on the 2x4 wall is paper faced, paper to the interior of course. Can I just leave it alone, and apply the same to the new interior wall? Can I go with (R-whatever, for 2x4 construction) - which will leave about an inch gap between the back of the new insulation and the facing of the existing insulation? Or, should I pack it in with insulation for a 2x6 wall (R-whatever it is) which would push right up to the existing insulation? Would it be bad to use greenboard behind the hardibacker, provided both had a waterproofing membrane applied?



edit: oh, and a second question. When I purchased my shower base/shower doors, I planned in tiling the back wall. When I looked at the showroom model, I noted the distance in from the rear edge of the base to where the shower door assembly attached. The salesperson said that when I tiled, I'd have greenboard, then hardibacker (or whatever), the membrane, then the tile. Is that correct? I glanced on youtube and saw one of the (pros?) use something similar to furring strips to build the wall out the correct thickness.

 

[Greenman]

You got me on the insulation, we don't use an inside vapor barrier around here. But since it's a paper face, I can't see it making any difference. On the thickness of the bats, a perfect instillation is going to be more important than the extra couple inches of insulation. Every penetration at the top of the wall should be sealed, and you should run a bead of caulking on the top plates behind the drywall. Stopping all air movement between the conditioned space and the wall cavity is just as important as the insulation you use. Get another opinion though, you folks have to do all sorts of strange stuff to buildings up there in the frozen north.

You can do the shower either way, green board and hardibacker or furring and hardibacker. But the membrane has to be directly behind the hardibacker (you mentioned the membrane as last, I assume that was a mild brain fart).

 

[3chordcharlie]

The best would probably be to insulate it all as one wall. You don't want a vapor barrier 'mid wall'. I don't think the paper face is an effective enough barrier to cause a problem, but it's not ideal.

Better not to have any spaces between the two layers, whether you use 2x4 or 2x6 insulation for the second wall.

Look up 'offset double wall' for some pictures. Building this way intentionally, to avoid thermal bridging, is a thing.

 

[DrPizza]

Thanks! The results of some googling showed that offsetting the studs only increases the R-value by about 1. I'm not sure it would be worth the effort, considering the studs would have to line up at windows, etc. And, in googling, I learned a bit about blown in insulation. Though, since the room was mostly insulated already, I don't think it would make financial sense to tear out the old insulation (which wasn't installed correctly anyway, negating what little vapor barrier it supplied).

However, I came to a 'well, duh' conclusion - having a space between layers of insulation, with the exterior insulation having the flammable paper vapor barrier would leave a fire hazard within the wall. So, I'll make sure that layers of insulation are touching, eliminating the ability for a fire to spread throughout an entire wall very quickly. Though, I haven't a clue what could cause something like that, other than perhaps arcing from a close lightning strike.

 

[3chordcharlie]

Right on - I didn't really mean in the context of r-value, just that double wall construction is a thing, so there's already a 'right way' to do it. And presumably plenty of keyboard warriors already arguing about that 'right way'.

Your solution sounds like it is reasonable and will give a good result.

 

[Greenman]

Thanks! The results of some googling showed that offsetting the studs only increases the R-value by about 1. I'm not sure it would be worth the effort, considering the studs would have to line up at windows, etc. And, in googling, I learned a bit about blown in insulation. Though, since the room was mostly insulated already, I don't think it would make financial sense to tear out the old insulation (which wasn't installed correctly anyway, negating what little vapor barrier it supplied).

However, I came to a 'well, duh' conclusion - having a space between layers of insulation, with the exterior insulation having the flammable paper vapor barrier would leave a fire hazard within the wall. So, I'll make sure that layers of insulation are touching, eliminating the ability for a fire to spread throughout an entire wall very quickly. Though, I haven't a clue what could cause something like that, other than perhaps arcing from a close lightning strike.

I used the BIB system on a place just a couple months back, it's a pretty slick setup, and very tight insulation.