[MrSquid]

Is there a general rule of thumb as to how far your 2x4 walls should be from concrete walls?


The builder framed the basement in my last house before they even listed it, and that distance there was all over the board - from barely an inch to almost a foot depending on what the concrete wall was doing at any certain point. Don't get me wrong - the concrete walls were straight, but there were areas where they jutted in or out to account for architectural features, and when they framed it they did it so there were nice, straight walls. (not to mention it was also easier for their framers)


I know you should stay out a bit to compensate for walls that aren't perfectly straight, but is there a problem with being several inches out from a concrete wall for an extended distance?

 

[Staffy]

I framed mine according to keeping the walls straight. It varies from 1 inch to 7 inches as the concrete walls are FUBAR.

 

[Brett Wilson]

For each section of concrete wall I marked two inches away from the concrete wall at either end, and then used a chalk line to create a straight line. After a couple of wall sections, I also found myself marking a chalk line for the outside of the sole plate - a lot easier to see the line, and the fact that I was following it


Then I prebuilt the wall on the ground, lifted into place. Made sure was level with chalk line, nailed into the concrete floor then leveled so that the wall was horizontal and the studs where also straight. Shimmed where needed and then nailed the framing into the ceiling joists.


So a more condensed answer to your question, I chose 2". Our walls were not too bad, and 2" seemed to be a real happy medium. Being away from the concrete wall is not a bad thing, you are not attaching anything to them, so don't worry from that aspect. I liked being as close as I was though, as once the insulation was in place there was a gap - but one that was not too big.


- Brett

 

[RhinoDude]

Depending on your local codes, there may be. You may have to fill the gap between the concrete and the framing with extruded polystyrene or other non-conbustible material in order to get rid of the "chimney" behind the wall. Or you may have to just put in a fire barrier on the bottom of the ceiling (firecheck sheetrock works great here).


In some locales you only have to do this if you have electricity running in the wall. In others, you have to do it no matter what. In some you can have no gap if there is electricity in the wall. In others you can have a 1" gap as long as there is no electricity. The key is you will have to check for your area. But independent of codes, for maximum safety the ideal is the framing is just tight enough so 1 layer of extruded polystyrene fits between the two. If your wall varies you can use more or less trimmed insulation to take up the space.


What they are trying to prevent is a fire in the basement racing up behind the wall and directly attacking the space above through unprotected ceiling joists. As such, some locales will allow the hollow space, but you need a fire barrier between the space and the exposed joists. 5/8" firecheck drywall provides the necessary protection in any codes I've seen.


I use this system when building here in Tompkins county.:



- seal the concrete wall (I finally broke down and bought a heavy-duty sprayer so I spray 2 coats of drylok now; backbrushing it in wth a masonry brush as I go).


- insulate joists all the way to the rim joists (ie past the where the concrete starts).


- install one layer of 2" extruded polystyrene against the concrete wall with the insulation tight up against the joists and the floor. Seal all joints wth appropriate tape.


- snap floor and ceiling lines for where the plates will be going. Doing the top line first and drop a plumb line for the bottom line. I build in place using this approach, not build and flip. It far is easier to get a straight wall out of not so straight lumber this way, even when working alone.


- where the wall plates will run between 1"+ to 2.5" away from the insulation I add a second layer of either 1" or 2" to close the gap to

- where the wall plates will run 2.5" or more out, I cut 5/8" firecheck sheetrock strips to width (the snapped lines show you how much) and install directly to joists and tight to the polystyrene insulation. Some inspectors will make you fire caulk the seams; some won't.


- frame the wall, and where necessary trim the sheetrock back a bit so there is a nice tight fit with the back side of the ceiling plate. If you were a bit short in some places fill in the void with fire caulk. Like I said, some inspectors may make you fire caulk it anyway.


- if you want more insulation or do not want voids in your walls (which is bad for HT's if any part of your HT is on the exterior walls), then fill with UNFACED insulation. If you use faced it creates a vapor barrier and you now have a potential if not guaranteed mold problem.


- drywall the walls with no more than 1/2" sheetrock. If you want more drywall than this because you have a HT room which is using an exterior wall(s), then put the extra drywall on the outside wall of the theater facing the other living space. That way all interior walls are balanced wth the same amount of 1/2" sheet rock (same frequency response), you still get your isolation, and your exterior walls can still breathe. I've seen lots of people make the mistake of putting too much drywall on the exterior wall. Let's hope they never get much moisture in the wall...if they do their won't be enough evaporation before mold starts to grow.


- If you paint the drywall do not use an oil or alkyd point. Stick with acrylic or latex. In short, anything that goes on the wall must be permeable and allow moisture vapor to pass.


That's it. The space behind the wall is now no more prone to fire burn-through to the floor above than the living space on the other side of the wall. Any moisture that gets into your wall can breath itself out, and warm air can not get to the foundation wall and cause excessive condensation.


- Rhino

 

[RaymondR]

Rhino,


I was considering using a layer of plastic between my block wall and polystyrene instead of drylok. What's your opinion?

 

[RhinoDude]

Bad idea. Very bad. I know at first the two seem equivallent but they really are not.


The reason you are using polystyrene is becuase it is tolerant to water, does not promote mold growth and it breathes. The drylok is there just to help try to keep the moisture which gets through to a lower degree, but some will still get through.


What does get through you do not wanted trapped behind a vapor barrier through which it can not breathe. That is where the mold comes from. Instead, you want it to naturally permeate through the styrene, then through the fiberglass insulation (if you have any), and finally through the sheetrock. Installing a plastic vapor barrier prevents this.


Above grade it's fine. Below grade it's bad.


I can certainly understand wanting to bypass drylok...it is a PITA to do right when doing it by hand. If you don't have moisture problems as is, then just skip it. It really is not necessary when using the above polystyrene system unless you have an already somewhat humid basement or if you fail the "foil" test on your walls.


If you don't know, the foil test is duct taping an 8" or so square of aluminum foil tightly on all sides against your foundation walls. Try it in several locations, especially where you suspect heavy ground water (near downspouts is a good place to test). Make sure the duct tape completely seals all edges. Leave it for a day and then peal off. If you have slightly visible condensate, you should seal your walls. If still bone-dry, let'em fly as is! If you have lots of water (you could pour several drops off), you have a problem that no traditional basement system will address and need to work on getting the soil around your basement wall drier by getting more water away from your foundation or re-sealing the outside foundation walls (yes, that's costly).


Keep in mind that in new construction it is very common for a good grade coming from your foundation at time of initial landscaping is totally inadequate 3 years down the road. This is very, very common. I have seen far too many people that finished off their basement within a year of finishing their house only to develop water and mold problems later because of the settling of the backfilled and untamped fill around their foundation walls. Sure, their basement was dry for the first year or two, but that was becuase most surface water was channeling away. Once that grade disappeared, the trouble started.


- Rhino

 

[Dwearly]

Rhino,

What material did you use to secure the polystyrene to the walls and what is an appropriate tape for sealing the polystyrene sheets? I am assuming that it would not be a concern to have the unfaced insulation touching the polystyrene since the polystyrene makes a warm surface and there would be not concern of condensing.

Doug

 

[RhinoDude]

Doug, you are correct. You can place UNFACED batt insulation directly against the polystyrene.


Mastic Ahesive is used to attach the polystyrene to the wall. You do not want to do the entire panel. Just place a 6" tall horizontal stripe at the very top, very bottom and middle and push well when placing it (but not so much as to crush the insulation). The panel is rigid enough so that those 3 stripes will hold it nice and tight. Don't glue the whole thing...remember the concrete needs to breathe and the mastic will prevent this.


For sealing the seams you can either use mesh tape and mastic, or you can buy an insulated and foil-backed tape made specifically for this purpose. Either varieity will work fine. The foil tape is faster but costs more. Tape all vertical seams from top to bottom.


Don't forget to properly seal your PT floor plate using Sill Seal gasket or similar. This will prevent most of the wicking of moisture into the PT piece and help eliminate any from going into the non-PT studs and drywall. Some people skip using PT plates and just use the seal, which is fine too. But either way you should use a seal otherwise the plate gets wet which gets your studs wet which gets your drywall wet. Normally this isn't a huge problem though if your walls can breathe.


Finally, when you attach the drywall , be sure to keep it at least 1/2" off the ground. I usually use 3/4" and instead of using screws to attach to bottom plate use construction adhesive. In fact, you should use a liberal amount of adhestive everywhere the drywall touches the wood framing since it acts as a sealant. Use a thick enough bead so that when the board is pulled against the stud all wood is covered by it.


- Rhino

 

[CaspianM]

What is the fire rating of polystyrene? Is it none combustible?