A double stud wall / insulation question...

[Fastback]

So my detached garage addition (see avatar)is built with 2x4 stud walls 24" on center, sitting to the outside of a cement block leaving a hefty ledge 4" off the floor 4" deep. I know its not the best way to build but I have a plan and I want opinions.

I am thinking about adding another set of 2x4"s to the inside of the room on top of that ledge and placing the studs in a stagger so they are not in front of the current studs.

I will use un-faced 24" wide on the structural wall up against the outside wood sheathing (the exterior will be wrapped in tyvek) and then another 24" wide insulation between the new studs also.
This way I will have 8" of overall insulation and my studs will have a thermal break. I can nail to the top of the 8" treated sill plate and if I use 10' studs I can nail to the face of the 4" top plate.

I realize I could weave the 4" insulation through the studs but that would still leave air gaps and only 4" overall...it may work if the budget gets cut back, but I think 8" is mo bettah.

I see this as the best way to prevent telegraphing of cold through the studs from the outside, my question is should I use faced for the insulation that is closest to the living space or would using plastic be better? Do I really need anything at all with the tyvek on the outside?

Do I need to close in the top of the new stud cavity or is it OK to allow it to be open to the attic area?

 

[kb2tha]

I would use either a 4 or 6 mil plastic vapor barrier on the inside with unfaced insulation. The faced insulation leaves too many opportunities for air to escape IMO. The type of construction you propose I have seen before. Using a 2 x 6 exterior wall plate. They then used 2 x 4 studs 24"oc staggerad as you suggest. This worked well in that application.

I'm sure you will get other responses that will help you.
Ken

 

[toymont]

8 inches is much better (ask any lady) lol

use unfaced against the outside wall and faced towards the workspace

Code calls for the tyvec so use it

use at least a single top plate for stability, since this is not a bearing stud wall I dont think it has to be doubled

You may like a little bit of the cement ledge so that when you roll a tool or something against the wall it has a bumper before the sheetrock

 

[jklingel]

Double-stud walls are done all the time, and there are countless ways to do it. I commend you on choosing the method. My house is double stud, and the addition will be, too. S-can the fiberglass; it is the worst insulation on the planet. Blow the walls w/ cellulose, or get cellulose, cotton, or mineral wool batts. You likely do NOT want a vapor barrier in Indy, but rather the ADA (airtight drywall approach). Google it, or see greenbuildingadvisor.com or buildingscience.com. j

 

[MoonRise]

Tyvek or other housewraps cut down on drafts from the outside (if done 'right') and act as a barrier to liquid water getting from the siding into the sheathing. They are NOT vapor barriers, in fact they are vapor permeable (almost always).

So the vapor barrier is something else -besides- the housewrap.

Staggered double-wall studs with insulation is a good idea, both for the thermal as well as the acoustic (so you can work in the garage, right? ) benefits.

Also remember that the drywall (or other wall) between the house and the garage typically has to be a 'firewall'. Local codes usually specify how much 'firewall' there has to be. Doubled 1/2 inch drywall or 5/8 inch drywall -usually- suffice, again check local codes.

 

[Fastback]

Thanks for the info.

Blown in sounds good, if I left the top of the outer wall opening open I could blow in from the top and cap it after the fact, but I wonder about getting proper fill? Or I could blow it in after the vapor plastic but before the plywood...?

How does the cost compare?

This is a detached garage that is getting it's own addition, so no drywall..., plywood walls and maybe even some galvanized metal from 4' down to the floor to radiate heat back into the building from the floor heat....

 

[MoonRise]

Blown-in in walls can settle. Then you have a 'gap' at the top of the wall.

Blown-in in walls may not fill properly under/around partial obstructions, like outlets or blocking.

Blown-in in walls will NOT get under complete stud-to-stud blocking, like for duct fire-blocking or backing for mounting things like towel bars or shelves or whatever.

Spray-in insulation will not settle. But the application has to be done 'right' or the expanding foam can bend or warp things and the applicator (person) still has to make sure to get complete coverage.

Double-wall studs in a staggered pattern can reduce cold /heat telegraphing through the studs, or just run something like 2x6 wall studs and then exterior foam board insulation sheets under the siding.

Your call as to how much time/money/effort you want to put into insulating the detached garage.

 

[tdkkart]

I'd go with 2 layers of 3-1/2" unfaced fiberglass in each of the walls, and then 4 or 6mil plastic.
Maybe I watch too much Mike Holmes, but lately I have a better appreciation of how important vapor barrier is, and I just don't feel like the faced insulation does a good enough job.

 

[Fastback]

I'd go with 2 layers of 3-1/2" unfaced fiberglass in each of the walls, and then 4 or 6mil plastic.
Maybe I watch too much Mike Holmes, but lately I have a better appreciation of how important vapor barrier is, and I just don't feel like the faced insulation does a good enough job.

By the time I get all of the first wall insulated around the windows and doors I may end up just finishing it with the unfaced and then sheet plastic, I will post a pic when I get that far.

 

[jklingel]

repeat: "You likely do NOT want a vapor barrier in Indy". Check on buildingscience.com. People are getting away from them in all but the coldest places (zone 7 and 8). Air seal. j

 

[MrMark]

Yeah, you really don't want a vapor barrier where you live. Maybe Alaska, but not there. They never made a lot of sense when you think about what happens with the condensation. I'm not sure why people seem to think they are a good idea.

 

[jklingel]

....Maybe Alaska...

What? Does it get cold up there?

 

[MrMark]

I don't know but how many hours are you guys behind California?

 

[Kevin54]

There are quite a few website addressing this. Here is one http://www.buildgreen.ca/2008/09/double-stud-wall-construction/ Do a search on your favorite search engine for "double studded walls"

 

[jklingel]

I don't know but how many hours are you guys behind California?

Just one. Four from the east coast.