Energy code outlet boxes VS drywallers

[wwhitney]

The IRC has a chart in it that tells you the minimum ratio of exterior continuous R-value to wall cavity R-value based on climate zone, and it's precisely for this purpose.

Oops, that's not the IRC chart (I was thinking of R702.7(3) on when you can use a Class III vapor retarder), but that is the correct approach for actually determining the temperature of the interior face of the sheathing.

If you have R_o insulation outside the sheathing, and R_i insulation inside the sheathing, and T_o is the outside temperature, T_i the inside temperature, then the approximate sheathing temperature is (T_i*R_o + T_o*R_i)/(R_o + R_i).

Cheers, Wayne

 

[electrofelon]

Oops, that's not the IRC chart (I was thinking of R702.7(3) on when you can use a Class III vapor retarder), but that is the correct approach for actually determining the temperature of the interior face of the sheathing.

If you have R_o insulation outside the sheathing, and R_i insulation inside the sheathing, and T_o is the outside temperature, T_i the inside temperature, then the approximate sheathing temperature is (T_i*R_o + T_o*R_i)/(R_o + R_i).

Cheers, Wayne

I always liked the idea of a layer of foam board on the exterior, for the purpose we are talking about but also to avoid the thermal bringing thru the framing members.

 

[wwhitney]

I always liked the idea of a layer of foam board on the exterior

Doesn't have to be foam board, could be rigid mineral wool or fiberglass; or could be a purpose built enclosure (see Larsen trusses) and blown in cellulose or fiberglass.

avoid the thermal bringing thru the framing members.

Well, reduce.

As a quick calculation, if we have R-2 continuous insulation (from the sheathing, the wall board, still air layers, etc), 20% R-4 wood (2x4s), and 80% R-13 fiberglass, the overall R-value is approximately 1 / (20%/(4+2) + 80%/(13+2)) = 11.5.

We could add two inches of depth either by jumping to 2x6 construction with R-6 wood and R-19 batts, or add 2" of R-7 continuous insulation. In the first case we get to 1 / (20%/(6+2) + 80%/(19+2)) = 15.8 ; while in the second case we get to 1 / (20%/(4+9) + 80%/(13 + 9))= 19.3.

Cheers, Wayne

 

[electrofelon]

Doesn't have to be foam board, could be rigid mineral wool or fiberglass; or could be a purpose built enclosure (see Larsen trusses) and blown in cellulose or fiberglass.


Well, reduce.

As a quick calculation, if we have R-2 continuous insulation (from the sheathing, the wall board, still air layers, etc), 20% R-4 wood (2x4s), and 80% R-13 fiberglass, the overall R-value is approximately 1 / (20%/(4+2) + 80%/(13+2)) = 11.5.

We could add two inches of depth either by jumping to 2x6 construction with R-6 wood and R-19 batts, or add 2" of R-7 continuous insulation. In the first case we get to 1 / (20%/(6+2) + 80%/(19+2)) = 15.8 ; while in the second case we get to 1 / (20%/(4+9) + 80%/(13 + 9))= 19.3.

Cheers, Wayne

Ok well, if I'm going to do something I'm going to go big and use an inch or two of polyiso which is like R 6.5 per inch.

 

[wwhitney]

Ok well, if I'm going to do something I'm going to go big and use an inch or two of polyiso which is like R 6.5 per inch.

Which IIRC is a class I vapor retarder when foil faced, so you'd want to avoid any class I or II vapor retarders on the entire face (so the sheathing can dry towards the interior if it gets wet).

Cheers, Wayne

 

[kwired]

I'm thinking half of the time they put the roto-zip in the box any way. So it would not matter.
I guess the other half of the time, y'all are right, they'd destroy the box or at least the foam liner.

I'm not required to use the box, but had thought about using that type of box in my own new home.

That is part of the concept of using roto zip while holding the sheet in place. You make a mark ahead of time that will end up somewhere inside desired cutout, you start there and "feel" for the edge of the box. Once you find the edge you jump to outside the box and follow it's perimeter around the cut out.

You can't cut out at the inside perimeter, it won't push back to the framing members without breaking out around the box, you have to cut to at least outer edges of the box for it to fit right.

 

[LarryFine]

When I drywall, I measure in each direction, make two marks that will clear the box by 1/8" each way, and draw a rectangle to cut out.

For recessed lights, I then draw a line from corner to corner and place the hole-saw pilot bit on the intersection of the diagonal lines.

 

[Buck Parrish]

That is part of the concept of using roto zip while holding the sheet in place. You make a mark ahead of time that will end up somewhere inside desired cutout, you start there and "feel" for the edge of the box. Once you find the edge you jump to outside the box and follow it's perimeter around the cut out.

You can't cut out at the inside perimeter, it won't push back to the framing members without breaking out around the box, you have to cut to at least outer edges of the box for it to fit right.

That's what I thought. That's why I asked if y'all had any problems if you've done this. As it seems the roto zipper would be perplexed.

 

[jmellc]

This is an issue born of the "hurry up syndrome". Buried and damaged boxes were rare when cutting was done ahead.

 

[kwired]

This is an issue born of the "hurry up syndrome". Buried and damaged boxes were rare when cutting was done ahead.

Hurry up syndrome is where they damage things or at very least end up twisting something that will flex.

I still use plastic boxes for most 1 gang boxes sometimes 2 gang but most the time 2 gang and larger I use fiberglass boxes. Had too many distorted multi gang plastic boxes because of drywallers that were in a hurry in the past.

Just yesterday I decided to walk through a house that the drywallers were hanging at the day before. They weren't done yet maybe 2/3 of it was done but what was up I never saw much issues with any cutouts for my stuff. Couple that will need some patching around, no missed cutouts that I am aware of. But I did see a couple HVAC rectangular vent openings that weren't all that square looking. Guess they will have fun putting registers in those at finish time?