Wall and envelope question

[dustyjay]

I plan to wrap my frame with a 2x6 stud wall with batts or dense blown cellulose, 1" rigid foam to prevent thermal bridging, sheathing, moisture barrier, spacers, and then B&B siding. I plan to put spacers between my frame and outside wall for ease of installing sheetrock and to allow the frame to shift.

After talking with my contractor yesterday, my specific question is where to put the 1" rigid foam. I originally thought on the outside of the wall, just below sheathing, then wrap, then spacers, then B&B. The problem I see with this scenario is that it is no over 4" from the outside of the battens to the stud wall. What is the best way to secure that? I want my siding secured to more than just plywood sheathing.


My contractor is currently finishing a remodel and he put the 1" foam INSIDE the framing. This was not a timberframed house. The only adjustment he has to make is to build out the door and window framing. Moving the 1" inside reduces the distance from siding to stud wall, but it means either cutting all foam and drywall to fit around framing members (and fixing cracks) or it means setting my stud wall 1.5" off the frame, cantilevering it further over the edge of the foundation.


 

How would I secure 4" of rigid foam, sheathing, wrap, spacers, and siding to the stud wall if I put the 1' foam outside like I want to? How have you done it? What is the best way to anchor the stud wall to the frame itself?

Thanks

[S.Hyland]

I would fasten your horizontal spacer board to the studwall with panel fasteners. I use Headloks with the pan head all the time for stuff like this. This will give you a rock solid connection point for your siding. Then you just use regular length fasteners for your siding.

You are best keeping the foam to the outside.

[dustyjay]

thanks. That's what I had in mind.

[jueston]

i have always installed the 1 inch of foam on the outside of the sheathing and then install siding right over it, or in your case horizontal nailers over the foam.

personally, thermal bridging scares me a lot less then a double vapor barrier. so putting foam out the outside of the framing, and plastic on the inside of the framing is worrisome to me. the architects always say "but there is no way for moisture to get inside the wall, so there is no problem" but in my experience moisture always finds a way into the wall, and when it does, we need to provide it a way to escape.

[frwinks]

Pretty sure you'll need at least 2.5" to successfully control thermal bridging and moisture up in NH on a 2x6wall.  Head on over to BuildingScienceCorp, they have great publications on this stuff with enough info to make an informed decision.

A few of us 'round here, used the w'ns and I for one, am very happy with the performance

https://forestryforum.com/board/index.php/topic,51272.msg740288.html#msg740288

[shinnlinger]

I'm down in NC for spring break, but check out my gallery.   I might have two reasons for you to visit.  My walls are 2x4 with 2 layers of used 2" polyiso on the inside.   

[dustyjay]

I'm down in NC for spring break, but check out my gallery.   I might have two reasons for you to visit.  My walls are 2x4 with 2 layers of used 2" polyiso on the inside.

Thanks. I'm traveling around a bit next week looking at houses for ideas. Maybe I can catch you in the next two weeks.

[S.Hyland]

Dusty Jay, were you planning on putting plastic on the inside of your stud wall like Jueston was saying. I would agree with him on no plastic on the inside.

[dustyjay]

no plastic on the inside. Plastic outside of sheathing, then rigid board, then spacers and siding.

Perhaps plastic could go outside of rigid insulation, but definitely NOT plastic on both sides of wall. One of my biggest concerns is getting the balance of tight and breathable correct.

[Thehardway]

Dusty,

You are on the right track.  I would go plastic, rigid board insulation gluing sheets together with foam glue or minimally expanding spray foam and then spacers/siding.
Rule of thumb, vapor barriers were typically recommended to be installed on the "warm" side of the wall.  Of course which side is warm may depend on the season and the geographical location and in some cases even which exterior wall of the home. So using this rule was likely to give you a sore thumb or worse yet a sick house.  Double vapor barriers, especially when using an absorptive insulation material like cellulose or Fiberglass batt, is a really bad idea.  The tighter the house the more likely it is that moisture will be drawn in through small holes and perforations and form wet spots that cannot dry out.

There are two ways to avoid this. 1. Avoid double vapor barriers that can entrap moisture. 2. Use a mechanical ventilation system  like an ERV/HRV to achieve positive pressure and dehumidify the house.

Better yet, if you desire a airtight home, use a thin spray foam coating (up to 1 inch) on the inside surface of the exterior wall sheathing before insulating and sheetrocking.  It has no ability to absorb water and forms a good vapor barrrier and thermal break.   Altough it is pricey it quickly repays in energy costs and will not move or migate in the wall cavity over time leaving hot/cold spots. you can then use a batt or dense pack insulation with no interior moisture barrier needed.

When using spray foam, and EPS/XPS ignore "R values".  They were not designed for rating foam. They were designed by the fiberglass industry to sell batt insulation.

[dustyjay]

if you desire a airtight home, use a thin spray foam coating (up to 1 inch) on the inside surface of the exterior wall sheathing before insulating and sheetrocking.

This sounds like a good compromise. The prevents infiltration/ exfiltration. I'm concerned about haing enough air movement to satisfy a good sized forced hot air furnace, but I'll be bringing fresh air to the furnace and home through an ERV/HRV. That set up should balance the indoor air needs of the house and keep the envelope dry.

[Thehardway]

Achieving positive pressure is of the utmost importance.  Works like a heat curtain to keep cold air out with minimal sacrificial loss. Much better than infiltration.  A good HRV should allow you to set the input/output flow balance.  You will need not only enough make up air to positively pressure the home but also enough to provide combustion air for the furnace and accommodate any exhaust fans for bath, stove and furnace.  Can get a bit complex.

If you can incorporate some thermal mass in the home it will go along way in achieving comfort.  Also pay due diligence to airflow and location of duct outlets/inlets and diffusers as the constant cycling of air can make things feel drafty and cooler than they really are.