So where I live right now it is getting to about 20 below at night and close to zero during the day. There is frost forming in the top corner of a bathroom where I am working. It is an outside corner on the house, where the double top plates are. Kind of interesting to see that even a house that is completely (ceiling included) spray foamed can have these issues. So I got to thinking about double top plates. I know there is an article here talking about framing without using a second plate, but to me that seems weak. I had the idea of using a smaller top plate. If you were to frame a house with 2×6″ walls, how come you couldn’t use a 2×4 for the second top plate towards the outside of the wall. That way you could put spray foam on the inside 2″ and still have the strength of a doubled up plate. Anyone ever done this? Why wouldn’t it work?
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Not sure why you would if you could simply learn the single top plate approach. Your observation, however should be worth it's weight in gold to nudge you to think more about the importance of minimizing thermal bridging (even if it is wood). Some guys sheath their house w/ e.g. 1/2" or more of e.g. Thermax sheathing in lieu of plywood/OSB. This takes that edge off the framing issues a little.
Single or double top plate ... you still have a bridge there ... it is just half what it was.
A single top plate can be used when the framing is stacked, A top plate has to be doubled if it's unstacked.
A 2x4 will still give you the required bearing, so a 2x6 top plate is not required.
Is your issue thermal bridging from the top plate, or a poorly detailed rim joist? If the top plate is thermal bridging to a large enough extent to frost the interior, then you should be seeing the same issues with the wall studs, etc.
I'm more along the line of thinking that there's a thermal weakness at the rim in that joist bay, and cold air is getting to the drywall. Interior moisture/vapor is condensing/frosting in that location on the cold drywall.
Not sure how the joist spacing falls, but often times at the end of a floor platform the joist spacing might be non-standard...less than 16" for example, as the floor platform terminates. Sometimes those skinny joist bays get shorted on insulation.
It could also be a poorly insulated studs at the corner. Maybe a 3-stud corner with a hidden cavity that they forgot to foam.
Just my thoughts.
What would it be doing if the place had been framed with thermally-broken 2x6s? What you see in the pic is from Nordic Engineered Wood Products, a Canadian supplier.
2x4s are glue-laminated to rigid foam, then the insides faced with 1/2" OSB.
And here I thought I was onto something revolutionary. Its not poorly insulated. As I said, the whole house is foamed. Its a single story ranch house, and the problem area is in an outside corner on the house where wall meets rafter. As far as a single top plate, to me it just seems weak. Look how much wall flex is eliminated with the second plate. I realize that whatever sits on top of the wall will stiffen it up, but it seems like an extra layer of sturdieness.
I am not aware of whether or not a second plate adds any strength whatsoever to the overall framing of the house. I use doubles as a means to lock intersecting walls together and for no other reason.
it helps eliminate a flex point in the wall when there is a break in the top plate
A single top plate has been an accepted method for a long time under the code. as someone said you do need to stack the framing.
I have framed both ways and personally I prefer double, with a single plate, the walls get bumped around a lot easier when setting trusses etc. which requires more bracing to keep everything straight untill the roof sheathing goes on.
If you use a single top plate you must be sure that your floor or ceiling joist spacings are in line with your stud spacings so the load is directly over the stud. As well as locking your corners, the double plate allows you to disregard this.
If you use a 2x4 top and bottom plate with 2x6 studs and set your rim and header joists back 1 1/2" to line up with your plates, you can then put a layer of rigid foam as a thermal break over top.
Even at 20 below or much colder than that you should not see frost on an inside corner due to thermal bridging in the top plates. There's an air gap in there somewhere. At a corner it may well be between the two layers of top plate if the walls were framed poorly and one sits higher than the other.
I don't see why the 2x4 2nd plate wouldn't work but I've never done it myself or seen it.
If you're working in that bathroom anyway it might be good to suggest fixing that issue. It would be a relatively quick fix with spray foam, acoustical sealant and a bit of drywall and paint. It would save a big headache with mold on the interior wall and maybe the inside of the wall too.
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