Rim joist insulation and air sealing with diagonal 1x subfloor

[EbS-P]

Time has come to insulate my rim joists. I have diagonal 1x subfloor. Some gaps between the subfloor planks that sit on top of the joists are 1/8” others are bigger. I will cut and place foam next to the joists but how do I air seal the small gaps between the subfloor planks. Most are too small for the applicator tip of a can of expanding foam. I took my FLIR camera seeping on a cold day and the cracks were colder. It was hard to say if it was air infiltration.

Is it even with trying?

Thanks

 

[John Galt]

Once the rim joist is insulated and air sealed with the expanding foam, everything is considered interior space and therefore doesn't need to be sealed. I presume you are doing the 2" rigid foam cut to fill the space and spraying canned foam around the edge. The expanding foam will fill in the seams of the subfloor 1x material, you might have to pause for a second when you hit each crack to ensure a good fill. I really like the "Pro Gun" setup because you can adjust the size of the foam bead but it does use a taller can and your space may be limited.

 

[EbS-P]

Once the rim joist is insulated and air sealed with the expanding foam, everything is considered interior space and therefore doesn't need to be sealed. I presume you are doing the 2" rigid foam cut to fill the space and spraying canned foam around the edge. The expanding foam will fill in the seams of the subfloor 1x material, you might have to pause for a second when you hit each crack to ensure a good fill. I really like the "Pro Gun" setup because you can adjust the size of the foam bead but it does use a taller can and your space may be limited.

I wonder if I can crimp/ smash the end of the plastic tube to fit in the crack. If it doesn’t get into the crack I don’t think it much sealing would happen.

 

[peakbagger]

https://aeroseal.com/aerobarrier/

I see this system mentioned frequently on hard to seal buildings.

 

[EbS-P]

https://aeroseal.com/aerobarrier/

I see this system mentioned frequently on hard to seal buildings.

Vaporized caulk. I’ll pass. I’m after low hanging fruit. Seems like the last resort to hit your AHC50 number. Or what you need to do to get some really low number, like less than 1.

 

[John Galt]

Smashing the tube might work, good thought . The new cans come with the improved spout that you can close off and store the can for a few weeks. The regular foam expands enough to fill those cracks even without squishing the tube, just hold for a half a sec in the gaps. You're hitting the low hanging fruit here. Enjoy the adventure and the work. The energy savings will take a few years to ever pay off.

 

[EatenByLimestone]

Cut the tube to an obtuse > shape. Too acute of an angle will spill foam out the top and bottom.

 

[stoveliker]

So rather than making a new thread I thought to piggyback on this one.

I am in the thinking phase of rim joist insulation (because the ceiling in the basement is next on the list to be replaced).

My rim joists have the standard glass fiber batts stuffed in them.

I was thinking of doing the foam board with spray foam around the edges, and rockwool boards on top for fire and r-value.

However!!, under my siding is some 1/2-5/8 " (I think not 3/4") foam board.
It's light blueish with a hint of green. I presume it's closed cell XPS, in which case it would have an r value of 2 or so.

But in truth I am not sure what its characteristics (r value but more importantly vapor barrier properties) are.

The importance of this is that I think it would determine whether I should air seal the foam board on the inside or not, or even if I should use semipermeable board or simply only add r value with rockwool. I should not have a vapor barrier on both sides of the rim joist.

(Some folks (experts or quacks...?) online advise open cell on the inside so that no moisture gets trapped. I think that's a fallacy because when it is cold out, the joist will be cold, and if vapor goes thru it'll condense.)

But having foam board outside may not result in a rim joist that's cold. (Indeed there is no indication of condensation there from past winters. Of course when my stove is running there it's very warm and low relative humidity.


Bottom line:
-is having some unknown foam board (some r value and potentially a vapor barrier) on the outside of the rim joist a situation where I should NOT vapor seal the joist bays

-i presume adding r value is good regardless. Should that be semipermeable? (And how does that depend on the properties of the outside foam board?)

 

[stoveliker]

I'm talking about this:

Rigid foam on exterior and interior of rim joist? - GreenBuildingAdvisor

I have 2″ of Polyiso covering the exterior of my house including the rim joist. I would like to insulate the rim joist and like the idea of keeping the […]

www.greenbuildingadvisor.com

Now that guy had 2" XPS.
I have 1/2-5/8" or so, of *presumably* XPS

Also look at the URL he posted in his OP...

 

[EbS-P]

My solution would be to use polystyrene with no moisture barrier. Foam the edges then stick a piece in. It should allow enough vapor transport to keep it dry. Second thing would do is make sure I have done everything I can to to keep it as dry as possible on the exterior.

I’d make a hot wire table saw.

 

[hockeypuck]

Time has come to insulate my rim joists. I have diagonal 1x subfloor. Some gaps between the subfloor planks that sit on top of the joists are 1/8” others are bigger. I will cut and place foam next to the joists but how do I air seal the small gaps between the subfloor planks. Most are too small for the applicator tip of a can of expanding foam. I took my FLIR camera seeping on a cold day and the cracks were colder. It was hard to say if it was air infiltration.

Is it even with trying?

Thanks

View attachment 312312

If you have a good wind barrier on the outside of the sill, I would not invest the time in filling every crack. Maybe lay a 1" bead of high expanding foam then drag a 4" putty bladed to squeeze it into the grooves.

 

[stoveliker]

My solution would be to use polystyrene with no moisture barrier. Foam the edges then stick a piece in. It should allow enough vapor transport to keep it dry. Second thing would do is make sure I have done everything I can to to keep it as dry as possible on the exterior.

I’d make a hot wire table saw.

But vapor transport also implies potential for water condensation on the inside of the rim joist when it's cold out... (Since the r value of my max 1/2".XPS outside may not be enough to avoid reaching the dew point on the inside...)

 

[EbS-P]

But vapor transport also implies potential for water condensation on the inside of the rim joist when it's cold out... (Since the r value of my max 1/2".XPS outside may not be enough to avoid reaching the dew point on the inside...)

Yes… so your choices as I see them, don’t insulate and keep the house really warm☹️☹️, keep the current batt insulation☹️, or air seal and insulate with something that allows some drying to the conditioned space😕.

It’s not perfect. open cell spray foam?? Do you want to collect temp humidity data for a season on a couple of different options this winter? That might give you some peace of mind. I don’t think there is a perfect solution.

 

[stoveliker]

It's interesting; because there is outside insulation, there is no perfect solution...!! (Without outside insulation it's clear, vapor barrier board and additional r value.)

Temperature data will fluctuate a lot, as I dehumidify with a dehumidifier unless the AC is on upstairs at which time I use the mini split in dehumidification mode for that. So it varies from 70ish to 55ish in summer.

In winter it varies from 55ish and less dry when I'm heating with the mini split upstairs if it's above 40-45, to up to 90ish and bone dry when the stove is on because it's colder outside.

All this depends on outside weather.
Too much variation, accessing all of parameter space to have more detailed data make a difference, I think.

 

[EbS-P]

I do wonder what the actual energy savings is? Air sealing and stuffing new batts vs air sealing and foam board vs doing nothing?

Is the savings worth the risk?

 

[stoveliker]

I couldn't tell. But I don't care about that in this case.
The dollars, that is.
The risk is why I'm asking here.

 

[zrock]

If it were me i would just spray foam the whole area and forget about sealing in rigid foam.

 

[stoveliker]

well, for that the cost would be rather large; either I have to buy equipment for a one time exercise, or hire folks for only the joist bays...

 

[bogieb]

well, for that the cost would be rather large; either I have to buy equipment for a one time exercise, or hire folks for only the joist bays...

I would think that you could rent the equipment from somewhere - maybe HD or an equipment rental place? Maybe your utility has a list that would be helpful (maybe not-I think they like to point people to contractors).

 

[EbS-P]

I would think that you could rent the equipment from somewhere - maybe HD or an equipment rental place? Maybe your utility has a list that would be helpful (maybe not-I think they like to point people to contractors).

I’m messy enough with a single can of foam. I can’t imagine what a mess the equipment would be if you gave me some that could spray a 1000 board ft🫣

 

[stoveliker]

Lol. Lots of plastic sheet and tape would be needed here, given that the space is already finished.

I also read that many amateurs don't use spray foam (mixing) equipment correctly, leading to less curing and more vapors releasing. I'm hesitant.

I posted on green building advisors and was advised to go with 1/2" of eps board spray can foamed in place (1/2" eps is most vapor permeable of rigid foam boards) and then R13-16 insulation in front of that.

 

[bogieb]

I’m messy enough with a single can of foam. I can’t imagine what a mess the equipment would be if you gave me some that could spray a 1000 board ft🫣

Same here

 

[EatenByLimestone]

The guys who do it every day are too. Don’t feel bad! Lol