# Should I insulate my sill plate?



## LVDIY (Mar 28, 2011)

Hi,

So I am in the process of finishing part of my basement. The house is built in early 60's. It has a block foundation. The sill plate seems to be untreated wood, but it is dry and appears to be in good shape. While it is resting on the foundation, there is a number of shims under the sill plate which looks to be roofing slate. This lifts parts of the sill plate up enough that I can stick a ruler in between the top of the wall and the sill plate all the way through to what sounds like metal flashing on the other side. So there is obviously the potential for a lot of air movement under the sill plate.

I have attached an crude drawing of what I'm planning on doing.

My plan is to attach 2" XPS to the foundation wall and then frame the finished wall on top of that. I will add the same 2" XPS to the rim joist between the floor joists. I will also add plywood as fireblock under my floor joists and let the XPS and Stud Wall run all the way up.

So my question is, what do I do with the cavity that's now created behind the sill plate? Do I insulate around it, or do I leave the sill plate alone? I guess my concern is that if I do seal up the sill plate, any moisture wicking up from the foundation would have nowhere to escape. What is the best approach for this?


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## Windows on Wash (Aug 30, 2011)

http://buildingscience.com/documents/information-sheets/basement-insulation

I would just use SPF there.


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## Gary in WA (Mar 11, 2009)

IMHO, add some metal flashing pieces under the plate where possible to stop any capillary wicking or moisture/air movement (convective loops) in the CMU's cells to your plate- especially if untreated or not cedar. Then caulk where you can to further stop air infiltration. Using 3/4" plywood to stop fire, per code; http://publicecodes.cyberregs.com/icod/irc/2006f2/icod_irc_2006f2_6_sec002_par018.htm

only leaves 3/4" or little more. Adding SPF, either closed-cell or open-cell is not an air/moisture barrier of much benefit at that thin thickness. I'd use some XPS foamboard- 3/4", wrapped in plastic sheeting to stop moisture from block to plywood/joists/wall framing. OR allow the metal flashing to cover the CMU completely, then unfaced XPS. Caulk/sealant the edge of the XPS sliver filler to stop any air movement to the wall foamboard. Any XPS on the wood rim joist should be foil-faced or plastic wrapped to stop interior/exterior summertime moisture drive as well. 

Use furnace duct cement ( with mesh tape) on the FB joints for it does shrink as it ages. If the CMU is sealed with metal/foam, the drying is to the outside in the above-grade portion of basement concrete wall. 

Caulk all the rim/wood joist joints to plate/sheathing or sub-floor before adding the ff FB/canned foam at perimeters to stop air/moisture ingress on exterior side of FB during seasonal humidity changes when framing changes size; http://www.thisiscarpentry.com/2010/09/03/moisture-content-wood-movement/

Use FB under the frame bottom plate for an air/thermal/capillary break from the slab, keeping your studs dry as I doubt there is a vapor barrier under slab?; http://www.greenbuildingadvisor.com...ressure-treated-sill-plates-and-building-code

As always, check local AHJ for local amendments.

Gary


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## LVDIY (Mar 28, 2011)

Gary, thanks for your very detailed reply.

I like the idea of flashing under the sill plate, and I think I'll just go ahead and let the flashing cover the entire top of the block where I can. I took a second look at my sill plate and it seems like I should be able to get flashing under most of 2 out of 3 walls. On the third wall, the sill plate seems to sit pretty tight on the blocks, and I also have a lot of plumbing on that wall to deal with, so I'll worry about that wall when I get to it after warming up by insulating and framing the other two walls first.

You mentioned foil faced XPS and I don't think I've seen that anywhere. My plan was to use the cut off pieces from the walls to insulate the rim joist. Is that not a good idea, or can I just add a plastic layer to it?

I also have some cable and phone wires running on the ledge next to the sill plate, so after I add the fire blocking, there won't be much space at all for insulation.

I have already checked with my local AHJ, and there seems to be no special local amendments. They did confirm that PT is OK for bottom plate and they reminded me to add fire blocking.


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## Theodore2 (Sep 4, 2015)

I have very same situation and similar question: how can I fireblock against the sill plate with cables running along and stapled to the side of the sill plate?

Also, if I spray foam this volume, woudln't I have to worry about de-rating the electrical cables encased within this continuous spray foam insulation?

Theodore


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## LVDIY (Mar 28, 2011)

Theodore2 said:


> I have very same situation and similar question: how can I fireblock against the sill plate with cables running along and stapled to the side of the sill plate?
> 
> Also, if I spray foam this volume, woudln't I have to worry about de-rating the electrical cables encased within this continuous spray foam insulation?
> 
> Theodore


I had to remove the staples and reposition the cables in order to get the fireblocking in place. I then sealed up between the sill plate and top of the wall before I repositioned the cables under the fireblocking on the lover part of the sill plate. After all that was done, there was not much room for insulation. I placed the XPS on the wall all the way up against the fireblock.


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