Insulation Details ..Getting them right!
Hi
So looking over the drawing of my new build I realized I missed a insulating detail with the stepped footing in the basement and the transition from stud wall to concrete.
The walls that are above the concrete walls, will have a 4” ridged foam on the outside then a sheathed 2×6 wall with Roxul, when the walls transition to concrete the plan was to have 4” foam on the inside, then 2×4 framed full of Roxul and drywall. Problem is on the walls with the stepped down concrete.
How can I transition between the 2 walls. ?
1. the inside has 4”foam against the concrete so if I continue this all the way to the ceiling the stud wall ends up with foam on both sides. I assume no good. Also I end up with walls 18” thick at the transition,with the 2 stud walls and 8:” of foam.
2. At the slab the concrete wall comes up min 6” if can’t use foam because it makes a sandwich. How do insulate that lower 6”section? See drawing
3. Details … My architect asked if I prefer the stud wall to sit inwards and have the 4” ridged foam sit also on the concrete wall or studs flushed out to edge and foam siting out bound. I have seen most details showing foam out bound , but is there a reason why?
Thank for any input in helping get the details right!
Dave
Zone 6
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Replies
sample of basement detail at the 6" wall .
Dave,
My first question is: the drawing shows brick cladding. Is that what you are planning to use? If you have brick veneer, that fact limits your options.
Assuming your don't have brick veneer, the two best options are:
(1) Insulate your concrete basement wall on the exterior with rigid foam, and make sure that the rigid foam on the exterior of the concrete is co-planar with the rigid foam on the exterior side of your framed wall.
(2) Insulate your concrete basement wall on the interior with rigid foam, and extend that rigid foam to the ceiling.
I wouldn't worry in this case about a "foam sandwich." If the wooden components of your building are dry on the day when the interior rigid foam is installed, you are unlikely to have problems with this type of sandwich.
For more information, see How to Insulate a Basement Wall.
-- Martin Holladay
here is the side view of the house, the red line is going to be roughly the new grade
Thanks for your response Martin.
No brick. sorry it was long day and I couldn't find a vinyl siding drawing.
So if I try to keep the foam inside, my walls then our stud will have foam on both sides, but your saying if they are dry this won't be a problem?
This would be the complete back wall and half the side walls in the basement. See drawing
I was looking at building science slab on grade image, can i notch the 10" concrete wall in 4" all the way up to the bottom plate for the above stud walls. Similar to what they show?
Then 2x4 stud wall in front of that filled with insulation, although now the upper part of the basement walls will be higher R-value then any place else. or could I just place fluffy insulation in the stud cavity as high as the concrete wall, then leave the upper section empty?
Anyone have an opinion on my 3rd point about exterior insulation out bound of the concrete wall?
Thanks Again, for all the help. ( my wife didn't think the $200cdn for membership was worth it. HA)
Dave,
I'm glad to hear that you think the GBA membership cost is worth it.
Whether or not the rigid foam on your exterior above-grade walls overhangs the foundation or aligns with the exterior of your concrete is up to you. If you are going to install rigid foam on the exterior side of your concrete, I think that it makes sense for the rigid foam to be in a continuous plane.
In most cases, you can reduce the thickness of your concrete wall above grade as you propose, as long as (a) the reduced thickness has been approved by an engineer, and (b) your concrete contractor is on board with the plan, and (c) you are aware of increased costs associated with the change, if any.
It's OK if some parts of your basement wall have a high level of insulation. By the way, you don't have to extend the interior rigid foam (the foam on the interior side of the concrete wall) all the way to the ceiling if you don't want to. You can just frame the walls to make the interior surface coplanar. You end up with a thick wall, as you point out, and you need to think through which type of insulation makes the most sense here. In Zone 6, where at least 36% of the total R-value of the wall insulation must come from the rigid foam layer, your 4 inches of rigid foam (let's call it R-16 to R-20) allows for a maximum of between R-28 and R-34 of fluffy insulation between the studs. (The value of R-28 for the fluffy would work for R-16 rigid foam, while the value of R-34 of fluffy would work for R-20 of rigid foam.)
For more information on these ratios, see Combining Exterior Rigid Foam With Fluffy Insulation.
If you want all of your rigid foam to be on the exterior side of the assembly -- not a bad plan -- you should cut-and-cobble a little more rigid foam on the interior side of the wall sheathing, thereby improving the ratio of rigid-to-fluffy in the direction of safety.
-- Martin Holladay
So I quickly put together a really bad drawing in "paint" but I just want to make sure I've got things right.
I``m not sure what you meant when you said if I do all exterior foam, I would need to cut and cobble some on the inside.?
In the drawing I will end up with 18" wall but if this works might be the simplest. Deep window t wells as I plan on outies.
Thanks for any advise
Dave,
There are lots of ways to do this. Ideally, you will come up with a way to reduce the thermal bridging at the top of your concrete wall. Your current detail doesn't do a very good job of slowing heat flow at this crucial area.
You have 4 inches of rigid insulation on the exterior side of your above-grade walls. Are these walls already built and insulated -- or are you still at the planning stage?
Assuming that the wall isn't built yet, and also assuming that you are determined to use 4 inches of exterior rigid foam, I was suggesting that you add a cut-and-cobble foam layer on the interior side of your wall sheathing (just in the stud bays of the above-grade walls in your basement). That extra layer of rigid foam will allow thicker fluffy insulation on the interior -- potentially solving the problem of the thermal bridge at the top of the concrete wall.
-- Martin Holladay
I'm still in the planning stages , which is why I'm asking so many questions. I read many times you and others saying get the details right before you start not after. Thank you for the input.
I'm not sure why you are saying I have a thermal bridge? The concrete is covered on the walls and on the top by the stud wall.
The walls above the concrete wall are roughly R44 depending on the R-value rating of of the 4" foam which meets the standard you mentioned earlier. R24/0.64=37.5*.36 =R13.5 of rigid.
If I go thicker on the fluffy R38 then I'm up to R21.5 rigid Which I assume this is why you mentioned cut-cobble..
The lower section concrete wall is roughly R34 .
Is the location of concern where the two meet? Could I just extend the interior foam up 4" or 6" past the stud wall.
Or is it you are recommending I fill the upper section above the concrete with fluffy and not leave it empty? If so would you put in against the other wall or leave it the 2x4 wall?
Can't say Thanks enough for your valued opinion.
Dave,
The thermal bridge happens at the corner of the concrete wall -- shown by the blue arrow in the image below.
.
Alright so could I just extend the lower 4" rigid foam up 6" to solve this?
or do you think cut-cobble and more fluffy is better?
I'm not cheap but want to make sure my ROI isn't 100 years.
I am still shooting for net-zero.
Thanks,
Yes, extending the rigid up another 6" would fix the thermal bridge.
The air gap behind the drywall may be a code violation unless fire blocking (not shown in the detail drawings) is included.
Thank you for your reply
I'm always impressed with wealth of information you, Martin and others give.
I was thinking of maybe running the ceiling 1/2" drywall over to the outside wall before I raise the 2x4 wall.
I think that meets code. ???
Do you see any issue with this setup?
I assume this would easier then trying to notch the concrete wall 4" (first dwg.)to have an even plain for the 2x4 wall.
The issue is the hidden vertical channel with high flame-spread potential. Some states only require a fire break at every floor transition between stories, others require fire-blocking at shorter intervals than one story, including partition walls and plumbing chases. Filling the void with cellulose works, since ~16" of cellulose is the fire stop equivalent of 2x lumber. I'm not sure of equivalent testing with high density rock wool, but I'm optimistic that it could meet the relevant tests. Your architects SHOULD be able to tell you what's required in your area. (If not, they would have access to people who do know.)
But filling the void with fluff of some type is going to provide at least some fire protection, and would also reduce any thermal bypass potential of the void.
My issue is if I add more Roxul then I either add more rigid foam or cut and cobble as Martin explained.
I am going for Net_zero home, so extra insulation isn't a bad thing, its just if the cost is worth it.
Thanks,