Foundation Insulation and Sill Overhang

[rautyasu]

I am a builder in Wisconsin, and we recently went through a code update, with the REScheck energy software now often resulting in requiring 1.5" to 2" foam for the exterior of basement foundations. We build homes with full basements in our region, historically using 1" exterior foam for the full height, down to the footing.

Our typical home uses 2x6 exterior walls with OSB on the exterior. We use I-Joist floor systems. I have attached a cross section for this home to this post.

Our inspectors (local and state) are holding us to a 3/4" to 1" maximum overhang for the sill over the foam.

The problem is that when we go to 2" foam, our exterior wall assembly does not line up with the foam, interrupting the drainage plane, and causing some issues with finishing this transition area.

We could always add 1/2" foam to the exterior of the OSB (new wall bracing requirements necessitate that we stick with fully sheathing the home with OSB), but I would like to avoid the added cost of another layer of material on the exterior of the home.

My questions are: Are they correct in the maximum 3/4" to 1" maximum overhang? If they would allow a 1" overhang for a 2x4 sill, why wouldn't they allow a greater overhang for a 2x6 or 2x8 sill? Is there something I am missing, or an alternate solution?

Thanks in advance for your help on this.

[oregonbuilder]

Re: Foundation Insulation and Sill Overhang

could you cantileaver the floor joist on the fist floor to account for the foam? We have the same issue when we build with ICF block. We seem to bevel the edge of the foam and cap it with metal, this can look ok on a stucco finished home but kinda funny on a lap siding home. do you have to foam the outside? can you foam the inside of the basement walls instead?

[rautyasu]

Re: Foundation Insulation and Sill Overhang

Oregonbuilder - Thank you for your reply.

The floor joists would need to be cantilevered both parallel to and perpendicular to the joist runs. I have not been able to get a satisfactory answer from our lumber supplier on whether we can cantilever parallel to the joist run. From what I can tell

Foam on the outside is preferred to foam on the inside. Foam on the interior would require a fire-retardant face to it or sheet rock applied over it in order to pass code. The basement of this home would not be completed living space initially.

We considered some type of cap on the foam, but, as you said, it would not look good on a lap siding home.

[David Meiland]

Re: Foundation Insulation and Sill Overhang

My guess is that if you could get a PE to stamp a drawing showing a larger overhang on your mudsill then the AHJ would accept it. Barring that, you could consider adding a rainscreen to the exterior of the building. A 1" overhang on the mudsill + 1/2" of OSB + a 3/4" rainscreen and you'd have 2-1/4" total before the siding starts, enough to cover 2" foam. Obviously there is extra cost in this but it may have some sales value depending on your buyers. I am increasingly running into customers who understand what a rainscreen is.... finally.

[mrd999]

Re: Foundation Insulation and Sill Overhang

I assume the overhang limitation is cited due to structural loads above - in which case, I don't agree.

If your band joist is flush with the overhanging plate, then your floor joists are effectively cantilevering over the bearing foundation wall. Just be sure you have blocking acting as a cantilever along foundation walls parallel to the joists.

If your band joist is held back (e.g. foam on exterior of band joist) then your bottom plate and studs are all cantilevered. Loads carried by this wall can be transferred at an angle rather than directly down. A sufficient portion of the wall would need to have bearing to avoid crushing of the wood. A 2x6 wall cantilevered 2 inches has the same bearing area as a 2x4 wall. The wall's center of gravity would be off a bit, but with ceiling joists and CJ blocking at parallel walls toenailed to the top of the wall, they are just as stable as a wall with full bearing.

Getting your AHJ to agree without an engineer's stamp, however...

[charles]

Re: Foundation Insulation and Sill Overhang

Pete Engle and I went back and forth about this on another forum a few months ago, maybe it was Materials & Techniques. Bottom line is that engineers will allow 2 x 6 sill to hang 2" beyond foundation.

[hammerone40]

Re: Foundation Insulation and Sill Overhang

I have to agree with everyone else- a 2 x 6 wall would still bear 3 1/2" on the deck. I know our inspectors have no problem with this in our area of WI. I would politely ask the inspector where that issue is located in the code. Sometimes it seems like they pick up on an issue from the latest lecture or class and then start to enforce things that aren't part of the code, while larger issues get overlooked until the sheetrock is hung and the problem becomes an expensive one.

Andy

[rautyasu]

Re: Foundation Insulation and Sill Overhang

Andy - I agree with the idea that the 2x6 would still bear the equivalent of a 2x4, but the 3/4" is something that is quoted by the chief WI UDC officials in Madison. Without an engineering stamp on the detail, this is the inspector's default requirement.

The 'rule of thumb' quoted by both local and state building code officials is that the bearing overhang can only be 1/2 of the thickness of material. (i.e., a 2x material could only overhang 3/4") In my research, I have discovered that builders that have been using 2" foundation foam have been able to double the sill plate in order to get a 1.5" overhang.

The alternative of creating a cantilever with the joists ended up being a costly solution in complying with the joist manufacturer's cantilever detail on the parallel joist ends.

Adding foam to the exterior above grade walls in order to match up the drainage plane was cost-prohibitive as well, as it requires two trips around the exterior. (OSB Wall sheathing plus foam)

In the end, I was able to modify my HVAC system by upgrading to a 95% efficient furnace, allowing me to pass the ResCheck calculation with only 1" foam. This required inputting each wall into the software separately and specifying the direction each faced, but it resulting in a passing score.

In this instance I was able to stick with 1" foam, but I am sure that in the future that will not always be the case with the new energy requirements.

Chris

[Top Notch]

Re: Foundation Insulation and Sill Overhang

Interesting thread. It makes logical sense to use a 45 degree material thickness as a shear max but in your situation I dont think that technicality should be law. A 2x6 would still bear plenty and no load from the stories above will really tranfer.

[riwiseuse]

Re: Foundation Insulation and Sill Overhang

so I concede that I tend to undervalue my own labor. But I don't see why an additional foam layer cannot go up while the siding is going on. Usually you are making a second trip around the perimeter for the finish siding anyway. I don't know if a well detailed layer of foam is equivalent to building wrap although I tend to think so. The isocynanate (yellow) foam actually has a higher permeability and a slightly higher 'r' factor than the pink ( if you don't get a vapor barrier on it ).

So it can function vaguely like a building wrap, and you could keep a can of spray foam going to seal the joints . . .

I know, time and materials.

But it adds to the 'r' so you're not getting nothing for it and, being on the outside, it actually helps cut the condensation in the walls by moving the averaged temperature outwards.

Or you could consider using a structural panel with foam and OSB as one piece. Pay for it in material, save - a bit - in labor.

I'm going 1.5" isocynate with no vapor barrier over the sheathing so that the outer wall slightly overmatches the foundations with 2" of pink. For finish near the ground I'm going to make a brown aluminum z flash with a short inside leg, maybe 3/4" to cut down on temp. transmission and a long outside leg to cover into the ground and keep the wood siding away from the dirt.

Aside from trying this to see how I like that as an aesthetic for finish, my understanding is that foam can be a pathway for wood boring insects, so I thought some kind of break like that would be appropriate.

I'm goin in there for dorothy, wicked foam or no wicked foam. I only want you to do one thing. Talk me out of it. (unless this sounds like an OK idea.).

thanks,

brian

[charles]

Re: Foundation Insulation and Sill Overhang

The isocynanate (yellow) foam actually has a higher permeability and a slightly higher 'r' factor than the pink ( if you don't get a vapor barrier on it ).

I'm going 1.5" isocynate with no vapor barrier over the sheathing so that the outer wall slightly overmatches the foundations with 2" of pink.

brian

My understanding is that polyisocyanurate can't be manufactured without a vapor barrier, either plastic or foil.

[riwiseuse]

Re: Foundation Insulation and Sill Overhang

My understanding is that polyisocyanurate can't be manufactured without a vapor barrier, either plastic or foil.

Well I've got a bunch on hand without. If you are using it to go over studs on the inside then the vapor barrier is necessary because the foam is not as innately impermeable as the XPS. But I have been able to buy it without vapor barrier for the outside no problem.

My issue with a detail that flows from the foundation insulation to the siding is the termite barrier. Some other threads are recommending "Termimesh" but apparently this is a system not a product so i can't buy it to do my own detailing. And their literature seems mostly self promoting but not specific on why it is going to out perform an aluminum, copper , or for that matter, stainless steel sheet flash or break of some sort. It does look like the Termimesh material is more easily maniuplated because it is a mesh, but if you can work a detail for brake forming lengths of sheet for install, I can't see why sheet goods wouldn't be just as good.

Maybe the concern with the more galvanically sensitive materials is that a flash that runs to the pressure treated sill plate is subject to corrosive failure. As you see above, my proposal originally was for aluminum, but I'm negotiable. I could use stainless, but I wonder what weight is appropriate.

What about using a piece of concrete clapboard under the sill that extends over the insulation. Like the metal, it would defeat the continuous insulation, but as long as it was detailed to prevent air infiltration, maybe hold it short of the inside edge of the sill plate and use spray foam to block air and break the conductive pathway offered by the concrete clapboard. Or are there any sheet plastics or other composites that are considered insect proof/resistant. I'm thinking all of this plastic decking composite stuff must at least not get eaten, but I don't know if it would provide an effective barrier and at what thickness.

An alternative would be to overhang the pressure treated sill as discussed here and let that be the insect stop and theoretical barrier as well. But then you still need some kind of barrier for the bottom of the foam on the outside of the above grade framed wall - I'm a big insulation on the outside fan.

Or, I suppose, I could use a pressure treated sill wider than the studs overhanging and let the foundation insulation come up underneath and then the insulation for the outside of the framing wall come down to it. But you still are going to have sheathing or siding dropping in front of that sill plate so I'm back to some kind of metal lip over the top and outside edge of the foundation insulation,

So maybe the timber framers and or engineered panel people have thought about this since they've been doing foam on the outside for a while. Maybe this is where the borate impregnated EPS comes in.

Or what about this new pressure treated that is supposed to be less corrosive. Guess I'll go look for a thread on that.

brian

[charles]

Re: Foundation Insulation and Sill Overhang

What is the brand name of this polyiso with no vapor barrier? Have you considered making the foundation insulation removable (install in the fall and remove in the spring) to avoid termites?

[gary robertson]

Re: Foundation Insulation and Sill Overhang

that is why i went to usen foil covered foam on the inside of the foundation if the basement is not finished off. chris do u have to put 2'' foam under slab over footings

[clydewater]

Re: Foundation Insulation and Sill Overhang

How about adding a water table at the bottom of the siding to make cover the 2" foam on the foundation then it steps back for the rest of the wall?
Very common New England detail
Not sure what the style or siding will be on this house, but it works, easy to do and adds some interest.