SIX inches of insulation under this basement slab

The slab is placed at the RI build and Nick walks through the insulation detail underneath and shows the framing progess.
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VIDEO NOTES
Site Visit walkthrough high-end new construction, renovation, and remodeling projects. Looking at architectural details, building products, framing, electrical, plumbing, staircase, rockwool, modern design, door hardware, repair, fox, replace, subtle details, modern staircase, mechanicals, and finish details. NS Builders shares building and project management insight while showing step-by-step progress of ongoing jobs. Site Visit is hosted by Nick Schiffer owner of NS Builders. Waterproofing your basement?
Building products, cabinet tips, new building secrets, and framing become architectural details. New construction and renovation, rockwool ideas start and finish with building and running successful builds.
NS Builders operates in Boston, Newton, Cambridge, and in the greater Boston area. Currently, 5 projects are under construction with a few new projects coming soon.

[@GoldsConcrete]

Thats going to be one comfortable basement ! We've done a few jobs like this and the customer is always in love afterwards

[@joshuasmith1215]

Man I hope to have the money one day to build a home that will perform like this. Can't imagine just how comfortable that home will ALWAYS be. Great job.

[@Patrick_Cashman]

Great to see someone doing everything right. ie, who understands the thermodynamics. Well done.

[@latitude48design]

Awesome details in this video, thanks for showing them off! The steel post & beam connection in the basement really show their value at this stage, I will definitely be referencing that in the future. Great work!

[@wesrobinson7366]

Thank you all the tips here. We are building a new area where our current claw space is today. I will be doing the same insulation under the slap but then will put drains around the edge and run the poly up the wall tied into the spray foam on the wall.

[@giovannifiorentino8947]

Site visits are the best.

[@billvojtech5686]

I'm not really sure I understand why insulating a basement is so important. My friend has a house built in the early 1900's. The basement floor is 5'-6' below grade. There is a thin layer of concrete that doesn't cover the entire floor - one corner has an area about 10' sq that has bare earth. I think it was a root cellar. The basement is about 68 degrees in summer and about 65 degrees in winter, (the furnace is down there). Being that far under ground produces a fairly constant comfortable temperature - cooler than outside in summer, warmer than outside in winter. That's what ground source heat pumps take advantage of. Of course, they concentrate the heat, but this involves no mechanicals and no energy input. Insulating the basement would cut you off from that. I can see doing it for a slab foundation at grade, but not one significantly below grade.

[@jeremiahjohnson6881]

Nice job,great Insight

[@kennethyoung3911]

I have floated a few radiant floor slab houses over foam with a separation from the perimeter as a thermal brake . Only issue is it's an easy path for underground termites to access the structure. Might want to consider a poison barrier applied prior to placing the concrete.

[@SailingSoulianis]

Rockwool has a major advantage over foam in that it is an undesirable material for termites. I'm not familiar with how much of an issue termites are in a colder climate like RI. But my guess is that despite that advantage over foam, rockwool was chosen for its numerous benefits and it's anti-termite properties probably did not contribute much to that decision.

[@kennethyoung3911]

@@SailingSoulianis as I am a builder in Rhode Island and have had experience with termites here. I posted from first hand knowledge.

[@canonicaltom]

@@kennethyoung3911 In Alaska where there are no termites, thanks for your opinion 🤣

[@jerryhuff86]

Love it

[@slickfast]

Wow, very very cool. Is this a passive house? Looks like you could probably be net positive with a house this tight!

[@rfauske]

Here in Norway all new homes have around 14inch of insulation in the slab. But we use EPS or XPS depending on the load on the slab.

[@TrionicSeven]

yep, us Nordic folks are always impressed that nobody else seems to insulate their slabs. 300 mm minimum for sure

[@kojimasan444]

Why build it this way, when you could've built it via ICF ? What are the benefits and cons compared to icf?

[@kevinmackfurniture]

Great video. Cool basement details.... But the Shop is too small.... They're Always too small.

[@ericbstudios9807]

This is pretty cool! Just curious about attic ventilation, specifically intake at the soffits. Will there be any?

[@TA-SEN]

so that main ridge beam stops the walls from spreading? also the blocking 2' down on all your exterior wall. what is that for?

[@CH-pp5py]

How did you detail the interior footings here? Is there no foam under them? Is there a thermal break between them and the slab? Thanks!

[@neilaleksandrov2655]

rockwool goes directly on top of the gravel? no water barrier? curious also what is rockwool resistance to termites?

[@randomrazr]

why not use spray foam insteda under the slab

[@welewisiii]

why no radiant heat in the basement and workshop ? nothing is more comfortable

[@nottherealjk]

I know you talked about having the plywood on top of the double layer of Rockwood on the outside, but how is it mechanically attached to the main home envelope?

[@greg925911]

With longer screws

[@greg925911]

2' on center wall studs that I would think would be hard to hang things

[@julianugentarchitect]

Plan A: long screws. Plan B: fiberglass z-clips. My bet is that we will end up with Plan B, but the team is buying samples of both to try out.

[@karbonphiber]

Great details. Thanks for sharing. Picking up bits of knowledge to arm myself when we build our next house. Question - why did the owner select this specific location? The view and surrounding homes seems to be incongruent with what will be a spectacular home.

[@NSBuilders]

The view is incredible, and it’s in a nice neighborhood with tons of potential.

[@johnnyhimburg6652]

Nick, another great build but I don't understand the headers over the window. There is rafters that falls over the top of the window and no real support for it. I just wondering how that works?

[@NSBuilders]

Rafter is essentially hung off the header as well. All engineered and designed.

[@julianugentarchitect]

It will make more sense when the hangers are in place.

[@nikfish1]

I don't know about America, but in Australia most of new construction is so crap. Everyone just builds to the lowest standards, which are very low. It's such a pleasure watching someone who cares about what they do, and is clearly actively thinking about what they're doing.

[@TheArtificiallyIntelligent]

It’s the same in the U.S.

[@LincolnLog]

We actually have homes that are built without external sheathing (no plywood/osb). From outside cladding > wrb > sheetrock/gypsum. Not even insulation batts either. That's not every build, but it can get that dumb. And people eat that sht right up and finance it 30 years

[@ImDembe]

@@LincolnLog I was surprised when i was in Florida years ago, some houses was built bad, everything felt hallow, they have storms and you don't bring a stick to a tornadofight and insulation and a sealed house makes the heating AND cooling bill much lower. People live in home that have the same/worse buildingstandard as my summerhouse in the woods that my grandpa built over 50 years ago.

[@LincolnLog]

@@ImDembe I hear you. Its the banking and finance system. Being able to swing the price of commodities like lumber, meats, grains, oil etc, ESPECIALLY by private and financial equity firms should be illegal. Its ridiculous that the average person has to watch the price of lumber daily since it swings like a pre-WW2 era Germany. The entire system is boinked and everyone knows it

[@safffff1000]

Why didn't they just encase everything with 2" closed cell foam, rockwool holds moisture and doesn't provide the structural strength which could reduced framing cost

[@kaspergruszczynski9919]

I suspect it's a concern for the environmental impact of the foam. And it's not that nice a product and requires an external contractor. The Rockwood batts are nice and easy to install and leave a nice even finish.

[@NSBuilders]

Comfortboard is different from comfort batt, and we’re eliminating the use of foam for this project.

[@safffff1000]

@@NSBuilders Whats the price difference including labor

[@NSBuilders]

@@safffff1000 didn’t run a coat comparison

[@RidgeLarick]

What tape do you use to seal the vapor barrier to the concrete? Awesome work, as always!

[@stevecrawford6958]

they probably used fentrim but unclear.

[@waiganliu9835]

Looks like stego

[@AustinCousineauEE]

If the vapor barrier under the slab is on top of the rockwool, can moisture get into the rock wool under the slab / plastic and reduce it's insulation capacity?

[@TA-SEN]

rockwool sheds water.

[@whiskey4553]

@@2brazy4ubitch Why can vapour sit around for longer with EPS/XPS?

[@BillyJ10]

👍

[@Real_Tim_S]

I'm of a mind that foundation walls sunk into the ground need the vapor barrier on the outside so that you don't get vapor drive through the concrete. Do your footings with galvanized bar and hydraulic concrete (designed to be wet), and do a vapor barrier/capillary/thermal break at the top of the footings - this way you expect the footing to be "wet" for the life of that building. Do the under slab insulation, then a vapor barrier, then the foundation slab on top - like you did here. For the foundation walls, external insulation, vapor barrier (fluid applied on the exterior), then the concrete walls tied into the slab floor - as you bury the walls, a french drain (and a pump if required). This way your continuous vapor barrier is outside all of the walls/floor, and the whole structure except for the wet footings are inside the conditioned envelope. This means you can build the concrete wall and floor with a known humidity for the life of the building.

[@julianugentarchitect]

The concrete walls are waterproofed on the outside and there are under slab perforated drains in crushed stone below the slab. This top layer takes care of any moisture that gets past those first two water management solutions.

[@Real_Tim_S]

@@julianugentarchitect I recall the exterior treatment - but is there a thermal and capillary break between the footing and the wall? If not, the footing will wick moisture from the ground, and "short circuit" the waterproofing barrier on the outside. @1:55 Nick is describing a vapor retarder going on the inside face of the concrete wall in the basement. Without the capillary break at the wall and footing, the base of the concrete wall will become saturated and emit water vapor through the retarder in the inside face of the wall. What I described in the above comment, is that the "continuous barrier" would need to actually be continuous. If you're doing external insulation - fine, but then here it is mixed and matched at one detail. The underslab insulation and the foundation wall insulation is on the outside of the vapor barrier - but it looks like the wall is connected to the footing without a thermal break or capillary break, making the vapor barrier discontinuous. The fact that Nick did a rim of Rockwool around the slab to "thermally break" the slab from the wall, indicates that he expects the base of that wall to get cold. With a warmer interior, and a wet cold concrete surface at the base of the foundation wall, why would we not expect hydraulic pressure from groundwater to diffuse water and vapor up through the foundation wall and behind the retarder layer? One of the worst things I recall one can do with water vapor control is squeeze it between two layers of vapor barrier/retarder with a delta-T.

[@julianugentarchitect]

Tremco was installed across the top of the footing, creating a capillary break so moisture can’t come up the inside of the foundation wall. The perforated drain is set in gravel at the bottom of the footingbelow this capillary break and drains water out downhill.

[@Real_Tim_S]

@@2brazy4ubitch Generally I think we are thinking roughly the same thing, Julia mentioned the Tremco at the bottom interface between the footing and the foundation wall, acting as a capillary break from the footing (which will equalize with soil moisture over time, which I why I advocate for considering the footing "wet"). I think we also agree in principal that some form of capillary break around the foundation to reduce liquid wetting and promote drainage is required. Fluid applied capillary breaks need to be able to stay adhered with high hydraulic pressure applied to one side, so the best capillary break is an air gap acting as a pressure relief. We do this all the time with rain screens. I find it helpful to envision what is done above grade with a rain screen while talking about this footing detail - you want a liquid barrier to shed bulk water like a brick wall which you say you expect (rightfully so) to get wet and have vapor permeate through, an air gap to act as a capillary break and provide an isolated drying surface. Behind that water shedding face and air gap, you'd want the insulation so that any moisture vapor present will meet the dew point in the insulation outside the vapor barrier, then the a continuous vapor barrier so that the whole structure is inside the conditioned envelope. When one does a penetration above grade for a balcony or mid-story awning, it must have a structure that mimic those layers, some form of vapor impermeable layer at the structure, a thermal break, and then out to the exterior structure for the appendage through insulation, and details to prevent liquid water infiltration at the joints. Where I'm a bit weirded out, is that as an industry, we don't do the same thing below grade as we would do for a balcony. Yes, I know structural thermal breaks are not free, but we seem to expend the budget and detail work for above grade fixtures, but not below grade - where it seems losses and water saturation of certain components are just accepted. We know we can't insulate below a footing due to code requirements that it either sit on bedrock or soil compacted to an engineered measurement, So why try to insulate it? We don't insulate the brick face of a rain screen. It seems to me that one should just get the joint detail right to continue the rain screen below grade, and the insulation below grade, and the continuous vapor barrier below grade... without interruption. We see insulation below the slab, and insulation outside the foundation wall - but there's this lingering thermal short circuit where the foundation wall sits on the footing and the slab joins the foundation wall. It's like a steel balcony deck outside being connected to a structural steel column inside without a thermal break and wondering why condensation is forming on the interior steel... That's where my head is at. If you aren't losing a ton of heat to the soil due to underinsulating, your foundation wall and slab should be around your average temperature for the conditioned volume driven by any thermal resistance through interior treatments and the thermal mass of the structure, so the foundation wall and slab will be the higher of the two temperatures between them and the soil's temperatures (so any condensation should form on the outer surface). For example, if your conditioned volume of 72°F is within about +4 degrees of the foundation wall temperature, you'd need to have almost 90% interior humidity for the dew point to cause condensation on the interior face of the concrete foundation wall and slab. With a peak "healthy" indoor humidity of 50%, your wall/slab would have to be ~20° cooler than your conditioned volume air to hit the dew point. If you have a 20° delta-T between your conditioned volume and the wall/slab you're losing a lot of energy to the ground - and if you're making up for this by insulating the inside face of the wall behind drywall, your dew point transition still happens on the wrong side of the vapor barrier (vapor condenses to liquid inside the conditioned volume). Any moisture that originates from inside the conditioned space will have to be handled by the building envelope's mechanical systems - liquid water needs to be drained, but the drying of a damp surface will need to be handled by the HVAC, and if there is insulation or a second vapor barrier inside the volume slowing that processes, that sounds to me like a great recipe for problem...

[@Real_Tim_S]

@@2brazy4ubitch The point of the "dew point in the thickness of the insulation" was more that all the insulation was on the outside of the vapor barrier, and thus the dew point occurred outside the vapor barrier - not necessarily that bulk liquid would condense in the middle of it. It's the very nature of the low thermal mass outside the vapor barrier that you would be after. Because the concrete foundation wall would be inside the "conditioned space" temperature, and generally ground temperature is always going to be lower - we'd be driving moisture toward the soil-side of the insulation where water would be inclined to condense. This is not something we can take for granted above grade where the hot and cold side of a wall can change with seasons. Moisture in the insulation nearer to the foundation wall would be inclined to transition from liquid to vapor due to warmer temperatures - thus the need to keep liquid water off the insulation layer in the first place (again keeping with the rain screen concept) as the energy used in that phase change comes from the controlled envelope of the building and has to be replaced by mechanical systems. The same physics that make a rain screen work above ground work underground (think of a solid surcharge on a foundation wall as the moisture laden "brick veneer"), we just need to be aware of and plan around the site water-table and drainage conditions to keep liquid water out in ways that don't apply above grade. As you point out in your Radon comment, there must be a negative pressure in the air gap underneath the foundation for this to work - but you also don't want that negative pressure to be made up by above grade air. Speaking of above grade air, you mention radon mitigation without moving parts. I wonder if a combination of a ridge vent and some venturi could generate a few inches of water vacuum at a decent CFM... Even a light breeze on a building should create enough pressure differential to work with. Most systems I see ask for 2"H2O vacuum on less than 200CFM as a general number (most well below this CFM), that's not actually that much. Wouldn't surprise me if we've been doing that "the hard way" too. ;-)

[@NickWelch]

Lil nit pick: heat from the concrete wouldn't cause condensation. Heat dries things out. (Think of a hair dryer or toaster.) The condensation is because the basement air is humid.

[@350oven4]

idk if you took a concrete/carpentry course but when the outdoor is hot, and indoor is cold, it causes concrete to sweat also known as sss

[@NickWelch]

@@350oven4 Condensation happens when the air is relatively warm/humid and a solid surface is relatively cold. Or, technically, when the temperature of the surface is below the dew point of the air.

[@kd8118]

Isn’t ground temperature always the same?

[@bobbray9666]

Pretty much around 50F eight feet deep in RI, I think. Adding that much insulation under the slab seems like a waste of money. Kind of like building a house in San Diego with R-30 wall insulation. It rarely gets hot or cold there.

[@bobbray9666]

@@2brazy4ubitch It seems there is a bragging contest going on with some home construction channels on who can make the most insulated and air tight home, even though they are well past financial ROIs for future energy consumption costs. Triple pane windows, R-60 walls, very low blower door tests using expensive air misting sealing etc. That's all great if living off grid north of the artic circle but otherwise foolish as you won't recover the investment including selling such properties down the road. Crazy to put a excessive money into residential construction when you could take that money and invest it. I had a four season porch built onto a home I had in 1989 and spent around $30k. We rarely used the porch after the first year and gained virtually nothing when it sold in 1999 before the .com bubble. Had I just put that money into a S&P index fund, it would have been worth over $750k in Jannuary 2022.

[@bobbray9666]

@@2brazy4ubitch I'm building a new home starting in May in zone 6. The sheathing will be Huber Zip-R12, 2x4 studs with 3" of closed cell insulation. The ceiling drywall in the attic will have 2" of closed cell and then fiberglass blown in on top of that. Aerobarrier will not be needed. Maybe Aerobarrier would be better suited for and existing home remodel where the siding remains. I got a quote of over $12k to see the cost. No thanks.

[@orangemanok5800]

You went to all the trouble of super-insulating an unheated slab? It's still going to be a cold concrete floor. Missed opportunity, unless I missed something.

[@orangemanok5800]

@@2brazy4ubitch For us it’s “once you go heated slab you never go back”. It’s like heated car seats. You don’t know how life-changing it is until you have it.

[@orangemanok5800]

@@2brazy4ubitch I'm not an installer, just a happy homeowner. We built a shop/mother-in-law apartment and it's all electric. Gas wasn't an option. We might add solar. Energy is expensive here in Northern California so we over-insulated everything. Well worth it. The best part is working on cars in the winter without wearing a heavy coat or running propane heaters.

[@MadsDKN]

I see some Velux boxes 🧐

[@NSBuilders]

Skylights.

[@akbaldman3731]

as a former Mechanical engineer turned contractor and builder, this wouldn't work for earthquakes loads here in AK. Why not use Zip, T studs and spray foam? You are using less dimensional lumber but more plywood. I bet your ACH would be better with Zip and foam. As far as rockwool under the slab, not a good choice imho. too much moisture can be retained. Blue board would have been better. But that's just me. I'm not familiar with your constraints where you are at. interesting choice of location to build a house.....

[@iq6840]

6 inches of insulation or 6 inches of Styrofoam to crack and settle into in 20 years? 😊

[@Zickafoose2024]

Honestly dude guy, why you holding the Boston accent back? Beer for blood bud

[@kylek9432]

Sounds great until it becomes a rental in a few decades and the rot is so prevalent in the kitchen and bathrooms that its damn near a tear down! Cannot beat 1920s houses with large overhangs. No rot!

[@NSBuilders]

Not following? House has overhangs and doubtful we’re going to see rot… nearly impossibly

[@kylek9432]

@@2brazy4ubitch Dunno about sawdust but the mineral insulation they used is top tier, even today!

[@Tom-sd5ru]

How the hell framers using pc pancake compressor.

[@NSBuilders]

You know Tom, I said the same thing. But it’s working, not a production crew, very thoughtful and taking their time.