Insulated Concrete Slab?

[DJF3]

I am about to start the foundation for my 34x60 shop build. I am in central British Columbia, Canada. I was going to use traditional forms for my 4ft foundation walls, and I've been told that I'm crazy for even considering it. I am now contemplating the ICF. At first, I didn't think it was something you could DIY. I was quoted $20,000 for someone to come in and do it. At that price, I would use traditional forms. Form rental will cost about $1000. ICF blocks will cost about $4500, and I'm finding out it's fairly simple to do. Just have to make sure it's braced adequately.

My question for the experts, should I insulate the slab if I go with ICF? I was going to put an R12 insulation under the slab at a cost of about $2500. I will be using overhead radiant heat for the shop, so the concrete should be warm. Will I see much benefit from the insulation with the foundation walls being insulated?

 

[ConCretin]

I used ICF's on some above grade walls on The Concrete Underground but opted for traditional formed walls below grade. In my case, it was less expensive even with a layer of rigid insulation with Z furring on the inside face. The determining factor for me other than cost was the exposed portion of the wall above grade. You don't want exposed foam and I've never come across a method of concealing it that I like.

The decision about under slab insulation doesn't have anything to do with the perimeter frost walls. Formed frost walls should be insulated to keep the frost out of the soils under the slab so there really isn't much difference from an ICF wall.

Under slab insulation will provide frost protection in the event the building goes unheated but the payback from reduced heating bills will take a long time especially if the heating system is other than in-floor radiant. I'd put 2" of rigid under the slab but not with the expectation of big savings.

 

[matt_i]

Just make sure that you have enough time with the form rental. A crew can setup the forms in a day, pour the next, and strip them down the 3rd. As a 1-2 person crew of DIYers it won't be possible, it could take 1-2 weeks.

ICFs are slower to build but there are no rental issues, as a tradeoff.

If you wanted the performance of an ICF wall you could easily put 2" rigid foam right down outside of the poured wall after the forms are stripped. I personally think the biggest gain is directly under the slab itself, but the more time you intend to heat it, and the warmer you intend the temp to be, the more you should spend and prep for insulation.

If you look at just pure conduction the slab itself has more square footage than the exposed concrete portion of the walls, despite a presumably higher ground temp than outside (maybe 50F vs 20F) the narrow band of concrete wall above grade is a fraction of the square footage.

If you use the basic equation R = sqft * deg F delta * hr /BTU and resolve that to BTU/hr = sqft * deg F delta /R you can see. R value for concrete is 1. Delta is difference in temp indoors to outdoors or indoors to ground temp differential.

I built ICF walls ~42" tall off the footer and used mostly 2x2s for bracing.

 

[DJF3]

This is going to be almost strictly done myself. My labor "free" so to speak. I think the ICF will be easier and possibly faster in the end, considering no crew will be involved. I'm trying to do a "budget" build, but not afraid to spend money where justified. This may be one of those places. The rebar will go easier too. I am leaning towards both ICF and fully insulating the slab.

 

[North Dakota]

I just got a quote for fox blocks for my 30x45 garage. I'm going from the stem wall up to the top of the first floor so 13'4" walls with 8" block it's right around the 10k mark.

 

[DJF3]

I just got a quote for fox blocks for my 30x45 garage. I'm going from the stem wall up to the top of the first floor so 13'4" walls with 8" block it's right around the 10k mark.

What is the advantage of this? I don't know for sure, but my initial thought would be stick frame 2x6 and R22 fiberglass would be cheaper and better R value.

 

[50pascals]

What is the advantage of this? I don't know for sure, but my initial thought would be stick frame 2x6 and R22 fiberglass would be cheaper and better R value.

Fiberglass is never "better" R-value. Especially compared to foam.

 

[pcmeiners]

Since the ground temperature is roughly the averages of the temperature it is subjected to, without insulation the slab id going to relatively cold all year round. Your heating system will take a long time to bring the slab up in temperature, as the ground below it will be absorbing heat from the slab with the heat on. Basically without insulation you will have a cold floor, you will be heating/cooling the slab (and earth below) year round. So pay for insulation now, or pay for it with heat/AC costs, cold feet and possible a condensation issue.

 

[Firebrick43]

I disagree. R value is R value. ICF has pretty poor R value. About R12 to R16 or so for most walls. In consistently cold climates, it's not very good. In climates with huge daily fluctuations in temperature, (think desert climates or sunny, high elevation) the thermal mass of the concrete evens out the swing, so it gives an effective R value that is higher. I've seen claims as high as R60. What it's really doing is making the apparent temperature difference between inside and outside appear to be less, so energy doesn't transfer as fast.

I've had similar or better performance between frame and ICF if the frame is well sealed. If you don't pay attention to sealing on the frame construction, ICF outperforms it because it's airtight. That's the primary advantage of ICF. Air infiltration loses more energy to a structure than radiation and convection, so it's to your advantage to seal the building well.

First many icfs have "true" r24 ratings. Eps is around r4 per inch and many forms now have 3" on each side. TherE might be some but I have not seen any with 1.5" walls personally .

Second r values are not equivent or very comparable. The r value test regimen was set up to favor fiberglass. It's tested at 70 degrees and the time measured on how long it takes to raise 2 degrees. The test is a function of conduction. First if temps are 70 or 72 what's the need for insulation?

In real world, as the delta t between inside and outside temps get larger, fiberglass creates air currents inside the cavity itself(convection). It matters not how well you seal the wall as its internal. The convection current carries the heat from one skin to the other and them conduction transfers it. High delta t such as -10 outside and 70 inside will cut the apparent r value north of 30%. This is not as much of a problem in moderate climate but the OP is from canada. Also fiberglass can not be installed perfectly, having gaps and such and unless one builds a double wall with offset studs there is significant bridging.

And while you mentioned the thermal mass, you over downplayed it's importance. In my icf addition is double the original house and has the same r value ceiling insulation as the original house) at no time did I "need" air conditioning during the 2 years it was under construction. Even wit some 100 degree daus the house never got above 77 degrees. The mini split that cools it is a 12k unit and is grossly oversized due to fear of being to small for a house that large. The original house
Has 2x6 walls and good air sealing as well as R60 in the ceiling. While very good performance the addition did not raise either the LP or electric bill (AC) more than 40 percent.

 

[DJF3]

I appreciate everyone's opinion. The more information the better. Just to clarify (now that I have more information), I thought I was asking related questions, but clearly ICF walls and insulated slabs are somewhat unrelated. My plan as of right now (and this may still change), is to build 2x10 footings, 20" wide with 2 rows of continuous 10M rebar. Wet set into the footings will be 6"x24" starter bars, 16" on center. Then ICF walls, 48" high with rebar 16" on center both ways. Then a 5.5" slab with 3" R12 insulation underneath. Rebar will be 12" on center. My building code requires an R6 thermal break between the slab and the foundation wall. This should be accomplished with the ICF. Then on top of the walls will be stick framed 2x6 construction with R24 fiberglass insulation.

 

[Trochu]

I am about to start the foundation for my 34x60 shop build. I am in central British Columbia, Canada. I was going to use traditional forms for my 4ft foundation walls, and I've been told that I'm crazy for even considering it.

What was their rational for calling you crazy? Also curious, I've spent time in Fort Nelson, FSJ, and PG, when you say central, do you actually mean central, or a Vancouver "central", so like Kelowna? There are many buildings built in northern climates on shallow footings and traditional frost walls, you just have to use a combination of heat/insulation if you don't want them moving due to frost. And also, maybe personal preference for a warmer slab or some such, but there is no design reason to insulate underslab.

 

[matt_i]

One important thing for a DIY foundation build was a rotating laser level with sensor. I could not have done the job very well without it. I bought it due to the length of time i needed it plus "future jobs".

Even though it seems like a 6ft spirit level is enough to get the footer-forms correct it has enough additive error over a bunch of feet to make a difference in the end product whereas the laser is extremely accurate over relatively long distances.

 

[DJF3]

What was their rational for calling you crazy? Also curious, I've spent time in Fort Nelson, FSJ, and PG, when you say central, do you actually mean central, or a Vancouver "central", so like Kelowna? There are many buildings built in northern climates on shallow footings and traditional frost walls, you just have to use a combination of heat/insulation if you don't want them moving due to frost. And also, maybe personal preference for a warmer slab or some such, but there is no design reason to insulate underslab.

I am in PG. The trend now is to go with ICF as it is much less labor intensive. Since I'm doing this myself, without a crew, doing the walls with traditional forms will be quite a bit less money, but much more time and labor. I'm finding out that there may not be a time savings with ICF, but that's ok. You're right, there is no design reason to insulate. I would prefer a warmer slab to work from. During the winter, I will have snow covered trailers and toys melting in the shop, and I believe the water will evaporate faster. There will also be a floor drain to help collect the water.

 

[DJF3]

One important thing for a DIY foundation build was a rotating laser level with sensor. I could not have done the job very well without it. I bought it due to the length of time i needed it plus "future jobs".

Even though it seems like a 6ft spirit level is enough to get the footer-forms correct it has enough additive error over a bunch of feet to make a difference in the end product whereas the laser is extremely accurate over relatively long distances.

Matt,
I read your entire build thread. You have given me the inspiration to tackle this on my own. There is a ton of useful information there. Thank you! I have already purchased a laser level. A Dewalt DW074KD with a sensor. I've already discovered what I thought was a relatively level build area has an 18" grade difference, and a 33" difference to the elevation of my driveway. There's going to be a few gravel trucks in my future!