Should I insulate my slab?
I’m renovating my 1850’s barn to be my new woodworking shop. The building is 30 x 40. I am pouring a monolithic slab under the whole thing. The slab will be 6″ thick and 16″ thick at the perimeter. I plan to pour on crushed granite. Only 1/2 of the barn will be heated in the winter. I have been planning to use “Insul-tarp” insulation. (1/2″ thick, R-10, about 65 cent per sq. ft.)
There’s the details, here are the questions:
1- Recently some friends have been saying that it is a waste of money to insulate the slab?
2- If I do insulate it how much better will the building hold it’s heat? My concrete guy says that this my main reason for insulating. It will obviously help protect from frost as well.
3- My concrete guy says a good comprimise would be to only insulating around the perimeter of the building?
4- Is it a good idea to run the insulation outside of the buildings perimeter?
5- Are there any problems raised when only one half of the building is heated?
6- I would also appreciate any feedback on ÒInsul-tarpÓ
7- WhatÕs the deal with fiber mesh? Do I need that? I am going to run rebar every two ft. in both directions.
I,m not trying to be cheap, I want a nice slab that wonÕt crack much. And I want a building that will hold heat well. IÕll spend the money but I want to be sure IÕm not just wasting it. As you know us woodworkers donÕt have much to waste!!
Thanks a lot !!!
Cushing, Maine
Replies
By fiber mesh, I assume you mean the fibers that are added loose to a concrete mix. They do not add strength. All they do is reduce the amount of shrinkage cracking. Since you are adding rebar, and you can control shrinkage cracking by keeping the mix on the stiff side, I woudn't bother if it were me. In fact, I didn't bother when I had garage and shop floors poured 3 years ago, and I don't regret it. Only the tiniest hairline cracks in the shop (20' x 30') and cracks no larger than 1/8" in the garage.
The fiber is not as cheap as you might hope. I think I'd put the money towards some insulation.
I can't talk knowledgeably about the cost/benefit of insulation, but I can say that the heated and unheated portions of the slab should be separated by a thermal break, say an inch or two of foam insulation. You'll need some expansion joints anyway, so you might as well make one there.
Eli,
I would be very wary of Insul-Tarp's claim to provide R-10 of thermal resistance. Such performance is technically impossible in a 3/8-thick product made of closed-cell foam and aluminum foil -- and that is what Insul-Tarp is. If you want R-10 -- and your probably do -- then, instead of Insul-Tarp, install 2 inches of extruded polystyrene insulation.
I've used the Astro foil under a slab and found the results disappointing. Learned later that you need space in front of the foil to benefitr from the radianbt properties which they include in figuring any R-value so the R10 (9.8) advertised is bogus.
Blue foam is better choice.
If insulating under a slab, I would do the whole thing but for a large slab, you do want a control joint or two also.
insulating the perimeter is the most important first idea, IMO Soils here in Maine run about 55°F but the frost can penetrate in under the perimeter, especially in the unheated portion. Insulate the perimeter with foam and you will keep all the underlying sopil between fifty and sixty degrees and thus control soil movement and keep the structure stable. This is not a compromise but an essential item.
Then, as a help for heating efficiency, you could add insulation under the heated portion also but this is more efficient if you are using radiant in floor heat.
Excellence is its own reward!
I put two inches of extruded polystyrene (Pink from Corning I guess) under my slab.
Entire slab...
I don't believe the foil/bubble wrap stuff at all either. The foil is all about low emissivity...which absolutely requires an air gap. IN a cavity space, maybe, against the ground, nope.
A good place for the stuff is with these old rubble stone cellar foundations, to staple up to the inside of the 8 x 8 sill and let fall to ground on inside of wall..
Excellence is its own reward!
Look in the Q&A of JLC this month's copy there is an excellent blurb on this
I put 2" under my 20 x 30ft workshop slab last year and insulated the rest of the structure with Icynene foam. There's a 1/2" thermal break around the perimeter to separate it from the perimeter block foundation.
Last winter it went to -10 in my neighborhood of SE Michigan and all I had in there was a little space heater. The room temp never went below 40F, and when the ambient was around 20 ~ 30F the shop stayed right around 65F. Someday I'll hook up the radient in-floor heat and save some money, but so far the insulation has made it possible to get half-way there with little electric heater.
The foam under the slab is the main source of that great performance. The floor is one big thermal reservoir and when it was 98F outside the inside stayed around 70 all summer long.
VERY nice to live with, and 2" of XPS foam is WAY CHEAP for this kind of benefit.
Don't think about it, do it! (is my advice).
Hope this helps,
Norm
If your weather is like Norm's and you often have big 20+F temp swings, it might pay to put insulation under the slab for comfort, but will not save you money in the long run for heating - this assumes dry soil with no water flow under the slab <G> . This also assumes 55 F is warmer than the outside and you don't live over permafrost -e.g lower 49!. However, insulating around the perimiter to frost line will save you LOTS. In the summer, not having insulation under the slab will actually help (55 F cooler than outside) , esp if you don't use A/C in the barn.
Did a detailed study years ago for missile silos, and in dry soil, the many feet of soil is an excellent insulator, it just takes time to get to the overall temperature gradient between 50-55 F deep soil temp or so and your interior - this is what earth sheltered and underground houses are all about.
Actually "heat" my pumphouse (by design) to above freezing in winter by having NO insulation under the slab, a well insulated slab perimeter(2" foam) and well insulated walls & ceiling (R-20+).
Comments on the 'tarp' insulation. As others have said, it MUST have an air space to acheive the R factor. As a matter of fact, R-8 insulation in a 2X4 wall with foil facing and a 1" air gap has a BETTER overall R rating (R-12) than R-11 that fills the cavity (per 1950s Dept of Housing actual building tests)
Did a detailed study years ago for missile silos, and in dry soil, the many feet of soil is an excellent insulator, it just takes time to get to the overall temperature gradient between 50-55 F deep soil temp or so and your interior - this is what earth sheltered and underground houses are all about.
Yes, but they work immensely better when topped by an insulating blanket that actually will adequately insulate, unlike the dirt with a tiny R value. Hait, in "Passive Annual Heat Storage" points out that 20' is the optimal distance from perimeter for the insulating umbrella to extend. This gives annual heat storage. Also solves pretty much all waterproofing problems. We are proof with our 20,000 cu ft, this year with a 10* annual temperature swing, unconditioned.
Even with a garden level basement, I've seen that a similar umbrella works well to moderate the basement temps while keeping the walls dry. Just needs to be a part of the overall plan initially.
For the slab on grade, I'd insulate out a few feet from the perimeter to allow heat storage in the ground directly under the uninsulated slab. This needs good ground contact with the slab. Thick gravel down there negates the system, as does a high water table. PAHS Designer/Builder- Bury it!
the ground is around 50 ~ 65F, all of the time. If you do not insulate your slab from it you will be fine if your living space is kept near those temps.
but if you want a warmer space (most people do) you'll be wasting money heating the rest of the planet that you don't need to do, unless you insulate.
Norm
but if you want a warmer space (most people do) you'll be wasting money heating the rest of the planet that you don't need to do, unless you insulate.
We do insulate, from the air. What you're missing is we don't heat year round. We spend as much time dumping heat into the dirt as getting it back. January ground here is nowhere close to 50* until you go a LOT deeper than our house is buried. We use annual heat storage. Emphasis on the "annual".
My next client is selling his conventional house due to high utility costs, one month's income every year. He's watched how our place does for several years. For our 20,000 cu ft house we burn 1 cord/year raising the temperature from 65* (the lowest it goes) to 70* in a 4166 degree-day climate. Simply ground-coupling won't do that.
Tell me where you are and I'll look up your heating season to compare.
If you'd like to learn more I'll post URLs. It's a simple concept, often misunderstood.PAHS Designer/Builder- Bury it!
I'd be interested in some general concepts and a few more specifics on what type of insulation you use, where you place it, and how you protect it.
Here you go: http://www.axwoodfarm.com/PAHS/UmbrellaHouse.html
Hait's architecture has nothing to do with mine. I only used the concept.
Insulation is extruded polystyrene within a polyethylene umbrella (multiple layers). Protection is careful backfilling. Recycled carpet works well for rock protection.
10 yrs here, we're still thrilled. A house should be inexpensive to build, keep you dry, warm in the winter, cool in the summer, have no exterior maintenance, and cost nothing to operate. We're close.PAHS Designer/Builder- Bury it!
Eli.. we insulate our basement slabs.. and our slabs on grade.
the whole slab..
our climate is not as severe as yours.. i would definitly insulate the whole slab of your workshop.. it will pay benefits in comfort and heating costs..
we've used extruded styren in the past ( Foamul-R.. Blue Dow Styro..).. but all of our future installations under slabs will be EPS with PerformGuard ( treated with borates for vermin control ).. with a density of 2.0 lb/cf..
i don't believe the ambient at rest temp under your slab is 55 deg.
i think it's closer to 45.. and as for earth being a good insulator.. it's not.. it is a good tempering medium.. but a lousy insulator..
if the temp is 45 at 4'... and 55 at 1' you can be sure the constant draw will be 24 hours a day during the heating season.. the heat loss calculation still remains the same... ( Area x U-value x delta -T = BtuH )
if your shop is 70.. and the earth is 45 , your delta -T is 25, the insulating value of earth ...... ( let's be generous and call it 1 )... so the U-value is 1/1 =1, x the area of your floor will be the Btuh loss.. change that r-value to 10 with 2" of EPS will give you a U-value of 1/10 = 0.1.. so your loss will be 10 % of an uninsulated slab...
naturally.... you have other heat loss to worry about besides the slab.. but they can be retrofitted... your slab can't
Mike Smith Rhode Island : Design / Build / Repair / Restore
My gut reaction is that insulate under the full thing - not just the perimeter.
The soil is a good insulator, but its pretty darn cool, relative to room temp, and it'll keep sucking out the BTUs.
But that's just a gut feeling - with some "common sense" thrown in - in an area which is often counter intuitive.
_______________________
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Many years ago I was working on a new house being built for a client. The homeowner specified that the concrete floor in the cellar be insulated. So when the day arrived to pour the floor, the GC covered the dirt with 3 1/2" of pink fiberglass insulation before the pour was made. You can imagine how much insulation value 3 1/2" fiberglass has underneath 5" of concrete. Later the HO asked the GC if he had insulated the concrete. Yes, he told the HO. There's 3 1/2 inches insulation under the slab.