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2680
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# Posted: 9 Sep 2012 05:09pm
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i was planning a new cabin (#3 for me) and was considering building on slab in order to beat the heat in the hot humid summers. my other two are on conc piers, so i have no experience with slabs.
anyone have any advice, suggestions on this subject?
thanks!
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OwenChristensen
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# Posted: 10 Sep 2012 07:37am
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I see your point. My house is on a slab and has been great. Always heated in winter it has held up great. But, for a cabin in Northern climates, slabs are a pain. In winter, if not heated they often crack from frost. A deep gravel base above any ground water can fix that, but they do freeze and take more that a weekend to warm up. I know of a couple up here that are not useable in winter.
Owen
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GomerPile
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# Posted: 10 Sep 2012 09:47am
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If you use a slab in cold climates you need to properly insulate.
I'm in the begining stages of buiding one (https://bodegabuild.wordpress.com/). I'm nearly ready to pour concrete. I'll be blogging my build at every step.
This guide was very helpful when I was in the planning stages:
http://www.toolbase.org/pdf/designguides/revisedFPSfguide.pdf
This PDF is an article by a builder from ME that does frost protected slabs. It has some great pics to give you an idea about forms etc:
http://bouldermountainresources.org/downloads/Super-Insulated-Slab-Foundations.pdf
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OwenChristensen
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# Posted: 10 Sep 2012 11:04pm
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Not to be a jerk , but insulation doesn't help much. Escaping heat into the gound is what keeps frost away, and will do nothing to keep the slab warm. I know it am a jerk.
Owen
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2680
Member
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# Posted: 11 Sep 2012 12:59am
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never thought of the bad effects in winter.... 
i guess what makes it good in the summer would make it bad in the winter....
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GomerPile
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# Posted: 11 Sep 2012 06:21am - Edited by: GomerPile
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Escaping heat from the building is not what keeps frost away...exactly the opposite in fact. The insulation actually insulates the earths constant temperature of about 50 degrees so frost can not form. This is being used all over the place and was first used in Sweden.
Worst case you have a temperature delta of 20 degrees from the slab to the house. In a small cabin that's only 300 BTU's or so.
This is light years better than post and pier where the temperature difference on either side of your floor insulation could be 60 to 80 degrees!
Frost protected slabs are a proven design and even have prescriptive methods in the IRC.
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OwenChristensen
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# Posted: 11 Sep 2012 07:51am - Edited by: OwenChristensen
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On a unheated slab the slab will freeze itself even more if you put insulation under it. It will make it even harder to warm from below 32 deg. If it is not insulated it might get some warmth from mother earth.
On a house that is heated all winter there is some heat loss into the soil and it does keep the frost away from the edge of the slab. It's simple logic. No disrespect this is just my opinion.
The reason for putting insulation under a slab is to cut some heat loss and save some heating dollars. It doesn't make the slab better. Ten thousand pounds of concrete is a huge heat sink and holds it's temperature for a very long time. This will make it hard to have a warm cabin after the slab is allowed to cool. Anyway I wouldn't use them for cabin.
This is all I'll say here as I want to be friends with you guys. You make up your own mind.
Owen
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GomerPile
Member
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# Posted: 11 Sep 2012 09:10am - Edited by: GomerPile
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There are two classifications for buildings heated and unheated. The unheated option uses more insulation. A cabin that is not occupied parts of the winter should use the unheated option.
In an unheated building you are looking at 3+ inches of foam installed on the sides of the slab with a 2+ foot skirt of insulation.
Couple other things to consider:
+ The base of a FPS is made from materials that are not affected by frost (gravel, sand, etc).
+ The insulated skirt and building divert water away from the building site which leaves less moisture to cause frost heaves.
Your point about the slab being a heatsink is a valid one. When you arrive and get the stove going its going to take a bit of time to bring the slab up to temperature. Once its up to temp it keeps the place warm (thermal mass).
The secret is to design the place to have a bit of solar gain (large south facing windows) you can keep the slab warm without heat.
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OwenChristensen
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# Posted: 11 Sep 2012 06:52pm
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I have thought about 2'' or so tubing in the slab to pump warm air though. You're right on about keeping water away. Frost without moisture is no problem.
Owen
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jjlrrw
Member
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# Posted: 24 Sep 2012 01:15pm
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If you have a choice (starting from scratch) I would do a craw space and wood floor. Our cabin is on a slab, yes cooler in the summer but in the winter it takes from Friday evening to Sunday AM to warm things up. Plus you can run the plumbing and have better access to it for any future repairs.
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TheCabinCalls
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# Posted: 24 Sep 2012 01:31pm
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We built slab on grade. We do heat it with radiant heat tubes, but we also built to take advantage of passive solar. This makes a huge difference. We lost power for a few days and the slab didn't lose much...in part due to insulation, but also from solar heat gain.
Something to think about...when positioning.
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MtnDon
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# Posted: 24 Sep 2012 02:35pm
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How the building will be used has a lot to do with how a slab will perform. As noted slabs are heat sinks. They will hold heat or they will hold cold. They have a slow response time much the same as a laog building. Logs are one of the worst things to build with when the occupancy is sporadic during cold weather. By the time they warm up the weekend is over. The same thing applies to a slab.
Our home is on a slab and We like it. The slab is also insulated under the entire area with 2 inches of foam that extends out from the perimeter as well. That helps keep the temperature of the slab within the comfort range as well as reducing heating and cooling costs.
But for a cabin that will not be used all the time, it is my opinion that large masses of any materials should be not be included in the living space.
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