Hi. Planning a small slab on grade house in Wisconsin. I'm trying to use as much insulation as makes sense (reasonable payback etc). Am leaning on 4" XPS foam (R-20) under slab. Walls will be around R-40, ceiling R-60 or more. I can't find a good explanation as to why the passivhaus people are using R-50 under the slab, while code doesn't require ANY. Quite a discrepancy. I'd like to know what the payback would be on using R-50 vs. R-10 or 20...?????? Any comments appreciated! -m

One argument is that with slab on grade, your floor slab is only a few R away from the ambient outdoor temperature. So it should be insulated close to wall or ceiling values.

"floor slab is only a few R away from the ambient outdoor temperature"

I'm not sure I understand what you mean....

Thanks for the reply.

How many feet does exterior cold have to travel to get from the air to directly under the slab? What is the R value of that path?

So you're saying that if one uses "wings" of insulation extending out from the perimeter that has a high enough R-value, then what you put under the slab doesn't matter?

With either wings or vertical insulation going downward, it will matter less. As frost protected foundations show - you can easily get to the point where the ground never gets to freezing. This is a "mild climate" and thus needs less slab insulation.

An important point with insulation - put some everywhere before putting lots anywhere.

See table 2, p10

http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

(thermal bridging is included on those numbers, not center-cavitiy Ron studwalls.)

Thanks to both of you for responding.

And thanks for the link to the article. I missed that one when I was on their website last week. They mention briefly that slabs don't require as much as walls or basement walls because of the lesser temp. differential. I guess maybe it is that simple. Seems like research is needed...

I did find a good one on their website on high-R wall systems:

http://www.buildingscience.com/documents/information-sheets/high-r-value-wall-assemblies

Thanks again.

Posted By pbrane on 17 Jun 2011 01:00 PM
"floor slab is only a few R away from the ambient outdoor temperature"

I'm not sure I understand what you mean....

Thanks for the reply.

Ambient stands for room temprature i suppose.....

Their has been lots of research on slab temperatures... There still needs to be more, but the PHPP afaik models subslab insulation quite well. The reason some of the passivhaus' go with such 'crazy' levels of subslab foam is to meet their energy targets. You will notice that Straube/BSC's approach is to go with insulation to the point where the cost of foam roughly matches the price of PhotoVoltaic.. I'm compressing that thought extremely, but that's the gist. Check out legalett or insulated raft foundation or frost protected foundation on google for some examples.. There's no real need to go with XPS.. EPS is fine in most cases, and you'll most likely need to go with a structural engineer anyways, so you might as well get the EPS.

Lots of good info there. I find this cost data interesting:

http://www.legalett.ca/0520.pdf

Also makes me wonder why one couldn't use clean coarse gravel under a slab and use the entire area as a hot air duct. A very low cost radiant system that uses a common air furnace.

Posted By boardom on 05 Jul 2011 10:24 AM
Their has been lots of research on slab temperatures... There still needs to be more, but the PHPP afaik models subslab insulation quite well. The reason some of the passivhaus' go with such 'crazy' levels of subslab foam is to meet their energy targets. You will notice that Straube/BSC's approach is to go with insulation to the point where the cost of foam roughly matches the price of PhotoVoltaic.. I'm compressing that thought extremely, but that's the gist. Check out legalett or insulated raft foundation or frost protected foundation on google for some examples.. There's no real need to go with XPS.. EPS is fine in most cases, and you'll most likely need to go with a structural engineer anyways, so you might as well get the EPS.

That's about as compressed as it gets- good work!

Decompressing it only slightly, the BSC folk argue the sub-grade EPS costs more than photovoltaic power lifecycle cost combined with current heat pump efficiencies & lifecycles, but the combined error bars on those assumptions are also quite large.  If using ~3 cents/R/square-foot used EPS (reclaimed  from commercial building re-roofing, etc), even PassivHaus levels of sub-slab R still make economic sense in Straube's net-present-value calc of PV + heat-pump lifecycle energy costs.  At the operating cost of resistance-electric-heating, even  using ~10cent/R/foot virgin-stock EPS there's still a long term NPV argument for PassivHaus sub-slab R values in higher-cost electricity parts of the US (but not where electricity is very cheap.)

But the error bars on discount rates & energy inflation built in to any present-value calc are also large.  It's generally better to err on the high-R side compared to code (where Straube et al are still reasonable), and sell the higher R on enhanced comfort and energy inflation hedge rather than a least-cost economic argument.  BSC's recommended R values meet the comfort challenge quite handily at the local climate extremes, and are at a level where window & door U-values and air-infiltration rates would then dominate the heat transfer in most houses.

Posted By Dana1 on 05 Jul 2011 03:59 PM

Posted By boardom on 05 Jul 2011 10:24 AM
Their has been lots of research on slab temperatures... There still needs to be more, but the PHPP afaik models subslab insulation quite well. The reason some of the passivhaus' go with such 'crazy' levels of subslab foam is to meet their energy targets. You will notice that Straube/BSC's approach is to go with insulation to the point where the cost of foam roughly matches the price of PhotoVoltaic.. I'm compressing that thought extremely, but that's the gist. Check out legalett or insulated raft foundation or frost protected foundation on google for some examples.. There's no real need to go with XPS.. EPS is fine in most cases, and you'll most likely need to go with a structural engineer anyways, so you might as well get the EPS.

That's about as compressed as it gets- good work!

Decompressing it only slightly, the BSC folk argue the sub-grade EPS costs more than photovoltaic power lifecycle cost combined with current heat pump efficiencies & lifecycles, but the combined error bars on those assumptions are also quite large.  If using ~3 cents/R/square-foot used EPS (reclaimed  from commercial building re-roofing, etc), even PassivHaus levels of sub-slab R still make economic sense in Straube's net-present-value calc of PV + heat-pump lifecycle energy costs.  At the operating cost of resistance-electric-heating, even  using ~10cent/R/foot virgin-stock EPS there's still a long term NPV argument for PassivHaus sub-slab R values in higher-cost electricity parts of the US (but not where electricity is very cheap.)

But the error bars on discount rates & energy inflation built in to any present-value calc are also large.  It's generally better to err on the high-R side compared to code (where Straube et al are still reasonable), and sell the higher R on enhanced comfort and energy inflation hedge rather than a least-cost economic argument.  BSC's recommended R values meet the comfort challenge quite handily at the local climate extremes, and are at a level where window & door U-values and air-infiltration rates would then dominate the heat transfer in most houses.

But dont you think going for photo electric effect and power would be a bit expensive..... ?

Of course PV is expensive, which is the crux Straube's criticism of PassivHaus regarding under-slab R values:

At the current installed price of virgin-stock EPS, going to the full PassivHaus levels on sub-slab insulation the additional heat saved is more expensive than what it would cost to replace that heat with PV + heat pump.

At the levels BSC/Straube is recommending (about 1/3-1/2 of where PassivHaus would end up in most designs) there is still a reasonalbe long-term economic rationale using current heating-fuel types & system efficiencies, even with very low fuel inflation and discount rate assumptions.

Although the  facts you mentioned are beyond the level of my understanding, any ways thanks for the information....

Is this suppose to reduce the energy bill on your home?  The people that hired us to renovate their home wanted me to looking into more efficient ways to insulate their homes.  If this helps create an even temperature in the home then my clients are all for it.