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Subsurface rigid insulation and boring critters

I was speaking with my passive house consultant the other week, and she expressed concerns about boring critters and subsurface rigid insulation. She had such an experience with some insulation in her garden (It was put in one season and removed at a later time as she got back to completing the project.)
Now, I know all about the ate/termite protection used in these insulation products, but what about larger critters that would love to have a warm dry home?
My consultant is thinking that having a concrete porch extending beyond the sub-surface insulation (of a frost protected slab may help - maybe having the insulation encased with gravel beneath the porch?
Or is this just a non-issue?
I would think that a area that doesn't freeze would be quite attractive to mice/groundhogs/moles etc.

Asked by Steve Young
Posted Aug 16, 2014 5:43 PM ET


25 Answers

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The wisdom of placing load-bearing elements, or increasingly whole houses on quite fragile materials like foam is something we won't know for some time. Animals, insects and foam-creep. Is it in most climates still a fairly safe bet? Have we created a time bomb which compromises the longevity of the structures? No one really knows, but it's fair to say that they aren't as robust as all less energy efficient methods that preceded them and should be looked at as a gamble.

Answered by Malcolm Taylor
Posted Aug 16, 2014 8:18 PM ET


Foam glass is an option.

Answered by Debra Glauz
Posted Aug 16, 2014 10:39 PM ET


Not to be too depressing, but the foam is unlikely to add or subtract from underground invasion. I have had a skunk dig under a front door pad (warm and dry), a possum under a patio slab(stone cold), and a very determined woodchuck that moved at least two cubic yards of pea gravel out from under my garage and at least as much from under the garage slab of the neighbors house. The entry point was on the downhill side -UNDER the footing! The neighbor's floor slab collapsed to the point that you could see with a mirror and flashlight that a large portion of the slab was floating above the excavation Ms. Woodchuck had done. Whether she or a relative was working on my own garage was never determined. I attempted to inject a lot of concrete down the hole I could see and under the unsupported edge of the slab. It only took a week or so before she had re-grouped and popped up through a gap I had yet to resurface. The house was eventually sold and demolished which settled the issue.
External attacks on my home included a raccoon that ripped off a gable vent to enter the attic, numerous woodpeckers searching for ladybugs under my siding, a mother squirrel that chewed through my soffits next an outdoor light to nest one year, then the following year mounted a direct attack on the roof where a gable valley met the main roof. Anti-pigeon roost wire and boards with nails in them didn't faze her. I finally stopped her with a Mad Max form of aluminum flashing I tacked in place. It looked a bit like a child's version of dragon teeth, but it worked.
If you don't allow trees and bushes to provide a natural highway to the roof it will slow them down and make alternate choices more appealing. We had food, water and shelter in abundance with convenient access to the ground. If you still have time for preventative measures, it MIGHT work to bury rolled up small grid wire mesh end to end around the perimeter of the patio. Take a roll and cut off enough to make two to three turns about 24-30" in diameter. Skew the inner turns by pulling on the inside corner so the grids cross each other to make the open size smaller. Pin the skewed coil with some zip ties and then squash the whole thing into a flat oval. Do this to as many sections as you need to go around the perimeter and bury them in ground as deeply as practical.
Vertical is best, but an angled trench going away from the bottom of the slab might be easier.
The mesh will snag the claws of the invaders and their attempts to move sideways or back will meet the same conditions. If they are especially determined they might ultimately find the edge, but hopefully by then you are aware of the attempts and might help dissuade them with a dose of fox urine (not a nice product), moth balls (kid danger) or common ammonia cleaner. There are of course methods more final, but we are the invasive species, so do what your Karma tells you to do. Have fun, I am facing pack rats now.

Answered by Roger Berry
Posted Aug 17, 2014 1:24 PM ET


Good luck with the pac rat. They are more intelligent than humans. I had a buddy who matched wits with a pac rat for eight months. He finally caught it live in a trap. He'd gathered so much respect for the pac rat he couldn't kill it. He released the rat, snow on the ground with a barn off in the distance. The rat headed directly for the barn.

Answered by Debra Glauz
Posted Aug 17, 2014 2:16 PM ET


Perlite and perlite concrete are the Most economical insect proof insulation!

Answered by Jerry Liebler
Posted Aug 18, 2014 8:29 PM ET


'Very interesting responses to my query, although no one actually indicated that they had vermin in their foam. I sure hope that Roger is no where near where I wish to build ;-)
I like the idea of the foam glass, but I would probably only use it in the frost-protecting-more-exposed-area, and use flowable fill and concrete board and wire mesh to protect the sides.
I like the idea of perlite , but when a asked about it a year or so ago, it seemed rather difficult to source outside the Pacific Northwest.

Slab Idea001.pdf 486.08 KB
Answered by Steve Young
Posted Aug 22, 2014 9:49 PM ET
Edited Aug 22, 2014 9:50 PM ET.


Anyone who proposes using spray foam or any foam under a footing, slab or anywhere in contact with the soil is asking for some very expensive trouble.



Answered by Richard Beyer
Posted Aug 22, 2014 11:26 PM ET


To me, foamglass is prohibitively expensive while perlite concrete or perlite geopolymer is more economical than even EPS yet much stronger, almost in the foamglass league.
There are dozens of perlite expander s scattered throughout the USA. With a bit of Google searching I'm sure you'll find one within 500 miles or so of anywhere in the US.. I've gotten prices from Illinois, Ohio,, Missouri, Pennsylvania, New Jersey (I'm in Kentucky).

Answered by Jerry Liebler
Posted Aug 25, 2014 12:04 AM ET
Edited Aug 25, 2014 12:11 AM ET.


Google has been my friend. I found the Perlite Institute, and they list a perlite expanding facility in Ohio and have documents regarding placing the material under slabs. This is pretty cool. It seems that the more perlite that one puts into concrete, the less load that it can handle (and the insulation value drops).
I saw, a while ago folks using the bags of perlite under a house. I like that idea, but want to see how it can be done with a frost protected slab (no frost walls).
I noticed in this drawing that they have two optional poly vapor barriers.


I wonder if one had well drained gravel under the perlite, whether that would be enough to keep the stuff dry. Yes, I know that a vapor barrier above the insulation is required. And I suspect that the material must be kept dry in order to maintain its thermal resistance.
More research ahead.....

Answered by Steve Young
Posted Aug 25, 2014 6:47 PM ET


Below Grade Rigid Foam:

Building scientists and the 2012 IRC has approved rigid foam insulation under slabs and not only that but they require it for Zones 4+. The sky is not falling when it comes to dealing with termites, rats, and whatever subterranean creature is lurking in the ground. Or is it the ground is not falling? Either way. The build-up is quite simple. If using rigid foam underneath a slab, one would use 4" of compacted AB under the rigid foam, which is also a deterrent to critters. Spraying the soil prior to installing the AB and rigid foam is good practice. Most importantly is using TREATED rigid foam. Most use Borate but treated foam is a must.

If what has been claimed is true then thousands of homes will experience major problems and someone should contact the building scientists and IRC committee and let them know they need to re-write their books. No more foam underneath slabs! Good luck in Zones 4+ with your cold energy-robbing slabs. Rigid foam is evil!

Let's not forget about rigid foam in SIPs. If rigid foam should not be considered as a long-term structural answer, then the entire SIP industry is in trouble.

Answered by Peter L
Posted Aug 26, 2014 5:42 PM ET
Edited Aug 26, 2014 8:10 PM ET.


"Good luck with the pac rat. They are more intelligent than humans."

I think the above is stretching it by a bit. Maybe saying they are more intelligent than "some" humans but I haven't seen any pack rats driving cars or engineering bridges but they day that I do, we are all in trouble...

Answered by Peter L
Posted Aug 26, 2014 10:37 PM ET


We have the choice of wishful thinking that plastic foam is durable and will survive termites and such or using thicker layer of a poorer insulator in perlite concrete. Even the 8 part perlite to one part concrete mix has more compressive strength than the best foam (most soils to) and it is definitely going to stop any termites.

Answered by Jerry Liebler
Posted Aug 26, 2014 11:06 PM ET
Edited Aug 26, 2014 11:10 PM ET.


Praise be to hardware cloth. It comes galvanized. 1/4 inch, 36 inches wide, laid full perimeter, pinned with geotech fabric pins, burried under the garden, with grass, geo cloth/gravel or pavers on top has solved my problem on the ground.

Rodents like to be close to the structure the dig under. Keeping the hardware cloths at 36"wide means a borrow hole is too far away from cover, so its exposed to predators and they will avoid digging there Even 18" will have this effect. The inside edge of the hardware cloth needs to be anchored to the buildings perimeter. The 1/4" will stop rodents and snakes.

Answered by flitch plate
Posted Aug 27, 2014 12:12 PM ET
Edited Aug 27, 2014 12:15 PM ET.


"Building scientists and the 2012 IRC has approved rigid foam insulation under slabs and not only that but they require it for Zones 4+. The sky is not falling when it comes to dealing with termites, rats, and whatever subterranean creature is lurking in the ground".
Peter L, you left out this section of the code. It covers a large section of the country and economy. Could you direct us to a thouroughly tested and approved method of protecting below grade foam.

R318.4 Foam plastic protection.
In areas where the probability of termite infestation is "very heavy” as indicated in Figure R301.2(6), extruded and expanded polystyrene, polyisocyanurate and other foam plastics shall not be installed on the exterior face or under interior or exterior foundation walls or slab foundations located below grade. The clearance between foam plastics installed above grade and exposed earth shall be at least 6 inches (152 mm).


1. Buildings where the structural members of walls, floors, ceilings and roofs are entirely of noncombustible materials or pressure-preservative-treated wood.
2. When in addition to the requirements of Section R318.1, an approved method of protecting the foam plastic and structure from subterranean termite damage is used.
3. On the interior side of basement walls.

Note: Lines defining areas are approximate only. Local conditions may be more or less severe than indicated by the region classification.

FIGURE R301.2(6) TERMITE INFESTATION PROBABILITY MAP Includes California plus 7 southern states.

Answered by Debra Glauz
Posted Aug 28, 2014 11:59 AM ET



Your argument needs to be taken up with the 2012 International Residential Code & the International Energy Conservation Code and its contributors. I don't write the codes and if they are contradicting themselves, then that needs to be taken up with them.

To date, the only protection for the foam is using Borate treated foam and spraying the soil area with Termiticide. There are no guarantees in life and this includes any home. Even if you don't use foam below grade you can have significant termite damage to a wood frame home from subterranean termites as well as flying termites. Plenty of flying termites have made their ways into gable and soffit vents where they destroyed wood trusses and sheathing. The homeowner doesn't see the damage because the termites came in from the top of the home through the attic and soffit vents. The solution: don't vent the attic and soffits. Well, we know that doing so can result in rot/moisture issues. So around and around we go...

Answered by Peter L
Posted Aug 28, 2014 1:28 PM ET
Edited Aug 28, 2014 1:30 PM ET.


Someone might want to let these guys know what they are doing is wrong...


Answered by Peter L
Posted Aug 28, 2014 7:11 PM ET


Those boys up north are pretty sharp. What might they be doing wrong?

Answered by Debra Glauz
Posted Aug 28, 2014 8:24 PM ET


Well one thing they didn't do was take much heed of the fun we have all been having with builders using absurd amounts of underslab foam - but perhaps that's for another day.

Answered by Malcolm Taylor
Posted Aug 28, 2014 10:04 PM ET


Well, this thread degenerated somewhat. No one pointed to documented cases of foam hosting beasties, but an ounce of prevention still seems to be warranted.
And why all the hate for perlite? It seems like a very green product, although there are still some questions. Presumably it is still a medium to be burrowed into. There also seems to be engineering questions. But isn't that what this forum is all about - to share knowledge and experience? I think there is a good case to use the stuff under a frost protected slab, or a layer of perlite concrete just beneath the standard concrete of the slab.

Answered by Steve Young
Posted Aug 29, 2014 7:10 AM ET


Here's something else to think about... http://aem.asm.org/content/79/23/7313.full

Answered by Richard Beyer
Posted Sep 2, 2014 12:33 PM ET


"why all the hate for perlite? " A very good question!.
Possible answers: The petrochemical mega industry has most of the building industry "carrying it's water". Simple ignorance of perlite's properties and advantages. Resistance to change, we've always done it this way. The herd instinct ie. everybody else is using foam.

Regardless of the why, once you know about perlite, the smart course is to use it!

Answered by Jerry Liebler
Posted Sep 2, 2014 8:19 PM ET


Jerry, It sounds interesting but I can't find much information referring to how you engineer it as fill. I keep seeing the same picture of a foundation full of unopened perlite bags. Can you compact it? What bearing capacity does it have?

Answered by Malcolm Taylor
Posted Sep 2, 2014 8:40 PM ET


Here is another way of doing under floor perlite:
From what I've found this is more or less the technique used under many European Passivehaus projects. This site clearly indicates it can be compacted which they recommend for thickness greater than 4"
Compacted and confined by perimeter walls and a concrete slab above it will have compressive strength comparable to sand just by what it's made of and easily tested with a penetrometer. While it's compressive strength has not been completely characterized it has been proven at least as good as the best of foams. and more than adequate to support a floating concrete slab.
FWIW I plan to combine the" leave it in the bag" with the loose fill methods filling the deliberate gaps between bags with loose perlite then compacting the result. I will follow the Shundler recommendation of cardboard covering the perlite followed by poly then poured concrete. The perlite will be above a "drainage" layer of pea gravel and separated from the gravel by a geotextile filter fabric.

Answered by Jerry Liebler
Posted Sep 2, 2014 10:46 PM ET
Edited Sep 2, 2014 11:24 PM ET.


I'm with you, but if it can be compacted and will test out at an acceptable proctor I'm just wondering why, unlike all other fill materials, I can't find any data as to what its range of compaction is.
I'd hate to get it in, call out my geo-tech before pouring the slab, and find out it couldn't meet the required loading.

Answered by Malcolm Taylor
Posted Sep 3, 2014 12:23 AM ET


This may help: http://www.idosi.org/wasj/wasj25%2812%2913/19.pdf
How much compaction is needed is dependent on the desired strength. Based on figure 1 of the above paper and knowing that 1 megapascal is approximately 145 PSI shows that 10% compaction is probably more than sufficient to outperform plastic foam. 25% compaction will achieve approximately 145PSI, 30% enters the elastic region at 435 PSI

Answered by Jerry Liebler
Posted Sep 3, 2014 12:18 PM ET

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