Best Insulation For Radiant Flooring

Hi all, and thanks in advance for the advice!

I'm currently designing an earthbag home with a rubble trench foundation and a high square footage (about 1700). I plan to do an earthen floor with a hydronic heating system embedded. The home will be built in climate zone 4A (coldish and moist), and I wanted to figure out a cost effective method of insulating under the floor. I looked into R-Tech by Insulfoam, but the cost for a high R value (35.5) is about $5/sq ft. I was thinking about using bagged rice hulls (R value of 3/inch) such as was done in this CoolBot project by HortCRSP and UC Davis in Uganda: https://naturalbuildingblog.com/diy-walk-cooler. Would this work if I built a wood framed subfloor and filled in between the boards with bagged rice hulls (along with probably some recycled cellulose or similar for moisture management)? This would be similar to straw bale walls, but I'm not sure if the boards will bleed heat (softwoods have an R value of about 1.4/inch). For reference, I'm shooting for a total R value of at least 30.

Thank you,
Alex

I also saw that Kelly Hart's foundation was just laid with some scoria. Does anybody have any knowledge of the thermal performance of such a foundation?

Hi Alex,
Welcome to Permies.

I am a fan of the mineral wool insulation. Roxul or Rockwool is brand name. It uses rock as well as slag from making steel to spin fibers into insulation. It is fire proof. Rodents and insects don't like it much, so there is no need to add chemicals for that. It can get wet, and when it dries it has the same R-value as before. It's recycling a lot of waste, so its better than most insulation in being eco-friendly. Oh... and it can breath, unlike most plastic/foam based insulation.

R-value is R30 in a 2x8 batt... so that is over R3 per inch. Than you can pair that with their comfortboard which is used much the same way as those big sheets of foamboard. You put it on the bottom to block the thermal bridging of the wood. If you used the 2x8 batt at R30, and a 5" board... you could get like R50ish on the floor. Or even a 2" board would give over R30 depending on how you want to calculate the supports.  

Downside is its more expensive than some, and it should not be in direct contact with the ground. Although I have put it right on the ground, the company does not recommend it.

That looks to be a good bit cheaper than solid foam. How well does it breath? Are there any extra precautions to make installing it in an enclosed wood subfloor? Also, do you know of any suppliers that will do a bulk discount?

Hi Alex,
Unfortunately I don't know of any places that do bulk discount, but I would check with your local lumber stores and home improvement box stores. Almost anyone up here who sells insulation also has that as an option. There are no extra precautions that I know of. It's less scratchy to install than fiberglass insulation. So less need for long sleeves and stuff. Also, it cuts with something that looks like a large bread knife (I've actually used an old bread knife). They tout it as a whole home insulation system that allows it to breath. I've had good luck, but I don't know any specs or what to compare it to.

Thanks so much, Matt! You've been a great help.

Using bagged rice hulls as insulation between the floor joists in your earthbag home is a creative and cost-effective approach. However, there are some considerations to keep in mind:

Moisture Management: Since rice hulls are organic materials, they can absorb moisture, which may lead to mold or rot if not properly managed. Adding a vapor barrier below the floor insulation can help prevent moisture infiltration from the ground.

Structural Support: Ensure that the wood-framed subfloor can support the weight of the floor finish, furniture, and occupants without sagging or buckling. Consider using sturdy joists and adequate bracing to provide structural integrity.

Thermal Performance: While rice hulls have a relatively low R-value per inch compared to other insulation materials, using a thicker layer or combining them with other insulation materials like recycled cellulose can help achieve the desired overall R-value. Additionally, minimizing thermal bridging by using continuous insulation between the joists and ensuring a tight seal around the edges can improve energy efficiency.

Fire Safety: Rice hulls are combustible, so it's essential to take fire safety precautions, such as installing fire-resistant materials around heat sources and maintaining adequate clearance from ignition sources.

Durability: Consider the longevity and durability of the insulation material in your specific climate and conditions. Ensure that the insulation will remain effective over time and won't degrade or settle unevenly, compromising its thermal performance.

Overall, using bagged rice hulls as insulation for radiant flooring can be a viable option, especially when combined with other insulation materials and proper construction techniques. However, it's essential to address moisture management, structural support, thermal performance, fire safety, and durability to ensure a successful and efficient insulation system for your earthbag home.

An alternative is a cob-perlite subfloor if you have dirt with ~40% clay. You could use a ratio of dirt:straw:perlite of 4:4:2 or less dirt if it has more clay. It would give you at least 1.7R per inch and the upper layers could add to the thermal mass of your floor.
It could be cost effective if you only have to source the straw and perlite (it should expanded, vermiculite could work too). You could add boron and lime at 1 or 2% per volume to fight critters/mold or high water table/spring melt, even though the best would be a water/vapor barrier

I wonder if making rice hulls into charcoal would stop rot and reduce flammability by increasing the ignition temperature.  

You could insulate the perimeter of the house and use all the earth under the house as thermal mass.