Insulation below subfloor heating plates

kavakia

We are near completion of a staple up radiant heat system using the heavy gauge aluminum plates (8” spacing) attached to the subfloor and 1/2” pex going through the groove. We are located in California climate zone 3C. Whole system is installed above a vented crawl space with piers and perimeter foundation walls.

We are determined to encapsulate the crawl space (assume 12-20mil thick vapor barrier). Contractors in the area don’t typically insulate the walls when encapsulating. 

My main question is whether we should 

1. insulate the interior foundation walls with EPS AND not use anything below the plates OR
2. no insulation on interior walls and use R19 batt insulation below the plates 

I have read a lot of different opinions on this website (ie use 1” space below the plates and use radiant barrier before batt insulation or using batt and then use foam boards below the joists but chatgpt insists on #1 above and my non scientific mind doesn’t seem to agree with it ie lots of heat getting wasted in the crawl space.

what do think? which option would you go with and why? any other option I am missing? 

HydronicMike

No space and no vapor barrier on the insulation-right up against the pex.

As far as the crawl space, encapsulation means floor and walls. But you’ll need to provide some air exchange.

kavakia

Thank you Mike so you do recommend option #2 above, correct? ie encapsulate the floor and walls (with no insulation against walls as most contractors offer in the area) and place batt against the pex between the joists with no air gap

kavakia

and yes i am aware of the air exchange requirement. We will need it for either option 1 or 2

Alan (California Radiant) Forbes

Yes, I like #2. We did a house in San Anselmo this way and used RockWool in the joist bays. Since plates heat by conduction, you can push the insulation all the way up against the PEX. There is another product called UltraFin that heats by convection where you need to leave a space for air to circulate.

Make sure you roll up the insulation at the ends to fully insulate the rim joist.

Kaos

Fist off, radiant foil does pretty much squat, it is about R2 when shiny, R1 once you have a couple of years of dust on it. Not wroth the cost or labor to install.

Unless you are in heavy radon area or flood zone, it is always best to insulate the perimeter of the crawl space and condition it. The idea of isolation the house from the crawl it is near impossible the real world even if you spray foam the subfloor. A vented crawl will always use more energy.

As long as you air seal the crawl space well (ie taped rigid on the stem walls, pieces of rigid sealed in place with canned foam at the rim joist), the crawl will have very low load. You still don't want to get it extra toasty though and without some insulation under the places, the crawl will get pretty hot. I would get the cheapest staple up batts you can find and put them right against the plates to limit the heat flow. R value doesn't matter, pretty much anything is good enough. Actually thinking about it, that radiant barrier might be good enough as well.

Conditioned crawls need to be conditioned, so don't forget either a vent to the house or a dehumidifier.