Climate Zone 7A – on the shores of Lake Superior – Lutsen, MN. 

I’m working on a crawlspace renovation for a cabin build in the 50’s as a part of an energy retrofit (project scope includes 3″ continuous rockwool on walls, triple pane windows, new heating system, bathroom addition).  It’s a 4-6 ft tall crawlspace, previously insulated with 2″ XPS on the poured concrete foundation walls, with 6 mil poly laid on the dirt floor, and a ramshackle sump pump of sorts placed at the lowest area of the dirt floor.  

We have removed fiberglass batting in the rim joist cavities, insulated with 4″ XPS and air sealed with expanding foam.  We dug an interior perimeter trench, installed interior drain tile and a proper sump and pump to the exterior.  We have air sealed all gaps and seams in the existing 2″ XPS on the walls, and taped 10 mil poly to the foam just above grade level and lapped over footings.  We are laying dimple matting on the uneven dirt floor and covering with 10 mil poly, lapped and taped onto wall poly.  

The owners would like to upgrade their existing and failing heating system (off-peak electrical Steffes thermal storage heaters).  They would like us to install under floor staple up hydronic heat, powered by an electrical boiler.  Their original request was an air-to-water heat pump but our local heating contractor is quite hesitant to install an air-to-water unit until the larger manufacturers start supplying them.  We have agreed to use an electric boiler for now and upgrade later as the technology becomes more widely accepted and used.  

I have read a fair amount about proper crawlspace renovation, both on GBA and on Lstiburek’s articles on Building Science Corp.  

I’m understanding that the above approach to encapsulate (air-seal) walls and floor and insulate the walls is best practice, though I’m not finding any advice on how to properly integrate first floor heat.  My heating contractor is requesting insulation batting under the staple up hydronic heat to encourage the heat to move upward.  The flooring above is subfloor and doug fir t&g.  

We will be installing an ERV as a part of the energy upgrades.  I have read in Lstiburek’s articles that we could either exhaust air from the crawlspace into air handler ductwork and install a passive air grille that allows air to move from the conditioned space above into the crawlspace below to condition it, or install a continuous bath fan to the exterior with a passive air grille to the upstairs for makeup air. 

The questions I have are:

-Does this seem like an ok assembly?
-Will the insulation in the floor joist cavities pose any thermal, condensation, or other issues that I’m not foreseeing?
-How much and what type of insulation would be most appropriate?  Would we want to send any heat downwards?
-To condition and ventilate the crawlspace, would it be best to exhaust to the exterior or exhaust into the ERV?

Thanks for your help!