Insulating open crawl space of straw&loam house

Hi permies,

I want to insulate the floor of my self-built small straw&loam house, but I'm having a hard time figuring out what materials to use best.
Would love some experienced advise!

The specifics:
--> The house is in a cold & wet climate (The Netherlands).
--> The first layer of ground is 5 cm gravel and about 30 cm of sand - no concrete anywhere and no foundation going into the ground.
--> The house is built on scribed poles, standing on big pieces of rock, see photo 1 that I took during construction. (there's a whole house on top now)
--> There's a 35 cm crawlspace, completely open on all sides. This means full ventilation ánd that rodents have full access if they want - enclosing is not an option.
--> The floorjoist are burned and oiled for extra durability.
--> The floor itself is buildup of 18mm underlayment, flax underfloor and 20mm planks. (it's been here about 7 years and no moisture problems on the exposed underlayment)
--> I'm ok with using XPS/PIR/rockwool stuff - as long as it is recycable. So sprayfoam is not an option for me.

I'm debating between these options:

Option 1: 15cm rockwool insulation between the joists, dampfoil and mice mesh on the bottom of the joists. My worry here is that the dampfoil encloses moisture..
Option 2: 15cm rockwool insulation between the joists, 3cm XPS board with foil on bottom of joists. For airtighness and makes batting inaccessible for mice. Plus mice mesh. Same worry as option 1.
Option 3: 15cm XPS between the joists. Will have gaps, for which I could use purfoam but I'd hate that so I'd take my losses. With or without: dampfoil / mice-mesh.

What would you recommend?

Hey Ivanka,

Welcome to Permies! I think I like option 1 the best even though there will be thermal bridging. Rockwool is resistant to mice and other pests but they will go right through XPS and . If you could somehow protect the XPS that would be great but involve crawling around under the house.

With our pier and beam cabin I did something similar but our crawlspace has more clearance than yours.
 

Rockwool does produce a rigid stone board product called Comfortboard 80. It's difficult to find around here. I don't know about its availability in the the Netherlands. If you could somehow attach that to the underside of your joists that would solve the thermal bridging problem and be pest resistant. In addition to its availability another downside of comfort board is it more fragile than foam board. If you don't support it well while installing it the panels have the potential to tear.

If you devise a very short sheetrock lift device to slide a panel under the house and then pin the panel to the underside of the joists that could work.      

Thanks for the replies!

@Aaron, with your pier&beam floor insulation setup, are you not worried about moisture getting trapped in your rockwool and joists? Or do you have a vapourbarrier between your joists and decking?

The rigid rockwool board is a great idea, there's one available here called "RockFloor Solid".
Any second layer insulation on the underside of the joists I will pair with anti-mice mesh, the dutch mice love even rockwool I can tell you.
The difference then would be rigid rock wool being more dampopen, than the XPS
I did some calculations on dampdiffusion, and think I'll go for a damp-open setup. Since my dampbarrier on the warm side is just the 18mm multiplex, that means no XPS for me since it would lock moisture in.

So I think I'll go for:
- rockwool between joists
- rigid rockwool on underside joist
- damp open climate foil
- anti-mice mesh

Ivanka Annot wrote:
@Aaron, with your pier&beam floor insulation setup, are you not worried about moisture getting trapped in your rockwool and joists? Or do you have a vapourbarrier between your joists and decking?

Not too worried. The primary vector for moisture getting into a house is through the foundation being in contact with the ground. With pier and beam the house is separated from the ground and the open crawlspace allows for plenty of air flow and drying. I think a vapor barrier under the deck would run the risk of trapping moisture and potentially leading to deck rot.

Ivanka Annot wrote:
I did some calculations on dampdiffusion, and think I'll go for a damp-open setup. Since my dampbarrier on the warm side is just the 18mm multiplex, that means no XPS for me since it would lock moisture in.

So I think I'll go for:
- rockwool between joists
- rigid rockwool on underside joist
- damp open climate foil
- anti-mice mesh

Sounds like a good plan. I think fully vapor open is the way to go in this situation.