Which of the three options should I choose for my subfloor?

[torparavgrund]

Conditions, wood house with plank frame from -29, 80 sqm 8x10 meters:

• Crawl space: concrete foundation wall, from ground to floor structure, at most about 1.6 m at least about 0.4 m. This will be covered with construction plastic film + insulation intended for the purpose.
• Ground floor structure: consists of about 10 beams across the width of the house, which in turn rest on 3 beams along the length of the house, which in turn rest on stacked natural stone. This is mostly stable and healthy, but need to be repaired in some places where there is rot damage due to condensation where wood meets stone.
• The beams themselves: round sides but hand-hewn flat on the top and bottom. That is, they have this shape: (_). A measurement like "cc" is not simple; the upper beams' cc is about 700-800 mm, but all beams are different thicknesses and wedge-shaped lengthwise, i.e., they are 200-300 mm at the thick end and 150-200 mm at the thin end.
• The shape of the beams means that a traditional method (trossbotten) with boards on the underside and then laying board or roofer’s felt from above results in both a triangular-shaped space at the bottom because of the beams' round sides, and extra work to get the width measurements right (because the beams are laid “offset” so that the ends are thick - thin - thick, etc.). Therefore, the traditional solution with “sections” would be a lot of extra work.

Below are three solutions for the blind bottom as alternatives to trossbotten. Only diffusion-open solutions, so materials like foam board, fiber cement, facade gypsum, etc., are excluded.

6mm oil-hardened "trossbottenboard" (Bolist), 122 x 55 cm boards, cost 7100 kr excluding home delivery
The commonly suggested solution.

• Method: screw or nail to the underside. Place roofer’s felt over joints across the beams for even better support from underneath.
• Advantages: small and manageable, I hate dealing with large sheets. Thick and robust. Proven material.
• Disadvantages: the most expensive option, and my budget is very tight. There is also likely to be a lot of waste because the size is completely worthless for me; with 122 x 55 cm it is either too narrow or too long to fit between the beams in a rational way, so the final price is likely to be even higher.

3mm oil-hardened board (Byggmax), 122 x 244 cm boards, cost 2645 kr excluding home delivery

• Method: same as the previous
• Advantages: the price (!), I can probably manage with less waste than the smaller boards.
• Disadvantages: feels quite thin, much greater risk of sagging but I hope to avoid that with the roofer’s felt.

Roofer’s felt 17x90 (Byggmax), 5752 kr (including 15% waste), excluding home delivery

• Method: nails or screws directly up against the beams, if there is a planed side it is placed downwards towards the foundation.
• Advantages: further stabilizes the floor structure. Easy to install with presumably less waste.
• Disadvantages: Requires a lot more screws, and consensus seems to be a much higher risk of mold growth. The previous trossbotten of mixed timber had no mold growth though, and I hope to further improve moisture conditions with insulation + ground plastification.

So, what would you choose builders? Do you also think I should cover the finished blind bottom with something like AC350 wind paper to further improve condensation safety + avoid cold drafts?

 

[torparavgrund]

I now see that I stated the wrong price for the råspont, it should be about 1050 linear meters, so approximately 7100 SEK.

 

[KnockOnWood]

Maybe you should consider construction plywood?
About 80 SEK/m2
A bit more stable than those thin sheets, but dense and good.
Tape the seams with duct tape for airtightness.
And then loose-fill insulation, which fills well.

 

[Autodidak1]

Otherwise, styrofoam and fibercement are diffusion-open...

But I would have done the classic with raw boards for your house. Personally, I have used foam plastic with good results on a couple of constructions.

 

[torparavgrund]

Otherwise, styrofoam and fiber cement are diffusion-open...
.

Well, I understand that they aren't completely diffusion-tight, but the question is how diffusion-open are they? I get the impression that wood materials breathe much better, and moisture safety seems more important than optimal insulation. (Fiber cement is also the most expensive option).

By the way, what do you think about wind barrier AC350 on the outside?

 

[torparavgrund]

Maybe you should consider construction plywood?
About 80 SEK/m2

I have considered both construction plywood (6450 SEK) and OSB (about 3500 SEK), but read in similar threads about subfloor that it is not recommended because the glue in the boards starts to deteriorate after a few years in the humid environment, causing them to eventually warp and/or fall apart. What do you think?

 

[KnockOnWood]

I used construction plywood in our 19th-century house when I renovated it in 1983. But I also replaced the half-rotten floor joists and installed real two-inch floor beams. And when we sold and moved in 1998, it was still perfectly OK. Before my renovation, there was regular masonite (3.5mm?) as a subfloor (or was it boards?), and it sagged and hung down, almost coming loose. On top of the masonite was newspaper, to prevent the sawdust, which was supposed to be insulation, from falling into the foundation. The newspaper pages were dated 1915, with horrifying war images from the ongoing World War I

 

[torparavgrund]

@KnockOnWood Yes, one has seen one's fair share of newspaper and sawdust. However, in this house, they ignored the newspaper, resulting in some sawdust here and there on the plastic in the foundation. The plastic has been there for maybe 5-8 years, and unfortunately, I seem to glimpse that in some places they laid the plastic on old semi-decayed sawdust, so it will probably be quite gooey when I replace the plastic (Replacing it for two simple reasons: 1. it was poorly installed from the start, so I would at least like to redo it properly, and 2. I see there are small holes and tears in some places).

The old trossbotten, by the way, was likely original from '29, as it consisted of all sorts of small planks of various poor quality, mostly with live edges and some with red paint left, while others even had the bark still on. However, I managed to find some lengths I will keep: one-inch boards that are certainly 400 mm wide, which look like new!