Remodeling the office (new floor, ceiling & adding insulation) passive house retrofit

[Daox]

I understand why you'd do this, but no superinsulated house I've seen does this. Do you know why?

[S-F]

Does what?

[Daox]

All the superinsulated houses I've seen must have the dew point in the middle of either a blown in cellulose or blown in fiberglass wall.

[S-F]

Yeah, that's fine. The dew point has gotta be in the wall somewhere. The problem is when that dew point is behind a vapor impermeable substance.

I just thought I'd add that the local up tight building inspector here let me put only 1" of polyiso on the outside of a wall that already had something like R35. He wasn't concerned with the ability of the wall to dry because of the massive hygroscopic value of all it's cellulose.

[Daox]

So, if I blew in say... 10" of cellulose for ~R37 on the inside, then put 3" of foam on the outside at some time in the future I would be fine.

[S-F]

You'd be better than fine!

But the dew point exercise is still very valid. There is a method of building called REMOTE or PERSIST which entails building a house, warping it in ice & water and then installing 100% of the insulation on the outside. There is no insulation in the walls. This all started in Canada or Alaska. Somewhere really cold. They wanted to completely eliminate the possibility of the dew point being inside anything that could rot.
That's where I got the idea for wrapping my board sheathed house in bitchathane. There will be some cellulose in the walls because cellulose makes for a healthier house but the brunt of the work will be done by the foam. It sucks but I don't think it sucks as much as tearing out every exterior wall. I'm not going to put myself or my family through that nonsense.

Also, you're talking about something like an R 60 wall. That's a lot of work you'd have to go through to get the 10" cavities in every room. It would be a gut renovation. Even if you have R 1 bazillion, you like an a 1,000 Sq. Ft. house with wall so thick you end up with a 1oo Sq. Ft. room, if it's not air tight, and I mean really really air tight, it's not going to pay off. The difference between R30 with 1 ACH @ 50 Vs. R 1 bazillion with 2.5 - 3 ACH @ 50 might not be noticeable.

[AC_Hacker]

Originally Posted by S-F

...So it's hard to calculate where a dew point will be at a certain temperature. There might be a table somewhere which lists dew points for specific R value walls at different temperatures and humidity levels...

Try THIS LINK for all your dew-point calculation needs.

-AC

[Daox]

So, even with the dew point in the cellulose there won't be issues?

As you can see, I'm still pondering what exactly to do. For the office I have as much room to do basically whatever I want. For the upstairs, as I said I have small rooms. I don't want to loose much room on the exterior walls. However, I think I can live with loosing 6" (though less would be better if you have any suggestions). With my rough cut 2x4s that gives me a cavity of 10" depth and I can get darn close to R40 that way. If I do the outside I can decide what to do then, its not on the short list of things to do though.

In the meantime I will work on sealing as that is still quite bad in the house. However, I wonder if that wouldn't be easier to do from the outside... Suggestions?

[S-F]

Originally Posted by AC_Hacker

Try THIS LINK for all your dew-point calculation needs.

-AC

I was thinking about saying something like "this is the kind of thing AC_Hacker would have" but I didn't because I didn't want to heap any responsibility on you.

Turns out you came through any way! Ha ha.

[S-F]

Originally Posted by Daox

So, even with the dew point in the cellulose there won't be issues?

As you can see, I'm still pondering what exactly to do. For the office I have as much room to do basically whatever I want. For the upstairs, as I said I have small rooms. I don't want to loose much room on the exterior walls. However, I think I can live with loosing 6" (though less would be better if you have any suggestions). With my rough cut 2x4s that gives me a cavity of 10" depth and I can get darn close to R40 that way. If I do the outside I can decide what to do then, its not on the short list of things to do though.

In the meantime I will work on sealing as that is still quite bad in the house. However, I wonder if that wouldn't be easier to do from the outside... Suggestions?

Like I said earlier, there is always a dew point somewhere in a wall. It doesn't matter if it's in cellulose of fiberglass or foam. You just want it to be able to dry out which doesn't work so well with a vapor impermeable material on the outside. This is why vapor retarders, like kraft paper and poly, are on the inside of the wall. If your XPS isn't air sealed too well you might be able to get away with 10" of cellulose inside. FWIW I wouldn't do it to my house though. If you're going to go through all of the trouble of actually, properly doing this from the inside you're going to have to actually do it properly, How are you going to deal with rim and band joists? If you're opposed to foam on the outside, and I couldn't blame you for that, then you might think about a Larsen truss instead as that way you take care of all those issues in one fell swoop. It's hard to deal with band joists without spray foam. There are reasons why these deep energy retrofits costs 80 - 100 K. And those price tags usually entail the easier method screwing 4" of polyiso to the outside. That's my recommendation in your case. Unless you are really willing to go for a gut renovation. You might want to invest in a blower door. all this work will be for naught if you don't get the entire house down to near passivehaus levels. Shoot for 0/6 ACH@ 50 and don't get heartbroken if you can't make it below 1.0. The top plate transition is going to be difficult. That area is tough to tackle with spray foam.