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Changing off grid house to grid and HVAC

mikeysp | Posted in General Questions on

My friend built a house for off grid living, but after his first summer in it, has run electricity to it with plans for A/C. He is in zone 4a near Nashville. His construction assemblies are not ideal with lots of air gaps and is on piers with no skirting. Currently, he DOES NOT have most of it sided, and very little interior wall covering. So what is seen on most of the exterior is the cheap tarp style woven housewrap and on the interior, the kraft facing of his R-19 bats (Wall and Ceiling). His ceiling is cathedral with the R-19 Bats and there is no planned gap or baffles between the R19 and roof decking, but there appears to be a little bit o gap naturally because of the R-19 in the 2×8 cavities. I told him that he will certainly regret it if hew just marches forward and covers what he has up.

I am unsure how he should proceed. He is doing it all himself or with friends and is ready to do what he needs to to remedy the situation, but his pockets are shallow, so I am trying to identify what he NEEDS to do, what he OUGHT to do, and what is NICE to do for a po man like him.
I have not offered any specific advice, but told him that fixing his assemblies for proper air tightness and insulation is a minimum necessity, or he will have a cup of ice sitting on a picnic table on a hot humid day – condensation, mold., rot… a big headache.
Here is an idea I have, but I really need advice, as I do not want to cause him any additional troubles:
1. Pull exterior housewrap and siding where installed 
2. Seal OSB.
3. Use Tyvek housewrap.
4. Remove R-19 from cathedral ceiling and air seal rafters to OSB from interior using a liquid applied sealant.
5. Scab the rafters with additional 2×4 for deeper channel and install 1″ rigid foam board 1″ air channels between foam board and OSB and then install R-30 Fiberglass batts.
6. Ignore interior wall air tightness since his tin and rough cut 1x will not be conducive to this. Count on an admiral job on exterior air sealing.
7. Underneath house, I have no idea on how to improve this, He is on the side of a hill in the back, sits on piers.
Other ideas I have mulled in my head:
8. To achieve interior wall air sealing also, use air sealed OSB inside, painted black under 1x finish walls or unpainted under his tin wall coverings… or use drywall instead of 1x and tin.
9. Put foam board over housewrap to give him thermal break and increase R-value.
Of course, each of these fixes increases his costs and he is not well off and progresses as he makes money since doing all cash out of pocket. 
What do you think of my ideas?
What are the needed vs nice ideas?
If you have another approach, please advise.

PS. his county has no building codes, so solutions just need to work and will not be inspected. 

Thank you.
-Mike P. on behalf of my friend Mike S.

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  1. Expert Member
    Akos | | #1

    You definitely need a semi decent air barrier under rough cut. Your idea of either interior drywall or OSB is a good one. You can get there with a well detailed smart vapor barrier but that might be more work than it is worth and easy to damage.

    For the roof you are correct that it needs venting. More than venting it needs an air tight ceiling though. Don't get caught up on larger elaborate vent channels and such, makes sure there is one ( cheap foam box store baffles are good enough), that it goes from the soffit to the ridge and vents are sized properly with more area at the soffit than the ridge. With R19 batts installed properly in the ceiling cavity it is probably good enough as is even without vent baffles.

    A ceiling that doesn't leak air, needs very little venting. No amount of venting will get you out of trouble with a leaky ceiling. So focus on air tight ceiling.

    For the ceiling interior air barrier, you could go with 1" foam with taped seams instead of OSB/drywall. That would bump up your whole ceiling R value, assuming 16OC framing, from R20 as is to R26. You can would still be able to easily nail the rough saw through the foam into the rafters and also some of the foams are black which would not show through the gaps.

    For the exterior, taping the sheathing is a good thing. There is nothing wrong with felt for house wrap, it is cheap and it works.

    Taping the seams of the subfloor in the house would also help. Generally an insulated shallow foundation is better but a lot more cost.

    1. mikeysp | | #3

      Ok, thank you very much for the advice. He is very grateful and was glad to hear the ceiling can be improved in both R and air by adding a meticulous foam board between the kraft bats and the 1x finish ceiling.

  2. Expert Member

    Mike P.

    There is no point air-sealing the roof sheathing to the rafters if you are providing a ventilation gap. Move the air-sealing to the interior, and as Akos said, do a meticulous job.

  3. Jon_R | | #4

    +1 on adding vents to this bat filled, unvented cathedral ceiling.

    Review how to find air leaks with a window fan (vs an official blower door).

    1. mikeysp | | #5

      Thanks for that Jon. A poor man's blower door test would be splendid for helping him tighten up his leaky house.

  4. mikeysp | | #6

    How critical is the interior wall air sealing? If his exterior wall sheathing is tight, is he good?

    1. Expert Member
      Akos | | #7

      Warm side air barrier falls into the belts and suspenders category. Since the air sealing wasn't part of the original design, it is hard to get perfect. If your sheathing is tight and your fiberglass install is good, you don't need it

      If it is a cottage that won't be occupied full time, I would worry too much about it. If it is a home, I would put up a layer of thin OSB or drywall.

      P.S. If your energy costs are high, going with a layer of foam instead of the OSB/drywall on the walls similar to the ceiling would be worth it.

      1. mikeysp | | #8

        Ok, great. Akos, are you saying can he use foam board on the "inside" wall between the kraft paper on the fiberglass bats and his finish wall?

        Ifr yes, can he use either "Expanded Polystyrene Foam" or "XPS"? I assume he cannot use Poli-iso?

        Thank you.

        1. Expert Member
          Akos | | #9

          Pretty much. You are in a mixed climate so foil faced (impermeable) insulation might not be the best. I would aim for either EPS (thin EPS is still somewhat permeable, XPS or un-faced/felt faced polyiso. If you have a tight budget you can get reclaimed roofing polyiso, which is usually felt faced, for pretty cheap.

          If you do go with foam insulation, remember to move your device boxes forward so they'll be flush to the finished surface otherwise you'll have to put extension rings on them.

        2. Expert Member
          BILL WICHERS | | #10

          Note that a lot of the thinner EPS out there, especially the stuff commonly stocked by the box stores, will have a poly facer that will make it vapor impermeable. You’ll want to be sure to get UNfaced EPS if you go that route and want the wall to have some drying ability.


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