We are in the process of planning our superinsulated dream home.  Around 2200 SQ/FT single story on a daylight basement.  As a DIY project, I really like the idea of ICF construction as it allows me to work at my own pace in assembly and a lot of followon steps are eliminated.  But I am looking for a little better than the R50 or so it will give me in the walls.  My plan is for the main living area to have a framed 2X6 insulated wall inside the ICF for around a total R70 wall. With around R45 in the floor and R74 in the ceiling it should take almost nothing to heat(Radiant heat, pacific northwest).  I did some searching on this forum and the interweb, but have not come across anything similar yet. 

Anyone have any experience with a project such as this?  If so I would love to hear about it.
Can anyone point out any obvious pitfalls(other than initial cost) to a plan such as this?

Thanks
Ron

Ron,
What type of finish do you want for the exterior - stucco, brick, HardiePlank, etc.?

I think the wall would perform better if most of the insulation was on the outside.  Dana1 would be the best to answer my question about this. 

Other than a cavity for insulation, is there another reason for the 2x6 wall inside?

The exterior will probably be Hardi.

The 2X6 would be for wall cavity insulation.  It might be a little more effective on the outside, but way easier to design and build on the inside and less material IMO.  My wife and I are also doing this project as much on our own as possible to cut costs, so building inside the ICF envelope on the main floor is a little more relaxed as we will not be feeling the pressure from weather...

 

Why use 2x6? the ICF layer surely is adequate structure! All you want the inner wall to do is insulate and maybe add air sealing. Suggestion use 2x4 spaced away from the ICF by 2 1/2" insulate the result with 2 layers of Roxul bats, the layer in the space r10 bats for steel studs (the extra width covers the outside of the studs) and r15 bats as the inner layer this would add about r22.5 to the whole wall assuming a framing fraction of 15% for the 2x4 wall which should be built without structural headers and only single plates and all the ideas of "advanced framing" even if you choose 16" OC If you built it out of 2x6 and used r23 Roxul and similar minimum wood it would only add r17 (r23 from a moisture issue perspective).   The savings in lumber cost probably pay for the slightly greater insulation cost and then some. The disadvantage is an extra 1/2" of wall thickness. With either choice you'll still need to air seal the interior, with air tight drywall or equivalent, a step that ordinarily is unnecessary with ICF. What you are doing is more like a wood wall with exterior foam from a moisture issue perspective. What you'll need to consider is condensation on the inner surface of the foam in wintertime.You can add too much, air permeable, inside insulation and create problems! Where in the great PNW are you, winter in Spokane are far more extreme than Seattle. Colder climates mean lower wintertime minimum temperatures and call for more insulation outside the condensing surface ( innermost surface below the dew point of inside air). For example a 40 degree dew point and 70 indoor temp ( about 35%RH and, kinda high for really cold weather but very comfortable) allows wintertime lows down to -20f if 2/3 of the total r is on the outside.
You would have this with r25 cavity insulation and r50 ICF.
From a cost effectiveness point of view ceiling insulation is a better deal, that's why it's usual to see r values of 50% or so greater than wall r values. With r 70 walls I'd expect to see r100 ceiling.

Posted By Ronmar on 11 Mar 2013 01:25 AM
We are in the process of planning our superinsulated dream home.  Around 2200 SQ/FT single story on a daylight basement.  As a DIY project, I really like the idea of ICF construction as it allows me to work at my own pace in assembly and a lot of followon steps are eliminated.  But I am looking for a little better than the R50 or so it will give me in the walls.  My plan is for the main living area to have a framed 2X6 insulated wall inside the ICF for around a total R70 wall. With around R45 in the floor and R74 in the ceiling it should take almost nothing to heat(Radiant heat, pacific northwest).  I did some searching on this forum and the interweb, but have not come across anything similar yet. 

Anyone have any experience with a project such as this?  If so I would love to hear about it.
Can anyone point out any obvious pitfalls(other than initial cost) to a plan such as this?

Thanks
Ron

Love radiant floors! Unfortunately in this house they would almost never come on so the floors would still feel cool. If you build a house you can heat with a candle - radiant would be serious overkill.

"Love radiant floors! Unfortunately in this house they would almost never come on so the floors would still feel cool. If you build a house you can heat with a candle - radiant would be serious overkill."

I Absolutely agree! Have you done a heat loss study? I suspect that the heat given off by even two occupants will be all you need, especially anywhere near the coast say Seattle or Portland where winter temperatures rarely even get below 20 f. Even in Spokane a Mitsubshi hyper heat mini split will be more than enough. Your floors will be essentially room temperature regardless of how you heat.

From a simple cost view you would be far better off with 2x4 framing for both walls. With r50 ICF you are looking at over $6/sqft just in the cost of the foam! Expanding my r40 wall, which costs about $4/sqft of material, to achieve r 70 is simply a matter of making it 7" deeper. My walls are 12" including drywall but no exterior finish material so r 70 would mean 19" deep walls but at really low cost ( the added cost of the insulation is about $1.50/sq ft of wall). With r50 of XPS your ICf must be a 14" wall or more adding 2x6 is already 20" or if you followed my suggestion 20 1/2"". It's such a shame that the very best windows are r7 and will account for the huge majority of your heat loss. FWIW from outside in, my wall is Tyvec, fiberboard sheathing, 2x4 stud wall filled with Roxul for wood studs, 3 1/2" space filled with Roul for steel studs plywood sheathing detailed as primary air barrier, 2x4 stud wall filled with Roxul for wood studs, drywall.

This is great guys, this is exactly the type of discussion I wanted to start to see how far out of line I am thinking.

Liebler, I am in Port angeles at about 350' above sea level, and winter temps rarely drop below 20F. The 2X6 definitely isn't needed for structural, and you are absolutely correct it could be accomplished with spaced 2X4 and enough structure to support the inner drywall. The plan is a vapor barrier immediately outside the drywall completely encompassing the living space. If I did it right, my first cut at heat loss numbers tell me just shy of 4K BTU/HR with 65F inside temp and 30F outside temp. That calc is for still air and was done without accounting for skylights though and a very rough estimate of window/door area(havn't finalized window selection) and no accounting for air exchange losses.

The delay of transfer thru the ICF mass was making me think that the inner ICF skin will have difficulty getting below dew point in this climate. Is the figure for typical adults heat output around 500 BTU/HR? If so I am figuring at a minimum I will need to makup at least 3K BTU/HR, or a little under 1KW worth? Not quite a candle, but low enough to allow me to run a lower temp heat storage scheme(lower delta and less loss). It will also make future solar augmentation a little easier.

The overall loss numbers are why I have not planned on the ceiling being significantly greater insulated than around R74. I don't see the need to have the ceiling in this case significantly more insulated than the walls, especially in a radiant heated structure with lower overall room air temps. I will afterall not be pumping in 120F+ air from a furnace that immediatly heads for the ceiling to escape thru that insulation... We had originally looked into a dome home with a superwall construction of near R75 overall. Planning and positioning inside a dome was a very conflicted process and I was repeatedly having to compromise. So we kept the only thing i really liked, the superwall, and sought to incorporate that into a more conventional construction.

I see the concern with the radiant not hardly needing to run though. I guess this would allow me to increase the air exchange...

All input greatly appreciated
Thanks

You really don't need any air sealing inside the permeable insulation at your low temperature of 20 degrees the indoor dew point could be as high as 53 degrees at 70f a relative humidity of 54% you should be running the HRV to dry out. Also I agree the concrete in the ICF will pretty well average out daily temperature variations and even on the coldest day the average temperature will probably never be below freezing. I used to live near Portland Or. and visited customers pretty much everywhere in the PNW.

Liebler, I am not familliar with the acronym "HRV", what does it stand for?

Thanks

HRV= heat recovery ventilator. A really well sealed house requires forced ventilation a heat recovery ventilator heats the incoming air by transferring the heat from the exhaust air. With an HRV only sensible heat is recovered, Most nigh performance houses either use an HRV or an ERV = energy recovery ventilator which also transfer latent heat as well as sensible heat.

HRV, Thanks, didn't recall that one. That is also part of the plan since back when we first started planning a dome. Thought it would be great to grab the warmest air from the top of the dome and pump it outside thru a heatexchanger to use that heat(wasted at the top of the dome) to heat the incomming fresh air. We are looking at a vaulted ceiling in the greatroom and I was planning the same warm air collection from the peak of that ceiling. The floorplan so far incorporates a vertical access trunk adjacent to the greatroom that will house this duct and provide an easy access pathway from attic to basement for future solar...

HRV, Thanks, didn't recall that one. That is also part of the plan since back when we first started planning a dome. Thought it would be great to grab the warmest air from the top of the dome and pump it outside thru a heatexchanger to use that heat(wasted at the top of the dome) to heat the incomming fresh air. We are looking at a vaulted ceiling in the greatroom and I was planning the same warm air collection from the peak of that ceiling. The floorplan so far incorporates a vertical access trunk adjacent to the greatroom that will house this duct and provide an easy access pathway from attic to basement for future solar...