Hello everyone!
I had a couple questions regarding a house that I am going to start building this summer. My goal is a low cost energy efficient home. From reading this forum I have learned that you get the most bang for your buck out of insulation. From talking to my contractor (who is also my best friend) and reading about High R-Value walls we have decided that we are going to build 2x6 walls with dense pack cellulose and 4" of exterior XPS. This puts the R-value for the walls around 40, real world probably in the mid 30's. My friend has built many houses, but never with energy efficiency as the focal point.

Question #1: Would adding a 1" layer of PIC on the interior walls be a good idea?

My attic will be trusses with an energy heel and blown in cellulose as deep as we can get it, hoping for r-60+. The second floor ceiling will have no penetrations. The exterior walls of the house will not have outlets or any openings as well. Air sealing will be done with pain staking care, by the person who cares the most, me.

question #2: What R-value should I reach for in my basement walls and sub slab? I also am thinking about using ICF's for the unfinished walkout basement. Is this a good idea?

The house will be three floors. Roughly 900 sq/ft per floor. The lowest floor will be the unfinished walkout basement. The home will have a HRV system. My wife has asthma and IAQ is another important factor. I would like to install a solar hot water heater. I would also like to install a fireplace woodstove with a back boiler to help with the hot water and heat loads if possible. My wife really wants AC but I don’t feel it is necessary in our <500 CDD climate.

question #3: Given this info what type and size HVAC unit should I consider? I like geothermal but is it overkill? I have read that radiant is a waste in super insulated structures. Would my house be considered super insulated?

I would like to avoid having any dependence on fossil fuel. What are my other options? Sorry for the long post. Any advice/info/direction will be greatly appreciated. Thanks for your time.

P.S. I posted this summer about building an EE duplex and got some great info. Plans have changed now after finding a really good deal for a house lot and now my goal is to build an EE home to live in for the next 10-15 years and raise our family in. I am an independent person and would love to be able to add renewable energy in the future to get my home to be net zero energy, but we can't afford it now. Unfortunately money is not unlimited at this point.

PPS I forgot to mention I live in Vermont and last year we had almost 8500 HDD

gmink21;

it all sounds good in theory, bt you won't meet the electric code with no outlets on your exterior walls unless you bring them up thru floor?

HVAC sizing cannot be guessed on this forum by insulation or sq. footage size, it must be done by a professional, who will consider other factors such as geographical oreintation of house, glass area/type, door openings.


We have installed Trane's whole house filter

http://www.trane.com/Residential/Clean-Air/CADR

for customer's who have family that suffer from asthma, it has been an effective system and a once a year filter replacement

The truss manufacturer should be able to give you a range that the height of the energy heel can be. The amount of overhang will affect the height of the energy heel. More height can be obtained by deepening the overhang.

Have you considered adding one-inch of closed cell polyurethane spray to the attic floor and then adding cellulose?

What kind of exterior finish are you going to hang on that 4" of XPS?  Can it support it?

What is PIC?

Where are you located?

What is your roof truss spacing?  If 24" OC, I would be concerned with R60 levels of blown cellulose (~16" deep @ 3.7R/inch) bowing the gypsum boards between the trusses, assuming you are using gypsum sheetrock, either 5/8" or 1/2" ceiling board.

Blown cellulose at 16" settled depth weighs ~ 2.7 lbs/sq ft assuming 2.0 lbs/cu ft density.  That will exceed almost all recommendations for 24" oc spacing of gypsum board.  Another option to accommodate these loads would be to install furring strips 12" OC perpendicular to the trusses and screwing the gypsum board 12" oc maximum.

See this link:  Loosefill Insulation

"Ceiling drywall can sag under heavy loads, such as those sometimes created by insulation. One drywall manufacturer recommends loads of no more than 1.3 pounds per square foot (6 kilograms per square meter) for 1/2-inch (1.3-centimeter) ceiling drywall with framing spaced 24 inches (61 centimeters) on center. The limit increases to 2.2 pounds per square foot (11 kilograms per square meter) for framing spaced 16 inches (41 centimeters) on center and for 5/8-inch (1.6-centimeter) drywall.

Loose-fill cellulose and rock wool, being heavier materials, could cause the ceiling to sag if installed at R-38 on 1/2-inch (1.3-centimeter) ceiling drywall with framing spaced 24 inches (61 centimeters) on center (see chart). Therefore, when deciding whether to use these materials for new construction, consider switching to 5/8-inch ceiling drywall or, if possible, changing your ceiling framing widths to 16 inches on center."

Thanks to everybody who wrote in! Here are some answers to your questions.

Outlets will be placed on the floor along the exterior walls.

I have considered spray foam but I know that it is expensive and with no penetrations on the second floor ceiling I feel some extra time spent air sealing will do the same job.

I have no idea what we are going to use for siding. Any recommendations would be helpful. I have read about other people using 4" xps on exterior and I assume their homes have siding. I need more research on this.

PIC stands for polyisocyanurate insulation which is a closed cell rigid board insulation with a silver lining that is approx r7 per inch.

I live in Vermont which has 8500hdd and 500cdd.

The roof truss spacing and all that weight may be an issue. Would screwing OSB to the second floor ceiling then putting sheetrock over this solve the problem?

Thanks again for your help!

I would glue and screw the drywall to compensate for additional insulation weight, OSB would be overkill

Gluing and screwing the drywall doesnt solve the problem, the drywall could still sag between the trusses. Buy insulation netting and staple it over the trusses to support the weight of the insulation and you can still fill between the top and bottom of the bottom truss chord by cutting fill holes into the mesh. This way the truss chord carries the bulk of the insulation weight, not the drywall. Make sure your truss bottom chord load is not exceeded.

Energy heel height can be whatever you want it to be at any pitch you want, with whatever overhang you want. The higher the heel with the same overhang the higher your fascia board rises and the more cost to make the truss.

Why not just use ICF for all ext. walls??

Great idea using the insulation netting, we will do that. How big of an energy heel should I get? At what point does adding attic insulation start to have diminishing returns?

I don't want to use ICFs for the whole thing because I feel framing will be less expensive. Concrete is not cheap up in VT. Also not sure if I can get the same R-value with a 10" thick wall.

Can anyone advise me as to what type of HVAC system I should look at? Since no one can guess as to the size of the system is there any direction I should look at in particular? I would rather not use oil or propane and NG is not available. Does this just leave geothermal, wood and electric? Is radiant a waste? Having no experience with these insulation levels I really could use some advice on what my options are. Thanks again.

Have you considered a double stud wall rather than just a 2x6 wall to reduce thermal bridging? 4" of EXS on the outside just seems a bit much to me especially if you are going to have anything attached to the outside of the house other than something lightweight like synthetic stucco. And it is much more expensive than cellulose.

Around here, wood framing lumber is cheap, especially 2x4 studs. You could go with the commonly used offset 2x4s 1' oc but with 2x6 or 2x8 top and bottom plates and the 2x4 studs alternating from side to side to give 2' oc spacing for interior and exterior attachment. To get maximum insulation and minimum thermal bridging, just go with double 2x4 walls including top and bottom plates spaced 1/2" apart for an overall wall cavity thickness of 7.5". With cellulose at ~3.7R/inch this gives you R27.75. Now put 1" of XPS on the exterior for an additional R5, except at corners where 1/2" OSB with 1/2" XPS over that for strength. That would give you an overall wall R value of ~30 that I'm sure would be stronger and less expensive that putting 4" of XPS over a 2x6 wall.

And if you went with a 2x3 or ripped 2x6 for the inner wall top plate, this would create a gap of ~1.5" in the top plate to allow cellulose from the attic to trickle down and fill any gaps in the wall insulation that may occur due to settling over time.

Down here in the south, roof truss spacing at 2' oc is common, but we don't have to deal with snow loads. Up north, it may be more common to go with 16" or 19.2" oc to accommodate those loads. This closer truss spacing combined with 5/8" gypsum ceiling board could probably tolerate R49 cellulose without any extra measures.

I second the suggestion about using two 2x4 walls rather than one 2x6.

Since you plan to use very thick cellulose in the attic you may want to consider building a catwalk and a storage area in the attic about one foot above the insulation.  The catwalk should be about 2 feet wide and run the entire length of the house.  The catwalk is very handy for searching for roof leaks and installing future items. 

gmink21, Is it really cheaper in your area to build a double framed wall, with multiple layers, and types, of insulation, than to just use a 6 1/2" SIP? Your actual energy efficiency will not likely be any better with a "DFW", and maybe not as good, as with SIPs. I suppose that if you can get good quality DRY framing lumber at a cheap price, and your labor is FREE or CHEAP, you might save some money. However, if the latter is true, that will also help you budgetwise if you decided to use SIPs for your walls. Anyone who has the skill to build straight, plumb walls, utilizing the DFW approach should have no problem putting up SIP walls.......in less than half the time.......with the bonus of interior sheathing for easier installation of drywall (less waste), trim, cabinets, etc.

In regard to roofs, it's hard to beat SIPs if you want vaulted ceilings, and/or conditioned attic spaces.

In any event, good luck with your project, whatever building materials that you decide to use.

Thanks for the advice everyone! After talking to my contractor he feels more comfortable with a 2x6 with the external insulation. Since I can install the insulation outside of the foundation perimeter it saves me some concrete costs as well.

I will look into the SIPS. Once I get my house plan finalized I would love to get a quote to see how they compare cost wise to my other option.

Has anyone had any experience installing/using wood stoves with back boilers?

Like these: http://www.stovesonline.co.uk/wood_burning_stoves/Aquatherm-Eco-insert-boiler-stoves.html

I would love some information from someone that has installed one of these over here (N.A.). Thanks!

Did you ever decide on what you are using for an exterior finish?

REM design is an excellent computer simulation program & valuable for determining how much insulatiuon to use in different locations. Rather than choose materials (ICF) calculate the R values you are looking for & look for the least expensive way to get there.. ICFs will give you R4 per inch of insulation.
Bob I

For exterior fininsh do you mean siding? If so then I would like to use a fibercement board like Hardiplank Lap siding. Need to talk to my contractor to see if it is possible to install over 4" of rigid board.

I am interested in looking into the double stud wall rather than the 2x6 with rigid board. Since I will be installing the insulation myself how difficult is it to dense pack cellulose in a double stud wall? My contractor is concerned that over time the cellulose will settle and I will be left with large gaps in the walls. Also how do double stud walls handle moisture in a 8500 HDD climate?

How do headers for windows and doors work with a double stud wall?

I have decided the only place I will consider using ICF's will be for the basement, and only if this is a less expensive option then pouring a traditional one then adding exterior rigid board. I know half of the concrete cost is forming and half is the concrete so will using ICF's and installing them myself be less expensive then the traditional route?

Thanks for the help!

As far as I know, Hardi only allows installation over 1" of foamboard unless they have changed things recently.

You cant dense pack with a typical cellulose rental machine, usually those Force 1 or Cocoon machines.

One wall is structural, the other is non structural.

I've never seen or heard it done before, but couldnt one staple insulation netting across stud bays at mid height to effectively reduce the height of the cavity and reduce the amount of cellulose settling. I never took physics so I dont know if its possible or not.

gmink21,

Google "Larsen Truss, Superinsulated wall design on Build It Solar". You will find excellent information there that will answer most of all the items brought up on this forum.

Rich Melius

Hello Rich, thanks for the input. I have read about the Larsen Truss built by Robert Riversong and was very interested in its design. In an old post it was brought to my attention that this is not an easy way to build and that Mr. Riversong is an exceptional builder. Complications may arise when he isn't directly involved, primarily in the cellulose walls. I would lke to build this way, but not sure if my contractor has the technical ability.

The 4" of XPS on the exterior which puts it well into the Class-II vapor retarder category with a permeance of about 0.5. If you add foil or vinyl faced insulation (permeance less than 0.1) on the interior you will create a classic double-vapor retarder mold farm, because the structure dries too slowly in either direction.  In your location as long as you have half of the R value in exterior foam you don't need (indeed shouldn't have) an interior vapor retarder. But with the same layup and only 1-2" of XPS on the exterior you would.

The additional thermal break of interior iso isn't going to dramatically affect your whole-wall R-value/heat loss.  You can gain more performance than that by shrinking the glazed area a few percent.

BTW: Most people in the trades call it iso, polyiso, and (rarely) PIR.  This is the first reference I've seen for PIC.