I am trying to gain info on Passive home construction. There has to be more to it than good insulation, window and doors. We are trying to make plans to build our next house. We will be building in Washington State, 4c for climate I believe. We want about 1000 square foot or so, livable space, not counting 15 inch walls. Many questions arise: 1 If I build with double stud wall will I need to vent it or can I fill it with blown in fiberglass? 2. Insulating the slab, floor,attic how much is optimal ? 3 If I use hardi-plank will I need furring strips on outside for better care of siding? To get an airspace behind the siding. We are open to suggestions..... If anyone has info on this type construction I would like to hear about it. It seems everyone wants 2400 to 3600 square foot homes. We dont want a home that big.

you can check out this blog for a similar project:

http://greenedmonton.ca/MillCreekNetZeroHome

regarding how much insulation is optimal - it depends - where you are, what kind of orientation your house will have, how much glazing, what wall to floor ratio....and the list can go on. luckly you can prety much model everything these days and you can see how your house will perform by using several pieces of software. for passive houses nothing can beat PHPP

http://www.passivehouse.us/passiveHouse/DesignTools.html

for hygrothermal behaviour WUFI is accepted as one of the best software simulations

http://www.wufi-pro.com/

PHPP is $225 and WUFI is free (a simpler version).

Ideally you'd have a professional taking care of these simulation and designs but if you can find your way with a computer and if you have some engeneering background and have some understanding of phisics you can design it yourself.

good luck
adi

"There has to be more to it than good insulation, window and doors."
There is - much more & too much to go into here. Two important items are air tightness and minimizing thermal breaks.

Seems to me that lapped siding always has some air gap. WUFI is of limited use since air in/ex filtration is the primary issue.

While there are many good simulation tools for estimating energy use on home designs, there's no substitute for the PassiveHouse tools for getting the design truly to PassiveHouse levels. They're pretty cheap (especially compared to what you might pay in over-design of some parts if you design it with other tools.)

Wintertime temps in most of western WA that dew-point calcs or WUFI simulations and interior vapor barriers aren't usually necessary for dealing with vapor-diffusion condensing on the exterior sheathing (except at altitude). But for yuks, what's your ZIP code?

Seasonal RAINFALL in most parts west of the Cascades (except maybe in the Sequim rain-shadow) is high enough that it's worth setting it up with rainscreen gap between the sheathing and siding though, since exterior weather penetration and exterior vapor drives are still potentially damaging to the exterior sheathing (and the siding). As a secondary benefit the rainscreen gap also releases wintertime moisture diffusion from interior vapor drives at a much higher rate. Having lived & built in western WA & OR I'd rainscreen any house I was planning on living in myself, even though it's not required by code. (A 10mm rainscreen IS required by code up in B.C- same climate, different country.)

You'll probably need to skin it in at least some exterior foam to get sufficient thermal break at the band joists & foundation. Rigid EPS is the cheapest foam in R/$ terms, and provides the highest drying-factor for the structural sheathing, but if you go more than 2-3" the fastener spacing on the furring to support fiber-cement siding is tighter, becoming it's own thermal bridge. In the forested PNW going with back-primed spruce or cedar would be half the weight, and would hold paint very well, protected by the back-ventilation of the rainscreen. (Peeling & blistering paint on wood siding is usually caused by higher-moisture content in the wood increasing in vapor pressure rapidly with daily temperature swings. Back-primered/painted it won't wick up water from rain events quickly, and back-ventilated it dries quickly and the siding stays MUCH drier than when it snug up against #15 felt or housewrap and sheathing.)

Air tightness and the quality of both primary & secondary air barriers is essential for achieving the necessary air-tightness spelled out in the standard (<0.6ACH/50). It's not particularly hard to implement (and is pretty cheap) if done all along as you go, but nearly impossible to achieve as an afterthought.

Passive houses can be constructed from dense or lightweight materials, but some internal thermal mass is normally incorporated to reduce summer peak temperatures, maintain stable winter temperatures, and prevent possible over-heating in spring or autumn before the higher sun angle "shades" mid-day wall exposure and window penetration.

The recent green home tour here in Seattle featured an owner/builder locally constructed passive house. He used larsen trusses for the wall system and blown-in fiberglass.

http://www.ecobuilding.org/gp/green-pages/dan-whitmore

http://www.ecobuilding.org/guild-chapters/seattle/green-home-tour

Hey BK.

I just built in the area using SIPs , I did a passive solar design with a slab and I also used fiber cement siding . I would also recommend using furring strips after the research I have done. THINK about how you will vent TOP and BOTTOM of your siding before hand and it will save you frustration during your building.  I tried a pre-made rain screen  fabric in the beginning and did not like it. I ended up using wood furring strips but if I had to do it again I think I would try a steel furring strip I found by an outfit called "Furring master". And on your slab ... Fully insulate it. And THINK about the details like.. How will you insulate above grade.. how will you cover the insulation above grade..  how will you protect the insulation below grade. etc. etc. The more you do your homework before you start -and make your decisions , the better off you will be. Read the archives on this site. The message I remember reading more than once is.. think about the details and then think about them some more . This is so true, because the people (contractors and suppliers) you will be dealing with mostly can't think outside the box,  so if you really want a house that is a cut above the rest. YOU need to know what you want. 

Would you build with SIPs again ? I am leaning towards double stud construction with 15 inches of fiberglass insulation ? All my building has been in Arizona, so wet climate is new to me, LOL.

So far I like the SIPs.  Time will tell if I have any problems. I thought about the double stud construction but my knowledge of building overall and vapor transmission was not sufficient for me to go with that type of wall. To my way of thinking (especially in this climate) moisture transmission is a huge issue, and I wanted to minimize potential missteps with building procedures that are relatively untried. Even standard 6x stick frame construction has ongoing debates on correct building methods and people are still learning how to keep their house from becoming a mold laboratory.

I am with VB.

Hey VB,
I have been recently pondering exactly what you brought up in your post about foundation insulation detailing. I plan on putting 4 inches of foam board on the exterior foundation walls and I'm a little confused about how to transition to the finished exterior. I plan on either zip sheathing (or OSB and Tyvek) and then 2 inches of foam board, then furring, then hardie siding. will a Z flashing take care of this? Thanks for the info!

Posted By vb on 18 May 2011 09:27 PM
So far I like the SIPs.  Time will tell if I have any problems. I thought about the double stud construction but my knowledge of building overall and vapor transmission was not sufficient for me to go with that type of wall. To my way of thinking (especially in this climate) moisture transmission is a huge issue, and I wanted to minimize potential missteps with building proceedures that are relatively untried. Even standard 6x stick frame construction has ongoing debates on correct building methods and people are still learning how to keep their house from becoming a mold laboratory.

The moisture transfer stuff is fairly well-modeled in a climate & material specific way using tools like WUFI. (See:  http://www.ornl.gov/sci/btc/apps/mo...sof161.htm ) There can't be too much real debate left on the vapor transfer stuff amongst those who have been paying attention.  WUFI is fully capable of garbage-in-garbage out, definitely not idiot-proof (you have to provide the correct data), but even non-rocket-scientists are capable of using it properly.

Design it to be resilient to air leaks, but build it to water and air-tight levels and the mold-factory issues go away.  Designing it  to be as vapor-permeable in both directs as can be allowed for the drives found in the local climate fixes a lot of construction faults.  Improper use/placement of unnecessary and highly impermeable materials seems to be the bigger problem in some high-R assemblies.

SIP construction is not immune to mold ('ceptin' maybe the steel-skinned types), nor are they inherently air-tight. Implementing the flashing correctly is the first 99% of the  "correct building methods".  The vapor transfer stuff counts, but it's the moldy frosting on the deluge-cake, almost never the ultimate driver of mold production.

In double studwall setups in most of the lower 48 placing the most vapor retardent material somewhere in the middle works. Exactly where in the middle is optimal varies with local climate, but nowhere does it need to be 6 mil poly.

To accommodate this, the assembly is
designed to be able to dry to the interior through the use of semi-permeable
latex paint on the interior gypsum. Drying to the interior is important,
therefore, interior vapor barriers should not be installed.


Dana,
First of all, thank you for the education! I am confused about vapor barriers in cold climates. The above is from Building Science Corps cold climate case study and they seem to suggest a vapor barrier is a no no in cold climates. I have read exactly the opposite elsewhere, but I tend to believe these guys. Can you comment on this one? Thanks again,
Scott

With regards to your question about requiring furring strips outside for better sliding, I believe yes you still need it. Should you need the step-by-step procedure in installing hardi-plank... check this site: ehow.com/how_7155211_installation-instructions-hardiplank-siding.html

______________

LogCabinDirectory is the #1 site for log homes.

Hoch, Maybe somebody else can be more specific with your question since I am still learning, even after building my house. But here is what I did. I used ICF's for the stem wall and stopped the pour 4" from the top. I then cut 4" off the inside of the ICF and poured my slab all the way to the foam on the outside, effectively insulating my slab. When I set my plate I set it to the inside of the foam so I have an un-insulated plate,which I consider a flaw. So what I ended up with is about 1 1/2" of foam that sticks out past my siding. I used custom made z flashing to cover this. So in essence I have a little shelf 10" up from grade. It's not perfect but it looks ok and it does the job. Oh and one other thing. Before flashing I used a Parge coat (kind of like stucco)on the foam. It was very easy and kind of fun to apply and looks good. I hope this is not too confusing a description.

Posted By hoch09 on 23 May 2011 09:55 PM
To accommodate this, the assembly is
designed to be able to dry to the interior through the use of semi-permeable
latex paint on the interior gypsum. Drying to the interior is important,
therefore, interior vapor barriers should not be installed.


Dana,
First of all, thank you for the education! I am confused about vapor barriers in cold climates. The above is from Building Science Corps cold climate case study and they seem to suggest a vapor barrier is a no no in cold climates. I have read exactly the opposite elsewhere, but I tend to believe these guys. Can you comment on this one? Thanks again,
Scott

It's only cold for part of the year, even in US zones 6 & 7, and putting a sub- 0.1 perm layer anywhere in the assembly impedes the ability of the assembly to dry.  Unless you are adding humidity to the interior air (or the house is so tight) that the interior is above 70%RH @ 70F, only the portions of the structure with average temps below 40F for a month or so are at all susceptible to moisture accumulation via air leaks or vapor diffusion from the interior.  If you build an air-tight interior, you can adjust the interior & exterior permeance to optimize seasonal drying in both directions, but placing a limit on the moisture accumulated during the colder months increasing the resiliency of the assembly to air leaks or modest bulk water intrusions.  The particulars will vary with local climate (Whitehorse is a lot colder than Winnipeg, which is lot colder than Chicago, etc.)  But it's possible to define a stackup that works for 99% of the cool-weather parts of the US.

A vapor barrier can be full of holes and leak quit a bit of air, but still behaves as a vapor barrier, impeding drying.  If the vapor retarder is ~ 1-10 perms (semi permeable), the moisture can still find a way out over a season of less-cold weather.  Moisture drives in summer from the exterior can be quite high when sun hits rain or dew-wetted siding (particurlarly for stucco, brick, or fiber-cement), leading to condensation on an interior vapor barrier and rotting studs, even in climates well north of the 48th parallel.  With more modest interior vapor retarders this doesn't happen (much).

In many locations 5-10 perms on the interior will be too permeable, but most can do fine between 0.5 perms and 3perms, depending on the rest:

Keeping the exterior layers above ~2 perms, and adding a ventilation gap (aka "rainscreen") between the sheathing & siding improves the exterior drying path considerably, helping the assembly keep up with ( stay ahead of0 moisture diffusion from the interior, even during the colder months.  The cold exterior air is much drier on average, with a much lower vapor pressure.  With rainscreened siding bulk leakage from the exterior dries quickly, and the dry air in the gap draws moisture from the assembly.

Cellulose insulation can buffer well over 15% of it's dry weight as moisture without damage or losing effectiveness.  With an air tight interior and semi permeable exterior even standard latex paint is an acceptable interior vapor retarder in much of the US, but in fat double studwalls the drying of the buffering cellulose can still take a long time.  In many US climates putting a ~1-2 perm vapor retarder (such as 0.75-1.25" of XPS) on the exterior if the interior studwall (where it is protected from accidental penetration, and can be detailed as an air barrier) with only high perm interior paint lowers the average & peak moisture content of the assembly.  Some superinsulation builders prefer to use 6mil poly in that spot, but that's less optimal in all but the coldest US climates.

Canadian code requires a sub 0.1 perm interior vapor barrier, even for insulated below-grade foundation walls.  This is a mistake, IMHO, since semi permeable stackups of foam + prermeable-insulation studwall protect the foundation sill (and studwall) better, and are easy to design for any climate.  (BSC has a bunch of good info on that approach too.)  But to their credit, Canada is one of the only countries to require rainscreens (10mm min) on timber framed structures.  This would be of benefit in both heating & cooling climates in the US that receive more than 15" of rainfall, or 5000+ heating degree-days per year independent of rainfall.

I forgot to mention: In thick cellulose applications it's best to not rely on it's moisture buffer too heavily. The primary cause of settling is creepage from moisture cycling- the less moisture it's asked to buffer, the less likely it is to sag. This can be overcome by increasing the installed density, but in super-permeable wall structures in cold climates the required density can become impractical. Some amount of vapor retardency would be required, but nothing so restrictive as poly.

Charlene, vb and Dana, thanks for the responses!

Wow. So much info to take in. I want to build a very tight well insulated house. I built the home we are in now, 2x6 and Hardi-Plank, it seems  pretty well insulated. We had fiberglass sprayed in. Now I am considering a double wall with hardi-plank siding and trim.
Will I be correctin building it with 2x6 exterior and 4x8 OSB sheer around entire home. 2x4 inside walls.I was thinking of 15 inchwall thickness. We plan on building in Washington State, near the Columbia or on the Peninsula.
I want to do alot of he work myself. I will have to pay to have foundation and slab in. We can do electric and some plumbing, cabinets, doors etc. I will look for help to frame it in and get gong on siding. Our living space wil be about 1200 square foot.
So will I need 1x2 to give me space between siding and OSB?
Insulation ? Spray fiberglass vs spray in foam or combo for best bang for the buck?
I plan on sealing all joints in the OSB. I have used Tyvek on top of OSB. Mu building experience is mostly in Arizona, warm and dry.