Designing for Cooling

Mike 870

Hi,

Hi, Im trying to design a thoughtful off grid home in Southern Ohio. It will be in a county that is over 70 % wooded with an abunance of wooded acreage on the property. This county experiences the full extremes of the seasons. Hot humid summers, cool snowy winters. The grid is not close to my favorite building sites.

It seems to make sense that with the abundance of wood I should design specificly for cooling. I've already got a lot of the basic stuff planned like desidous trees shading the south side of the home, a white metal roof, and a plethora of insulation.

Some of the things that are stumping me are:

1. To insulate or not to insulate my slab.

2. Venting in the attic space. I am leaning towards a scissor truss roof with an excess of eave vents and ridge vents. In the summer hot air should exit the ridge vent whiile cool air is pulled in the eave vents right? I also plan a DC whole house fan to speed up this proccess when the days are hot but the nights are fairly cool. Should I go overboard on venting or follow the code guidelines? (FYI the insulation will be against the inside frames of the scissor trusses).

I am not too woried about ice dams since I am planning a white metal roof with a 12-12 pitch.

I am also interested in hearing from people living off grid in a similar climate, and any other design aspects in regards to designing for cooling.

Thanks!

niel

Re: Designing for Cooling

sounds good. i am grid tied, but climate wise similar to you. keep in mind that you can't roof mount any pvs for power if the home is shaded by trees. they can be located elsewhere though on poles or ground mounts and give plenty of ground clearance to allow for snow accumulations and snow runoff.

the slab imho should be insulated. cool in summer is good to have, but it needs to weather cold winters too and an uninsulated floor will suck the heat out of the living space area to a degree and make for cold feet.

the venting sounds good to me too as long as the insulation does not block any of the airflows, but i'm assuming you to mean the insulation is to be on the bottom side of the scissors and not near the outer roof portion. i suspect the fan assist to not be necessary of done right. the vast area of ventilation would create a situation akin to assisting an open window with a 4 inch muffin fan. it won't do much.

icarus

Re: Designing for Cooling

Absolutely insulate the slab, at the very least insulate the perimeter.

I would have as much attic venting as you can accomplish. Consider a "high heal" truss that allow extra attic insulation (over the wall) while allowing for good ventilation.

Intake soffit vents, along with continuous ridge vents, or even extra gable vents. Keep the heat in in the winter, keep the heat out in the summer,

Tony

PS Home work done now will yield best results later,

T

Mike 870

Re: Designing for Cooling

Thank you for the responses.

Niel:

I'm not planning on roof mounting. To me, shading the house is more important than roof mounting since AC is out for off grid, I'd rather spend extra money on wire guage than lose any advantage for cooling nature could give me. I will admit that I have not decided on a system voltage or PV array size yet.

Icarus:
Having scissor trusses should allow for plenty of insulation and also plenty of venting. I was thinking I would vent every "space" on the soffits between the trusses and have a continous ridge vent. I had not considered extra gable vents, but I will from here on out.

Some of the other design criteria I have considered and then nixed are earth berming (resale potential) and earth tubes (too tricky, and controversial).

Does anyone think building on piers might be better than a slab, since you would get airflow under the cabin?

mike95490

Re: Designing for Cooling

Not a cooling aspect, but here's a nifty design for winter heat:

Solar Shed
500 gal storage @ 180F
http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm
http://tinyurl.com/6hypyt

mike95490

Re: Designing for Cooling

What are your summer nights like - do they cool off ? You might try some sort of
evap-o-pond, shaded daytime, open to sky at night, and circulate chilled water.

Cariboocoot

Re: Designing for Cooling

An insulated slab gives you a "controllable thermal mass" to work with.
As for air flow under the cabin ... I've got that. You want it? You can have it! It's also known as "cold drafts through the floor boards".
You always want to keep your "heat envelope" sealed up so you can control the air changes. This goes for cooling as well as heating. If you build with any sort of basement and insulate the floor above so that there is unheated space below you will want some ventilation to reduce moisture that will condense in the cooler, unheated space.
Sufficient eave/ridge venting should eliminate a need for any fan to move air. 1/2" of air space under the roof is a minimum for keeping the temp down (to prevent degradation of the roofing).

Brock

Re: Designing for Cooling

I would agree, insulate under the slab. I would suggest 2 inch of pink foam under the slab and around the exterior and then add 3/4" PEX spaced on 12 inches to 18 inch centers in 250 foot lengths. The cost of adding the pex and insulation before you pour the slab isn't much (about $1 per sq ft) if you can do this yourself. Then later you use just about any method to heat up water, solar, wood, propane or whatever and use the slab as thermal mass. And it’s comfortable

I am not sure if you use more heating or cooling, but if it is more heating you would be better to allow for solar gain for winter, but decrease it in summer. If you have 4 foot overhangs and good attic insulation direct sun won't add much heat or direct solar gain in summer and in winter when the sun is lower the sun will shine in giving you nice solar gain. That’s just me though.

stephendv

Re: Designing for Cooling

Controlling ventilation is just as important as insulating well, this means building to a high degree of airtightness and then using a ventilation unit to circulate air. Noticed you mentioned a whole house fan - there are versions which include a heat exchanger to pre-heat the fresh incoming air with the heat in the stale air that's leaving the building. And this works in both directions, so on hot days you can leave the exchanger on, so that the cool indoor air cools down the incoming hot outdoor air. This only works if you can then cool down the interior of the building at night, with cool night air.... and of course it also only works if your house doesn't have uncontrolable draughts all over the place. These unit typically use DC fans, the European versions consume between 20-100W depending on the speed setting.

High thermal mass on the inside of the building envelope, cermamic brick, marble, stone, etc. help to moderate temperature swings and absorb excess heat during the day. Over here in Spain, they typically leave a good gap between the roof insulation and the arabic style tiles to encourage ventilation - in some areas they build with a reflective metalic insulating material just on top of the regular insulation layer to further reflect the heat.

There are quite a few different ideas on what constitutes enough insulation. In our new build home (-5C in winter, 38C in summer), we've decided to build according to the German passivhaus (www.passivhaus.org) specification which limits the total heating load of the house to 15kWh/m2/year. To achieve this, we are having to using 28cm of celulose insulation in the walls, 35cm in the roof and 20cm of XPS around the basement walls and under the slab. The interior will be built with ceramic bricks and concrete which act as good thermal mass during the summer to reduce cooling load. Even in 38C summer, we should not need an AC unit due to the tight control over airflow, high levels of insulation and high thermal mass.

icarus

Re: Designing for Cooling

I would not build on piers unless I needed to. If I did, I would then create a solid skirting around the perimeter. The problem is, while you need ventilation, you need to CONTROL that ventilation. I the winter closeable vents to prevent uneed drafts, as to be able to control the venting with hot humid summer air, condensing ON TOP of your vapor barrier in the summer.

A well insulated slab, with proper vapor barrier under it might possibly be the best alternative. On the other hand, concrete footings and stem walls make a good alternative.

Tony

FL SUN

Re: Designing for Cooling

A lot of consideration into building envelope and type has been put forth due to the rising cost of energy.

I can't say enough about ICF construction and closed cell foam under the roof deck. This type of construction cuts heating and cooling loads by half. I have been in a 2,400 sq ft ICF structure with underdeck insulation for almost 3 years, and I cool it with a 2 ton 16 SEER heat pump. A similar wood framed home would need 4 tons.

If you build off-grade, closed cell foam under the floor deck will do a world of good.

There are many benefits to this type of construction. Since living in one, you couldn't pay me to live in a wood framed home ever again.

Dave Angelini

Re: Designing for Cooling

Mike,

It sounds like your plan to design for summer heat is what we do out here in the Sierra mountains of California. Plenty of blue oak for heating. If you do a stick house with 2x6 plan on using all the soffit spaces on each side up to the ridge vent. Orient the house so that afternoon breeze (if you have one) can go in one side of the soffits and out the other. This is the key to summer venting and very few builders have a clue here. Use r60 in the attic no matter what color the metal roof is. There are colored metal roofs now that are "suppose" to reflect as well as white.

There is a thread on the energy section of this forum on the Sanyo mini split AC that you might check -out. A tracked array can be 200 feet or more from the battery. Good Luck!

benthere

Re: Designing for Cooling

The great thing about passive solar design for heating, is that it is also passive design for cooling! Properly orient your house, put most of the windows on the south where they will be in shade of your calculated roof overhang in summer, keep west windows to a minimum, insulate well, etc.

It sounds like you could easily shade the west side of the house. The east is of some benefit too.

PS. I just installed a white metal roof in place of a dark asphalt shingle one and it made a HUGE difference in the temperature of my attic. Sorry, no numbers - but I can tell you that it was miserable, and is now comfortable to work up there in the middle of the day.

Edit to add: http://en.wikipedia.org/wiki/Passive_solar_building_design

Mike 870

Re: Designing for Cooling

I appreciate all the input. The "tight envelope" Is something I have been thinking on. I'm hesitant to use SIPs because I'll be an owner builder doing almost everything myself. As much as I am a planner, I have a feeling I will be doing a lot of tear down and do over as I wire electric, run plumbing etc. I have heard of a method of "poor man's sips" using polyethelene in conjunction with fiberglass or denim bats.

I just have to make sure I get it right because of the wide climate swing in my area. Moisture generally wants to travel from wet to dry and from (edit) warm to cool right? Since I will have no AC, and will be heating in the winter, I it would seem that putting polyethelene on the outside of my insulation would trap moisture inside, during the winter when I am heating but keep moisture outside in the summer if my passive heating/cooling design works. Which takes precedent?

As far as wall cavity, I had been planning to frame with 2 by 6s on 24 centers, to minimalize thermal breaks and allow R19 in the walls. Someone mentioned R60 in the ceiling, which sounds good to me, I have a bunch of R30 bats already and I can criss cross them to get R 60 and get rid of thermal breaks where the rafters run.

I'm decided on a insulated slab, and I will run the pex even though I wont use it right away.

I have used seer software to find the ideal location and amount of glass, and ideal overhangs.

I assume if I do my envelope correctly, I will need an Air to Air heat exchanger, I don't know much about them though. Would it be a power hog in an off grid location? I've always just cracked a window if the wood heat is having trouble drafting.

Mike 870

Re: Designing for Cooling

I edited my previous post, I had some stuff wrong.

benthere

Re: Designing for Cooling

Mike,

You need advise from someone who knows more than me about condensation and vapor barriers, but thought you might have some luck searching for the answer with the term "vapor barrier"

niel

Re: Designing for Cooling

the moisture barrier is always between the living quarters and the insulation which is nearly always attached to the insulation face. i supoose there's nothing wrong in having both inner and outer barriers.
i wouldn't count on a r30 fiberglass batt on top of another r30 fiberglass batt adding to r60 as the weight of the upper batt on the lower one will compress it some and reduce the r value somewhat.
i think you should consult a professional for advice on your project. although most everything you have in mind may be ok, it is something little than can cost you big like a vapor barrier facing the wrong way.

GreenerPower

Re: Designing for Cooling

The moisture wrap/vapor barrier is normally outside your insulation. It's permeable. It prevents condensation when inside is cold and outside is warm or vice versa to prevent mold/mildew, it won't stop/seal moisture in or out.
Down here is SE texas, with a radiant barrier roof and R30 roof insulation, my attic temp never gets 15F above ambient. So, with your metal roof and adequate passive ventilation, I think R30 is sufficient. R60 does little improvement.
There are also radiant barrier dry-walls if you want to improve the themal envelop.
GP
PS: to add, the vapor barrier should be placed where there is most temperature difference which is normally the outermost walls before external panels, sidings or brick veneer.

Dave Angelini

Re: Designing for Cooling

R 60 made a measured difference of 6 degrees inside over my R 30 blown in at 4PM when it was 90F outside with a metal roof. It may not sound like much to folks that are tied to the grid (with all it's options) but it can be the difference between comfort and discomfort when starting out in a new house offgrid. That said it is not hard to add the blown in supplement later.

Your biggest concern would be windows if the house is oriented for a view rather than least solar heat gain. Even large overhangs won't help when the west/northwest sun is an issue.

Probably the worst problem I have seen is a house that is oriented properly and the only views of 10,000 foot granite mountains are from rooms that only are used for sleeping. What a sad failure!

niel

Re: Designing for Cooling

"PS: to add, the vapor barrier should be placed where there is most temperature difference which is normally the outermost walls before external panels, sidings or brick veneer."

if you did that up here in the winter it would ruin the insulation, the walls, and ceilings because the moisture inside is trying to escape to the outside and stopping on the other side of the insulation making the insulation collect moisture. this will kill fiberglass insulation's r values and create molds and mildews. the situation reverses somewhat in the summertime if you have a/c and then it is advantageous to have the vapor barrier on the outer side. both sides is best imho and that is only available with your sheet type insulations.
now the insulation sheets on new housing is on the outside of the studs, but their is a barrier on both sides of that so as to not get moisture into or past the insulation. the reflective portion on such sheet insulations is where what you said of the temperatures is true as i can see facing the foil to the outside in your state.

GreenerPower

Re: Designing for Cooling

Maybe I'm not really clear here. From inside to outside - drywall, insulation, plywood or OSB, Tyvek/vapor barier, sidings. I've seen construction sites this way in northern VA and further north also. When outside is warmer and more humid, the moisture at the colder vapor barier surface tends to condensate at the outer surface, the barrier minimizes this condensation, any condensation on the outer surface would easily vaporized out. When the house is wamer and more humid, the moisture gets out to the colder outer surface of the vapor barrier. Similar thing happens. Moisture always tends to condensate on the outer surface of the vapor barrier to easily vaporized out.
GP

Solar Guppy

Re: Designing for Cooling

http://www.bdcnetwork.com/article/CA6450462.html


For Northern Climates , Vapor barrier is between drywall and the studs/insulation, reversed for warm climates like we have in the south.

The key is to prevent moisture migration at the point of the insulation were dew point drops so that prevents the insulation from getting wet.

In the North, that's the inside face of the insulation, for the south its on the Outside

icarus

Re: Designing for Cooling

Northern Climes,

Siding, Building paper (Tyvek) plywood/osb or wood sheathing, insulation, VAPOR BARRIER, interior wall finish (gww/plaster/panelling etc. (At -40, warm inside air condenses right now. Any tear or rip in the VB brings real trouble.

Floors the same, over crawl space, vented with VB over earth.

I suspect where you have real problems is in temperate climes where sometime you have warm in, some times warm out.

Tony

Mike 870

Re: Designing for Cooling

I think I need to clarify terms here. We're really talking two types of vapor barriers here. Water resistant yet permiable (Tyvek), and almost not permiable (Polyethelene).

Although I'm designing with cooling in mind. I think what I have is really a heating situation as far as vapor retarding diffusers (almost non permiable barriers) go.

The discussion came about because I'm trying to produce an envelope similar to SIPs without using SIPs. To do so I tossed out the idea of using polyethelene sheeting to seal the envelope.

Since I am in a mixed heating and cooling climate it gets confusing. In very cold heating climates you put the barrier on the inside. In hot humid cooling climates you put the barrier on the outside. This is where confusion seems to be coming about.

In mixed climates usually you have a permiable barrier (tyvek) on the outside and a semi permiable barrier (painted drywall) on the inside.

What I am thinking of doing is considering myself a heating climate, since I will have no AC. I'm considering a tyvec type material on the outside and polyethelene on the inside (to make the envelope as air tight as I can).

As far as R60 in the attic, the rafters will prevent the top bats from compressing the bottom bats which will be in between the rafters, the top bats will run criss cross.

niel

Re: Designing for Cooling

sounds viable to me. do know that a painted drywall surface is not considered a vapor barrier, at least not with normal paints and primers anyway.

icarus

Re: Designing for Cooling

Perm rated primers are an rated Vapor barrier.

Tony

Dave Angelini

Re: Designing for Cooling

The building department in the area where the house is going to be built will tell you exactly what needs to be done. It would be risky to design without their advice. For instance the zone where I live does not require a vapor barrier only a house wrap such as Tyvek or Typar. Go see the building department and get the scoop.

Mike 870

Re: Designing for Cooling

Unfortuneately there is no building department to consult. There are also no inspections, and no permits aside from well and septic. I have talked to a local builder and have consulted several other sites. I figured I'd try here since I thought there may be some people off grid who went through a similar dilema when they built. It is a bit of an off topic question for this forum so I apologize if it doesn't belong here.

I will return when I start hammering out details of my electric system.

niel

Re: Designing for Cooling

what was the advice given to you by the local pros?

Mike 870

Re: Designing for Cooling

I asked him about SIPs and he had a very negative response. He said they end up costing a lot more money than you think. Said even though they pre cut channels for electric and plumbing it's generally not adequate. Said the electricians and plumbers generally spend more time standing around trying to figure out how to do stuff than they do working.

He said SIPs for the roof might be a good idea.

I never got to talking about trying to seal the envelope because I hadn't had the suggestion by that point. The next time I see him I will try to pick his brain more about local building proccesses.

He was a big fan of timber framing, said anyone can build a stick built house.

Solar Guppy

Re: Designing for Cooling

Mike 870 wrote: »

He was a big fan of timber framing, said anyone can build a stick built house.

So he puts down a technology he can't provide? seems the motivation, not build the house to the requirements you want.

SIP is a very very good way to build homes ... find a builder that has experience, Its all about planning with SIP, the people standing around is a contractor issue and nothing to do with SIP.