Hi there,

Sorry if I'm posting in the wrong place but I thought that since this is such an active forum there might be someone here who could answer my question...
I'm building a 4300 square foot stick frame house with suspended thin slab over framed up subfloor. I am also sealing in my attic (no vents) and insulating it with spray in foam  right up against the inside of the roof deck,  I'm wondering what's the right thing to do at the ceiling level, insulate or not??  also, would the spray in foam be recommended for under the sub floor or is that overkill?

Thanks

George

We can always try.

Are you heating the attic? If not, why are you insulating it? If so, there is no need to insulate the floor below it.

Some have made a case for heating the attic space if there are mechanicals installed in it. If there is no real need to heat the attic, you are increasing your heating load for no apparent reason(and wasting insulation). 

I live in the Columbia River Gorge, world famous for wind sailing and wind driven rain and snow... I don't see any reason for it driving through my attic depositing moisture on it's way and scrubbing away the heat from my insulation. I'm just not sure if I should insulate at the ceiling level as well... My thinking is that this added layer would simply delay heat loss through the ceiling (and eventually through the roof) creating a semi or "slightly conditioned" space from the ceiling level up, not warm but not outdoor ambient either... am I missing something??

I will be running ductwork (primarily for cooling} either through the crawl or the attic... my plumbing will run through the crawl.

Thanks

George

Ok, I'll defer to the experts. See: ORNL Marine Climate. Hopefully this will answer your questions.

My suggestion (recommendation) would be what I'm doing in the house I'm getting ready to build in SW Idaho. 1st, seal and condition the crawl space. There's less area to insulate on the crawl space walls than on the floor. Also, there's less area exposed to cold ambient temperatures when you keep it out of the crawl space. On my house, which will be be 11" ICF (6" concrete) with 2200 sf inside the walls and 176 ft of crawl space wall, not counting garage common wall, varying from about 24" to 60" in height, the heat loss calculation shows 1000 Btuh less heat loss with the sealed and conditioned crawl space than with R19 under the floor and vented crawl. Plus, I don't have to worry about moisture in the crawl space.

Then, consider sealing and insulating the attic space. If you are going to have ducting up there, do it. If you have no ducting in the attic, and do not plan to use it for storage, then make the decision based on economics of insulating. When I did the heat load calc on my house the best answer I could come up with was the heat loss through a well insulated attic was about the same as through an equally insulated ceiling. I think this is due, in part, to the fact the roof gets warmed by the sun and so heat may actually move into the attic during winter days. The roof area will be greater than the ceiling area.

I chose to seal and condition the attic for four reasons. One, as I said, the heat loss would be about the same. Two, I don't have to worry about air leaks from the living space into the attic via canned ceiling lights, cracks in the drywall, air paths through the interior walls up into the attic from electrical boxes and wiring holes, air leaks around bathroom fans, and so on. Three, I want to use the attic space for storage. If there is 16" of fiberglass or cellulose on the ceiling it will either get trampled down or I have to built a floor 12" above the joists. With all the insulation on the underside of the roof I won't have to contend with it on the ceiling. Four, it will easier to circulate air through the house and make sure air that gets injected into a closed room, like a bathroom or especially a bedroom, will return to the furnace return duct. I will do this by putting registers in the ceiling throughout the house that open up into the attic. I also will have registers in the floor to allow air to move into the crawl space. That is required by code for sealed crawl space. Makes sense to follow the same rule for sealed attic. Air circulation in closed rooms is a problem with usual building practices.

Now, insulation on the ceiling. I will do it only if I go with my plan to install radiant heating in the ceiling, simply to push the heat downward. If I go with a high efficiency air to air heat pump that puts out heat down to 5F, I will not insulate the ceiling.

I plan to use a full time heat recovery ventilator to assure indoor air quality. My current plan is to exhaust from the attic and bring the fresh air into the crawl. Hence another reason for vent between the living space and crawl and attic.

Hope this gives you some things to think about. Go to www.buildingscience.com as a starting point for web research on this whole topic of sealing and insulating. You'll find we've been pretty sloppy in a lot of our building practices in years past.

Mac

I have a sealed attic space in my house, I have seen a minima of difference in my heating/cooling costs compared with others in my neighbourhood with similair models as mine. I am slightly cheaper (Maybe $ 10.00/mth) but I am unaware of their heating/cooling temps so this is not a fair assessment. I can say that I am much more confident that I have a completely sealed house as all areas are filled with foam and no voids are present. This system highly compliments an ICF house.

Chris

Do not insulate the ceiling.

I agree with drogers: Do not insulate the ceiling. Insulate the roof rafters! In NC we call it "hot roofing". Asphalt shingle manufacturers are now providing guarantees for their products concerning hot roofing.

Do not condition the attic. The attic temperature is regulated by means of air osmosis. Osmosis is a movement, in this case air, of high energy to low energy. The membrane in case of construction is comprised of building aggregates that seperate the habitable space from the attic. I have had great success with hot roofing.

I recommend to use 4" of closed cell product and top that with a 2" cover of open cell product. This will eliminate having to paint the closed cell product since the closed cell product is flammable and the open cell is not.

Don't insulate the ceiling, assuming that your spray foam meets the R factor required by code. I "cheated" on this a little. I put up 2" styrofoam between the trusses using lengths of spring steel wire to hold it in place. That's only about R10. Then, I sprayed 2" of foam over it, removed the wires, and it looks like 4.5" of spray foam. Supposed to have R30. The inspector didn't say anything (how could he know?). I probably have an actual R20-25 (the same as I'd get with R30 fiberglass bats). So far, I haven't needed to turn on the heat. Just the lights keep the place warm.

The scary part of foam is making sure that you really, really have it sealed up. The thought of having a cold spot condensing moisture on the roof deck and hidden by the foam made me check the job several times before I used up the last of the foam.

You should have an air exchanger if you spray foam (I also sprayed the framed wall areas) because you'll have such tight construction. Be sure to have an air exchanger exhaust vent high in the roof peak to continually replace any moist air up there.

Mark Fleming

Search around the following site for some good reading about the subject of sealed or non-vented attics:

http://www.buildingscience.com/bsc/

What system are you guys using to spray your foam? I want to go with self-applied soy-based foam. It is a surprise to read that closed-cell is flammable and open is not.

Thanks for the Great posts! http://www.buildingscience.com/bsc/ is a great source of information!

Mac, I did not intend to insulate my crawl when I built the house so I might be too late for that one... I don't want to dig out my foundation to insulate it, Also the floor is dirt... 4300 square feet of dirt... If I knew of a an efficiient way to do it from the inside I'd consider it... right now I'm just going for plastic film on top of the dirt and batt between my joist...  I've considered a vapor up against my joist but I worry about trapping the occasional plumbing leak or whatever...

About the spray on insulation... closed cell seems to be heavyly recomended (on buildingscience.com etc...), but I've also heard that by using closed cell  I could end up trapping a roof leak between the membrane ( My entire roof is covered with Certenteed wintergard HT) and the closed cell foam and in time it would spread just rot out my OSB roof sheathing... I suppose that if I use open cell I am then taking a chance on moisture vapor eventually migrating up against the roof decking and condensing there... (why wouldn't it?)  it looks like you take your chances either way... hmmm...

It's nice to know that I'm not the only one out there "pushing the envelope"!

Thanks

George

Just back spray the ceilings above any porches, not the entire ceiling.  Slab floor I don't know about.  I'm in Fflorida.