Should I be seeing the outline of my rafters on a frosty morning? I have open cell foam insulation sprayed directly to the decking.

Note - The first picture shows heat loss around the fireplace. Building inspector says that I must maintain at least a 2" airgap between the fireplace venting and the spay foam ( I assume this is the source of the "hot spot" in the first picture).

My opinion:
If your roof deck is insulated with polyurethane spray foam like most that I have seen, then the foam is only between the rafters and not on the rafters.  Although wood is not a great conductor, it will still conduct enough heat to keep the frost off of the deck above the rafters.

A better way to insulate a roof deck would be to use nail base insulation on top of the rafters to insulate the deck and rafters.  A thin layer of spray foam between the rafters would reduce infiltration.

Do you think this will reduce the energy efficiency of the foam?

tsu,
this condition is fairly common for 'cold roof' applications, be it spray foam, SIPS, or any other type of insulation where the rafters/splines are 'exposed'.
The wooden rafters have a lower R value than the insulation. It is not unusual to 'see' the rafters (or splines) in this situation.
Your overall energy efficiency should not be overly compromised, at least in comparison to traditional methods of insulation.

Alton;

 

you are right on , most people don't  think of wood as  a thermal conductor, but it is less thermally efficient than the foam, so it acts as a thermal bridge, if we get another frost this year I will take a picture of my steel sips roof with a solid white coat.

Thats why I insulate top level ceilings and leave the attic space unconditioned, the cellulose covering your ceiling joists acts as a thermal break.

My neighbors house, brand new 4000sq ft spray foamed house does the same thing every frost. They built heir house right and have lower utility bills than we do in our 1948 ranch of 1700 sq ft. I think you will be pleasantly surprised with your utility bills.

TSUFAN;

the picture below was taken at 7:30 am this morning - my home on the left , the roof is 6" steel SIPs with shingle cover - solid frost cover

The house on the right is a conventional roof with vented attic and plywood /shingles.

the insulative properties of each is obvious.

Chris,
You mean it actually frosts in Florida?
Seriously, how did you attach the shingles to the steel sips, screws? nails?

wes,

those white bird of pardise are brown from the 2 hard freezes we had, my house is the only in the neighborhood that gets solid frost and this morning it appeared at just 45 deg., I wish I knew why that was, I think it has to do with temp, dew point, etc.
shingles are attached with waferhead screws, for cosmetic purposes, panels were water tight before shingles went down

Chris, it must have something to do with your steel SIPs! Kind of like my bathroom mirror fogging when I take a shower on a cold morning.

But, the shingles are over the steel? if you had a covering over your mirrors the covering doesn,t fog?

The bottom side of the exposed steel does not frost

Your neighbor appears to be lacking any insulative properties in parts of his roof. I would bet that your utility bill is lower than his.

tsufan;

you bet right! by about 60% . he also does not have insulated windows a typical FL home

per wikiAnswers:

Frost is deposition of water vapor directly into ice crystals on a cold surface.

Water molecules in the air are constantly bouncing off of surrounding surfaces. If a surface is cold enough, the molecules will stick to the surface rather than bouncing off. Once a layer of frozen molecules form, new molecules will tend to align themselves with those already there, forming regular crystals.

The temperature of the surface is determined by the net heat balance on it, which is a function of air temperature, air movement, radiation, and solid conduction. Usually the ground is warmer than the air, and the air tends to be the strongest effect, but a surface isolated from the ground on a clear, still night will be dominated by radiation to the sky, so it can get colder than the air. That's why you see frost on top of the mailbox or on the tips of the grass when there is none on the ground.

Regarding the air gap around your chimney, you can and should stuff that cavity with rock wool or fireproof insulation, otherwise you get a thermal chimney affect happening there, it is impossible to make the roof deck ie;flashing completely air tight. The code says that chimney must be a minimum of 2" from combustible materials that does not rule out fire proof insulation, drywall, or metal flashing as in preventing the roof from leaking.

Drywall is not fireproof, it has a paper face

Chris K,

That picture you showed with your commentary is a very powerful testamonial to metal SIPs, you should post this over in the SIPs forum. To people that understand what that picture is showing, it is screaming energy efficiency.

Simon;

be glad to talk to you, I met with some people yesterday for a house in Mims, FL. and gave your name as a designer. We are also now licensed in Georgia and your connection there may be beneficial.
Call me on the toll free # tomorrow and I'll PM my cell # to you

Thanks