I have a 1938 cape. Attic is insulated (combination of fiberglass batts and rock wool) but poorly-vented. I have ice-damming in winter and it’s hot upstairs all summer.
Insulation contractor says he can spray foam between the rafters and seal off the gable-end vents. That means there will be NO ventilation at all in the attic. Also, he says there’s no need to remove the current insulation in between the joists on the attic floor/upstairs ceiling because he’ll be sealing off the attic.
Questions: Am I ok not venting this attic if I go with the Icynene? Should we be spraying the attic floor as well as the roof’s underside? Anything else I should know about before I hire this guy?
Replies
It would be best to remove the insulation in the ceiling.
The installer will probably recommend using 5 inches of Icynene. I recommend using 8 inches. From a cost recovery standpoint alone he may be close to right, but there are other issues that come into play on this.
Energy costs will rise in the future and you won't want to go in and add more foam later. When you replace your HVAC system in the future, you may be able to downsize the unit due to the added insulation and that will save money that the installer is not calculating on.
Adding R-value does have a factor of diminishing returns but the breakover comes well beyond 5 inches. Covering the bottom of the rafters will also help prevent thermal bridging through the rafters.
> It would be best to remove the insulation in the ceiling.
Is that so the Icynene will go on the attic floor and up on the rafters? (essentially cocooning the entire attic space?).
If you're currently using some of your attic space for storage, how would that work? (Put down plywood sheets on the attic floor on top of the Icynene?)
Thanks!
Only insulate at the rafters. Do not insulate the floor of the attic. Confirm a seal to avoid exterior air from infiltrating the attic.
Here's my take on this. Insulate the ceiling joists by spraying down from the attic. Icynene is the perfect choice for this application because it also acts as an air barrier, eliminating the need for a vapor barrier (i don't know if there is one there already). Remove any insulation on the rafters and allow these to vent from soffit to ridge. This will leave you with an attic that is unheated (saves you money) and cold roof sheathing which should eliminate most ice damming. agree? disagree?
You could do it that way. Here are the pros and cons for insulating at the ceiling.
Pros- There is less square footage. If all of your mechanical equipment and piping and ductwork is below the ceiling, then there is less volume of air to heat.
Cons- There are more difficulties in air sealing at the ceiling because of light fixtures, plumbing stacks, interior wallplates, and furrdowns. There may be mechanical equipment and plumbing in the attic that will be outside the conditioned space. There may be plywood decking in the way. I think I'll just cover the rest in the pros for insulating at the rafter.
Here are the pros and cons for insulating in the rafters.
Pros- No need for attic ventilation. Plumbing and mechanical equipment can be safely placed in the attic. The attic can be used to store things of value without regard for temperature and moisture issues. Lighting at the ceiling can be of the inexpensive non-IC type. Insulation is not underfoot. Decking is not in the way of insulation application. Attic is valuable without being included in property values for taxation. Adding wiring or fixing equipment at a later date is easier since insulation is not in the way and the attic is more comfortable to work in. Roof penetrations are less numerous than ceiling penetrations. Attic floor can be easily vacuumed to remove dust if insulation is overhead. More insulation space available at wall to roof intersection when there is no need for vent channels. That's a biggee. The experts agree this is the way to go.
Cons- More square footage to install. May be prudent to cover with a fire rated covering such as drywall or intumescent paint.(usually not required by code "interpretation") May obscure a minor roof leak until damage occurs. (Any large amount of liquid water passes through.)
By the way, Icynene is not the only choice. Other products are nearly identical and less expensive. I do like the owner of Icynene and he has made a significant contribution to the industry. None of them are making a killing at this due to the high cost of raw materials and plethora of lowcost (though inferior) competing technologies.
a well-articulated list of pros and cons for both ways. thank you.
I'd like to say one more thing on the subject of the cost of conditioning additional footage. The heating and cooling load does not come from the volume of air within a space. The load comes from the perimeter enclosure and from interior heat sources. In conditioning to the rafters instead of the ceiling, we look at the square footage of enclosure area and the air tightness of the enclosure. The roof has more footage but fewer penetrations. The other factor is the effect of the attic inventory. If there is HVAC equipment and/or ductwork in the attic, the losses through this equipment and the degradation from being in a hostile environment far far outweigh the additional surface area.
My home is in central TX. My attic is unvented and is insulated in the rafters to R-30. The air tightness of the home is phenomenal at .76 ACH50. The square footage of the living area is 3400 sq ft. The area of the attic that is decked off and walking height is 800 sq ft for a total conditioned space of 4200 sq ft. I have an old refrigerator, incandescent lighting, rainwater collection which must be pumped and sterilized with UV, a hot water circulating system and still my energy consuption for the last 12 months totals to 12,000 KWH and 120 gallons of propane. That is a total operating expense of just over $100.00 per month. The additional volume of conditioned air is obviously not a penalty. The fact that all my HVAC equipment and ductwork is within the conditioned space means that any leakage is within the enclosure as well. This fact eliminates the differential air pressures that causes outdoor air to be drawn in to a home with a conventional vented attic.
The code requiring attic ventilation was written in the early 40s to respond to testing that was done in minnesota in the late 30s. The reason it was used was to overcome interior moisture that collected on the bottom of the roof deck in cold climates, leading to premature rotting of the wood frame and deck. It was the best solution that they had back then. When we started using ducted central HVAC systems, attic ventilation caused problems that we did not understand at first. We now have better strategies for dealing with interior moisture in the winter. This new strategy is far more appropriate for the systems that we use in our modern homes.
I install Icynene insulation in northern Illinois and think you are headed in the right direction.
BUT I would always remove the old insulation. My reasoning is:
There are other foam alterative but do the research and find out:
If after that you feel there is a better foam out there pleas use it. and pleas let me know.
I'll answer your list of questions based on my knowledge of Demilec. I will first state that I like Gabe Farkas(Icynene owner) and I have no financial connection to Demilec.
does it shrink and if so how fast? Only if improperly installed. Same as Icynene
does it feed mold ? No
hold or wick water? Gabe says does and Icynene doesn't but I have not found that to be the case.
if it gets wet will it maintain its original form and maintain its R-value? ( I prefer performance over R-value) Same performance as Icynene
will it feed the local bugs, birds ext.? No
does it off gases? A little more than Icynene but for a limited time.
is it a fuel source for fire? Same btus per pound as Icynene
will it stay in contact with the substrates if the house settles or moves? Same performance as Icynene
How long has the company been making it, and what is there track record? I have not researched that. What is the answer to this question for Icynene?
Insert any Question you can think of . How much does it cost per board foot? Retail cost of Demilec in my area is about 35 cents per board foot for half pound foam.