Spray Foam in Roof Rafters - Not Sure if it will Help

12 years ago

So my builder wants to spray foam all the rafters in my attic. We live in new england and will have 1 finished attic room (guest room). Maybe use the other room for storage and HVAC. Out house has a ground floor, second floor and an attic floor.

With the finished room being occasional use, and given this is very limited info. Would spray foam in the attic help the main living space? All the other walls we have preexisting blown in cellulose and batt.

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Comments (8)

User
12 years ago
It is possible that closed cell foam is the only way to achieve the building code required min. R value without furring the rafters and losing headroom. Can't say more than that without seeing the design.
energy_rater_la
12 years ago
spray foam is a good thing, when it is correctly installed.
an unvented attic (foam @ roofline) puts ductwork in a semi conditioned space, and really adds to the air tightness of the house.
read thru a few posts about upstairs being hot/cold
and you will see what difficulty folks have with comfort
in rooms that are built in attic space. the problem is the air leakage and heat/cold gain from attic space into these rooms. the walls, the floor and ceiling of these rooms are all surrounded by attic temp air. when you move the thermal boundry to the roofline, and make it air tight
then the leaks in the walls ceilings and floors are not
pulling extreme attic conditions into the house.
things like recessed cans, oversized cuts at bath fans
and even duct leakage become less of an issue because the
air and thermal boundry is at the roofline.
visit www.buildingscience.com and look for unvented attics for your climate.

best of luck.
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foam installer is responsible for the roof to attic floor connection. shouldn't be just a little foam... if foam is not installed to fully make this seal then your attic is not unvented. the install at this junction is critical. no voids, roof to attic floor must be continuous, or else you have a semi vented attic. at the cost of foam, ideally the installer would have learned the importance of this seal. but itrw....I take a can of spray paint and mark the missed areas. foam co comes back and makes it right. but it is your choice. best of luck.
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Hi, I think that the approach of spraying foam under the roof rafters, and making the attic space part of the conditioned space of the house is a good one if done carefully. Seems like it would be a time consuming job to block all the current attic ventilation carefully. If you use closed cell foam, I don't see why there would be any moisture or condensation problem with the roof sheathing. Some of the people who make roof shingles don't like insulation right under the sheathing because it makes the sheathing and shingles run hotter and may reduce their life -- some people think this is bunk. If you have ducts or AC equipment in the attic, moving the insulation line up to the roof rafters will make your AC and duct system more efficient. The FSEC has some papers on what kind of saving results from this. Have you thought about just blowing additional cellulose insulation on the attic floor? Seems like this would be a lot cheaper, and may accomplish about the same result? http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/97/970504.html Gary
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A couple fo things: Open cell foam. Note the word "open". It will absorb moisture vapor and hold hold that vapor. It can eventually saturate and hold liquid moisture, just like a saturated sponge. Open-cell foam is indeed classified as an air infiltration barrier. But it still needs a vapor barrier to prevent the open-cell structure from holding moisture. I use both open- and closed-cell foams. But I'd never use open-cell in a roof. Never. In my opinion, and it is my opinion, closed is the better option. If you are going to do an unvented roof, then the foam does need to be applied directly to the underside of the sheathing. To the house a a whole... When the thermal envelope of a house is tinkered with, you need to take a holistic approach. You sealed the lid. If you didn't already do so, you need to address the basement as well as the living spaces below the lid. For most houses, moisture is generated in the basement as moisture wicks through the slab and/or foundation. That moisture can work its way into the living spaces through stairways, doors, and utility penetrations in the first floor platform. That moisture, plus moisture generated in the living spaces, can work its way into the attic. In older leaky houses, or houses with vented attics, the moisture can be whisked away through air movement. That same air movement gets rid of moisture vapor, but it can kill the thermal efficiency of your envelope. You sealed the lid of your house, eliminating an escape route for that moisture vapor. Your open-cell foam may now be acting as a holding medium for that moisture. Thus the high humidity levels in your attic, as well as liquid moisture draining out of your soffits. Address moisture sources in your basement as you can. Seal penetrations in your first floor platform. Address moisture generated in the living spaces; cooktops, showers, etc. Seal penetrations in your attic floor. After a dry spell when your open-cell foam has been able to go through a drying cycle and it's no longer holding water, get a vapor barrier on that foam. An easy way might be to use a topical coating to "paint" right on the surface of the foam. The coating can act as a vapor barrier as well as an intumescent barrier.
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Yes, it's a "conditioned attic", which is why the soffit penetrations (above top-plates) have been foamed shut. They're actually backed with with some of the original fiberglass batting to give the outside surface of the foam something to cure against. I'm in CL3, so recommended ceiling R-value is 30 or 38 depending on which version of the IECC specs you use. Unfortunately, I fell pray to the "foam performs better than permeable insulation, so even 3-4 inches performs better than the most of your neighbors" argument, so strictly speaking, the R-value requirements were likely not met. In fairness, I probably have around R-25 once all is said and done and any inefficiencies are accounted for. I'd love to go back 6 months and pay a little more for the extra pass (1-2 inches) to bring it up to the rafter surface and all the rafters, but I didn't. Thankfully, the argument that the biggest benefit is the near perfect air sealing is still true and I will get the lion's share of the benefits even if it's slightly below the recommended thickness. If I ever have my other roof elevation sprayed (over the garage, still a "vented attic"), then I may have the contractor "skim" another inch on top of the existing job. The cost of getting them out there to set up the job is most of the cost. $/foot goes down somewhat as the job gets bigger, so it'll only make financial sense to add foam if there's other work being done. Thanks for all the comments! Dan
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TonySak
Original Author
12 years ago
If the room/room in the attic isnt used too much, nor is it a priority when the whole house is taken into consideration, and with the assumption everything will be done correctly.

In general, could/will the rest of the house (all the non attic rooms) benefit from spray foam?
david_cary
12 years ago
If the HVAC is up there, you will see a whole house benefit. Whether it is worth the cost is another question but you will definitely see a benefit. It is also unlikely that you have adequate attic insulation already so this will just add to your effective attic insulation.
User
12 years ago
Will there be insulation in the floor between the guest room and the main living spaces? If so, how much?
PRO
Epiarch Designs
12 years ago
what kind of spray foam? open or closed? I typically never recommend closed foams in rafters as it prevents the water from coming through if there was a leak in the roof. With closed cell, your roof could be leaking and you will never know it until your sheathing rots away.
Open cell will be cheaper as well, with a slightly less r per inch.
energy_rater_la
12 years ago
Yes the rest of the home will benefit as well.
I also recommend open cell less R-value means you have
to put more, but on the roofline unless you have a cathedral ceiling this isn't an issue.

ROI is higher, but still much less than if you had
foamed the walls (a waste IMO) as well.
you'll prolly have a 10-12 ROI depending on roof height
and size of house. You do know that you don't foam over porches or garage? foam installer, builder or you can build a wall even attaching felt paper for them to foam against to exclude these areas.
Mark Leach
9 years ago
From what I have heard, foam insulation is a really effective solution. As long as the foam is applied properly it should work as you need it to. I have been doing research and foam insulation has become pretty popular lately. http://www.armstrong1234.com/roofing/foam_roofing/residential/

Spray Foam in Roof Rafters - Not Sure if it will Help

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Spray Foam in Roof Rafters - Not Sure if it will Help

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