attic insulation and ventilation
hello,
wonder if folks have suggestions on a product or technique for maintaining an air gap between insulation bats and roof decking in a gabled attic. seems like i remember seeing a product to do exactly this mentioned in FH 1-2 years ago but haven’t been able to find it.
thanks in advance,
ziptied in denver
Replies
There are many different "baffles" available to maintain an air space.
If at all possible look at the long term cost effectiveness of your insulating approach. Batts cost less now and more over the life of the structure in most situations.
Do some research on effective r-values, air was, foam options and cellulose.
Heating cost are going up, this is the time to invest in insulation and air sealing.
Garett
Grott,
thanks for your reply. here's the dilemma:
the attic spaces are presently unventilated and experience lots of solar heat gain-great in the winter bad in the summer. summer temps can easily be 15/20 deg higher on the second floor than the first my goal is to ventilate, insulate the rafter bays and then cover the insulation with rfoil. in this case, it seems that limiting solar heat gain is more crucial than further maximizing r value for the benefit of restricitng heat loss from the living space because:
1879 story & 1/2 wood frame farmhouse, gabled roof. the attic space is living space. 1st floor ceiling and 2nd floor knee walls are well insulated. it's a hydronic heating system with infloor on the first floor and radiators upstairs. the radiators don't work very hard in the winter. observing the roof as well on cloudiest coldest days there are not hot spots visible on the roof so i'm pretty sure heat is not escaping from the living space.
any further insights on attic baffles is appreciated. thanks.
ziptied in denver
Venting an attic does little to cool it. It's one of those practices that seems like good building, but in fact is largely a waste of effort. The primary heat transfer mechanism is radiant; that is, the sun heats the roof, and the roof radiates heat inward. FG isn't a great radiant barrier, so much of this heat travels through the fg to warm your ceiling. Since radiant heat doesn't warm air much (the air up there is warmed by the framing which warms the air through conduction), venting that air isn't going to help much. Stand on the roof on a hot day, where there is unlimited ventilation, and you'll find it's still hot because of the radiant heat.
The best way to keep hot attics from heating the house is the right insulation. Cellulose is mostly opaque to radiant, so blowing some in on top of the fg can make a big difference. Even better is foaming the roof deck from below. Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Ziptie,
Are you insulating the Rafters running along the slope of the roof?
I'm just trying to get a feel for your problem here?
How much space do you have to work with?
Garett
Garrett,
yes, i'm insulating the rafters along the slope of the roof. they are 2x6 dimensional fir and roughly 20" oc. the "slope" is about 8' in length.
i posted 3 photos here:
http://photos.yahoo.com/kensingcourt
then click on rafters and decking
thanks for reading.
ziptie
If you NEED an air space I would look into Dense-Pack Cellulose. A chute or channel made with Thermax and tacked in place before you DP with Cellulose will give you air movement, great R-value per inch and a reflective surface to reduce heat gain.
How much of the rafter has ceiling attached directly to it? I couldn't tell from the pictures. If it's less than 6' feet foam chutes and cellulose will work well if you need the air space.
It will be labor intensive however you go, if the airspace is required.
Spray Foam would also be a good choice but the price will be higher, but not as much if it can be applied to the roof deck in your area.
Garett
thanks Garett
sorry it took me 3 messages to finally spell your name correctly.
after doing some more research-since i'm not replacing the roof at this time, i don't necesarily need to do ventilation to meet code. the question is would ventilation help cool the space? i'm not so sure it would. that said, i'm thinking of doing the spray foam to the decking/as deep as the rafters and foiling over that now.
will let you know.
ziptie.
thanks again.
Looked at your outer attic (kneewall) photos. You have an energy loss disaster going on there!! I'll send you a private E-mail if you widh to contact me about it.
Full insulation of the rafter cavities with dense pack cellulose from the top of the exterior wall to the attic flat area is the best way to reduce the attic heat gain!!! Go back and re-read Andy Engel's post. This issue has been studied by a colleague of Andy's. He came up with conclusions much different than you hear "on the street".
Hi experienced,
thanks for weighing in on my "situation". i'm going to go the spray foam route. the tricky part is going to be getting to the actual peak of the roof.
cheers,
ziptie
If you do not have much space to work in, spray-on foam can be a disaster. Just before Xmas, I consulted on the worst spray-on job I had ever seen (due to restricted work space). Told the owner to withhold payment of $2700 (which she did) and to contact Icynene head office with my pictures and comments.
Spoke to the homeowner about 2 weeks ago. Insulation contractor nor head office appear to be moving to correct job or collect $$$$.
Spray-on foam is a good product and has certain applications where it cannot be beat. But there is a cult like following developing around it not seen since white sliced bread. Some situations where it is used are not as cost effective as cellulose.
Questions:
How are you going to fill the cavity in the finished sloping ceiling between the outer attic and the upper flat ceiling? there appears to be some partial insulation in the slope now.
Is there enough room for a spray on applicator to easily move a spray nozzle/wand for even application?
What type of spray are you going to use? Closed cell would be recommended as open cell foam does not provide vapour barrier properties.
ok,
i'll bite. what is your recommended clearance for spray on foam applicators? (the portable kits seem like they're more manageable in tight spaces)
correct me if i'm wrong-the tradeoff here is that cellulose will be easier to apply assuming we don't pull down all drywall,plaster in the living space adjacent to the roof. however, it will be less dense than a high density foam.
ziptie
Minimum work space dimension:
vertical walls: 4-5 feet
Your attic triangle: 5-6 feet
Cellulose will be easier and cheaper to to apply!!! Remember that insulation values work under the "Law of Diminishing Returns"!!! Don't get into R values & costs that have 25-50 year paybacks. Under 10 years is great!!! 10% or better simple return per year
They sell a stirofoam air baffale at all the home centers for about a buck. Look in the area that they sell the shingels.