Venting and Insulation
Apologies or te double post - i have the same question posted in the construction section - i just thought I would get a different point of viewin the roofing section.
Live in Chicago and am looking at finishing an unfinished attic space without any current insulation in it. House was build early 1900’s. From my research attic & cathedral ceilings in my zone require R30 to R60 insulation.
My attic framing is 16” on center and I was planning on going the DIY spray foam route (soythane). My issue and question is related to venting. From what I have read – an area, such as Chicago, with major temperature swings would require me to vent behind the spray foam. My initial plan was to run Durovent baffles from the soffit to a ridge vent (not yet installed). And from there – spray foam into the cavity. My intent was to drywall the ceilings all the way to the top of the roof and leave the collar ties exposed for a lofty look.
BUT…. I have also read about sealing the soffits and cavities with spray foam and going the “hot roof” route. This just does not make sense to me – but I’m not opposed to it if it is the right thing to do. I just think having airflow makes more sense – but what do I know.
The plot thickens….. If I go the durovent route – that takes about an 1-1.5” of space for airflow and only leaves me with about 2.5 – 3 inches of space for insulation. With the spray foam – that depth only gets me about a R9-R14 r value. I do plan on zoning this space with its own furnace and AC units but given my location - im thinking that I need to address the insulation aspect of this project differently.
Again – I have heard both the hot roof/total attic seal will work as well as the venting behind the spray foam – that said – I just don’t know how to approach this.
Question 1 – hot roof or Durovent to a ridge vent? Stop at the collar ties (will that alleviate some/any issues?)
Question 2 – Is insulation at the R levels described even worth all this trouble?
Question 3 – open to suggestions/comments/ideas
Another alternative would be installing polyurethane foam board (R-Value 8.7 per in.) on top of the rafters. Preferably foil faced foam board to bounce sun heat back out and indoor heat back into the new room for more energy efficiency.
The unconditioned part of the attic needs to be well vented, and the conditioned area needs to be perfectly air sealed, for better results and comfort in the living space.
And depending on the type of roof you have and what you plan to do with the attic, you will probably have a kneewall you need to treat properly.
For all of the above reasons, I would strongly suggest that, before you spend any money with your attic project, call in an energy conservation expert and have a home energy audit performed.
They will evaluate not only the attic, but the whole house, and give you the best options in terms of both energy savings and cost. This way you will be 100% sure you are achieving the results you expect and will not have any problems in the future. An improperly treated attic can be the source to a myriad of future problems, including energy waste, discomfort, moisture and mold, and ice damming in the roof just to name a few.
Thanks for the advice... still doing a bit of research on this.
As for the attic - I have attached a photo.
Awesome space! Full of possibilities! Good luck with your project and I commend you for doing your homework on this. :)
I just found a thread in this here forum with a lot of links and information you might want to look into:
As for the space... it was one of the deciding factors of the home purchase... that photo is only about 1/2 of the space I have to play with!!!
Lots of room up there - cant wait to get this all figured out and get the project started (that's after im done with the 1st floor gut! no worries... contractor is handling that - i don't have the time to tackle that one on my own).
Most shingle manufacturers will void or limit your warranty if there is no ventilation.
How old is your current roof, and what type of roof is it? if it is ready to be replaced you could install furring, new deck, and a new roof. This would allow you to insulate directly to the deck as the insulation would be between the new and old deck.
Roof is going to be replaced in about a year or 2... its your standard architectural
shingle application. Its newer but looks like its on its 2nd layer.
Thanks for the suggestion - i didnt even think about this... I was looking into furring on the inside to create more space - but didnt think about the oppisite.
How does one do that and channel the soffit venting to the new furred in vent space?
Again - thanks for that suggestion... this may be the way to go.
Good to hear this may work for you. You could also add rigid insulation (polyiso or styrofoam) on the outside for extra insulation. They sell a product which comes pre-assembled called 'ventilated nail base' http://www.atlasroofing.com/tabbed.php?section_url=51. However, you can just as easily make it yourself.
If you do not have current soffit venting, you could use a ventilated drip edge http://www.airvent.com/homeowner/pro...ventedDE.shtml
If you do have current soffits, I would block off from the inside and cut a couple inch wide strip out of the current sheathing above the existing soffit.
The only downside to this method is on your gable ends the fascia will be larger (which may look awkward, depending on what is existing)
As for the ridge, you could add a ridge vent or again cut a couple in wide strip to tie into your existing gable vents.
So here is what I have decided to do (i hope someone in the same situation finds this helpful). I have researched, researched and researched more...
Taking costs and overall effectiveness into consideration - here are my thoughts and what i plan to do.
Fiberglass has less r-value per inch than rigid foam. Spray Foam would require me to install the Durovents to the roof deck (extra cost) and to get to an appropiate R value - I would have to somehow extend out my rafters as they are 2x4's.
It's not recommended to either have the insulation in contact with the roof sheathing, or to compress fiberglass.
What is the solution you ask... Not sure if this is right... but this is the best I can come up with:
Rip pieces of 2" or 2.5" rigid foam to fit between the existing rafters
- Keep the foam flush with the lower edge of the rafters leaving an airspace @ the roof sheathing (removes the need for the durovents)
- Caulk and Tyvek-tape the seams (can even tyvek wrap the entire interior if you like - i personally think this is overkill.) I have also thought about spray foam to seal but cant figure a way to make it flush and the added cost, in my opinion, cant be justified.
Next, apply full sheets of 2" foam to the whole ceiling, using long screws and washers.
Now, being aware of where the rafters lie under the foam, chalk out lines 16" on center from the peak to the plate, perpendicular to the rafters, parallel to the ridge and side walls.
Next, using long screws into the old rafters, install 1" furring strips along these guide lines. To this you will attach the drywall or whatever finish.
Overall r value will be about 28, but possibly even better because of the 1" dead air space with the furring strips, and the fact that the ceiling is thermally isolated.
I plan on starting this sometime in Nov and if anyone is interested - I can post progress and photos if you like.
Seems to be similar to what people in this thread have done. If it were me I'd get the foam that comes molded with channels for furring strips.
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