Roofing,ventilation and insulation woes
Hello seasoned veterans and the like. As a Fine Home Building reader and a home owner I’ve learned a lot in the last few years about what makes a home work but I always seem to come across something I haven’t been able to find anything written on.
My question is on rafter ventilation and insulation. My home is an older one, built in 1872 it has a full cross gabled roof, there is no ventilation for the rafters as there are no eaves, each elevation rises to a gable the from the full width of the exterior wall. The rafters on the inside are covered w/ t&g pine. I’d like to finish the attic to expand my living area but would need to insulate the spaces between the rafters (I live in the Pittsburgh area) before adding heat to the space. I’d also like to leave the T&G in tact. My concern is that the addition of blown insulation will lead to heat and moisture problems w/ the sheathing and shingles. I’m looking for expert opinions, advice and any knowledge gained from experience. I see no practical way to vent the rafters and can’t fully utilize the finished space if I don’t insulate them.
I also had a query concerning roofing and radiant barriers. The manufacturers proclaim that the use of radiant barrier on the interior side of the sheathing will only mildly affect roofing material temperatures and a quality shingle shouldn’t be affected by it. I cannot access the underside of the sheathing to run a radiant barrier but was wondering if the radiant barrier could be laid under the roofing felt during the installation of a new roof? I reason that this would only mildly inhibit its performance as roofing nails will be driven through it but could think of any other potential side effects of using it in this manner…..
Thanks in advance for any advice you can offer……
Replies
welcome to BT! if I wasn/t so bust eating fried spaghetti, I 'd tell ya that ya need more info than I can tell ya..and welcome to the ride..this is a subject that can be re hashed by going to ADVANCED search hard upper left as you see the menu bar..don't even try the bottom one.
If you want to post a new qwuery about this subject, post it to the guru of all "AndyC "..he'll set ya right up.
Welcome ,,again.
Thanks for the welcome, I wasn't aware the site had a search feature. I'll be sure to give it some use......
The search feature is a bit like throwing darts. Some skill, some more luck, and sometimes it bounces back and hits an innocent bystander
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Hi RemoH,
Couple of facts first.
Radiant barriers do no good unless they have an air space in front of them. so there is little likelyhood that one can serve you in your application.
Second piece of info that is critical for your understanding of this situation - ventilation is not necessarily needed.
The key to doing away with the ventilation detail is that the insulation must be done in a way such that no dew points exist. A dew point is where warm moist air contacts a cooler surface and condensation appears.
Your roof design precludes any sort of effective ventilation scheme anyway because good venting requires that air flow in at bottom equivalent to air flowing out at top. Otherwise, you can create a negative draft that is more than likely to pull more wet warm air into the space from below.
So what you need is an insulation that has enough R-value to overcome the cold spots, and dense enough to prevent air flow through it. Plain FG battts are notorious for allowing convection currents within them so current venting models, codes, and assumptions, are often based on that fact. The insulation for your house would be one of three - sprayed or poured foam in place, BIBBS ( blown in blanket systems - chopped fibreglas), or Denspacked Cellulose.
The final choice comes down to who and how it can be installed and at what cost. Foam is best, but I have my doubts that you can do it well without removing the T&G. If as implied, you are going to re-roof, any of these could be done from above when openning things up. The Cellulose choice would probably be the least expensive for several reasons. one is that there are more guys doing it than any other so availability is a factor, depending on where you live. Two drawbacks with cells is that I imagine some of the dust cellulose fibres finding a way to drift thru the T&G into the living space, mostly in that first week. The other is that if moisture does enter the cells it is slow to dry back out again, and has been known to encourage rot. The back side of this point is that if you have conditions that get cells weet, you have other problems with the house anyway.
have fun and good luck.
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
Well that makes more sense than the Steelers play calling did today!
Many thanks for the informative reply. I'll probably go DPC, its feasible and I know where to find some folks who can do it. I don't want to attempt any effort to remove the T&G, I doubt it would come down without sustaining some damage and I can't replace the character of 150+ year old quarter sawn pine, as far as pine goes its about as pretty as it gets.
Thanks again......