batts only for zone 3 ventilated attic? r60 fiberglass vs r49 with 2x8s?
foremost, i know that this is not cost effective in the slightest in comparison to blown in. i have a borderline irrational dislike for blown in insulation and will be living here for a decade ++ unless a magical windfall happens.
i have a ventilated attic that is comprised of 2×8 floor joists. to help with wind washing i have installed plastic baffles in every single bay and completely covered the base in spray foam. i also have zero hvac equipment in the attic.
the issue is the 2x8s;
if i go with two layers of r30 fiberglass, the base layer will stick out about 2″. i am not sure if the weight of the perpendicular layer will compress the base layer enough make contact with the joist. how important is that for thermal bridging? according to the internet, r30 fiberglass compressed into a 2×8 still has an rvalue of r25. so not bad.
if i go with r19 base + r30 top layer, then the top layer would make contact with the joist, but there will be about a 1″ air gap between the two layers in between the joist.
what would be the lesser of two evils?
alternatively, r30 mineral wool fits in a 2×8 bay perfectly flush. would there be any performance issues with stacking fiber glass on top of mineral wool?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
You can get R25 fiberglass batts, which are about 8" thick, and would probably squish into a 2x8 cavity pretty well with little effort. I would not try to stuff 2" into the cavity, especially since you won't have anything rigid to hold them in place -- the upper layer of fiberglass batts would likely just "ride over" the top, leaving gaps that could carry air currents -- and provide mice with a nice little freeway system.
If you use R25 fiberglass batts in the cavities, and R30 over the top, you end up with R55 which is pretty close to your R60 target value. You can use mineral wool in the cavities with fiberglass over the top with no performance hit too, the R values in that case would just add in the usual way. You can mix and match different types of batts without issue. The only downside to using mineral wool in the way you describe would be the increased cost to purchase the material.
Bill
dunno how i missed the r-25 fiberglass. :)
i don't really have any end goal for an r-value, but anything is better than what i have now. an unhealthy portion of the attic does not have insulation covering the floor joists.. flush at best.
r-25 fiberglass is a bit over $1000 cheaper than r-30 mineral wool for the coverage i need. so that is a tough call.
but as far as the top layer goes; the pricing per square foot per r-value is not consistent between products. e.g, 24 inch wide rolls might be way more expensive per square foot compared to 16 inch wide batts. just a generic example, but i will have to make a spreadsheet to figure out the most cost effective solution for the top layer.
thanks for the input
I usually just work out the cost per square foot for things like this, then go with the cheapest option for materials while also considering how much will be wasted, and the additional work to install more "passes" of the narrower batts. Between joists though, you pretty much just have to use whatever fits, so you have fewer options there.
I'd probably not spend the extra money for mineral wool here (and I like mineral wool). You could run two layers of R19 rolls over the top of the joists instead of one layer of R30 batts, which might be similar in cost depending on your material prices -- unfaced R19 rolls are usually some of the cheapest insulating you can get next to loose fill.
Bill
so i didn't go into complete detail on my reasoning, but a main reason is that i want to have an easier cat walk for low voltage junk in the attic and because one portion of the house has 8' ceilings and the other is 10'. there are some areas of the attic that blown in insulation will be fine.
if i had soffit baffles extending up further than i do now -- only have one per bay, would it be unwise to take a lot of the blown in insulation that is above the 10' ceilings and move it into the 8' part of the attic as seen in the photo?
i will have enough insulation to where the insulation could be the same elevation as the 10' ceiling insulation.
but with that, there would be some insulation coming in contact with the roof decking, so i'd need to add in more baffles which is no big deal.
ended up buying an r-30 fiberglass roll to give it a try, the manufacture says its r-27 when compressed into a 2x8 bay.
they were a bit cheaper than r-25 batts, which were also all close to a month lead time due to being a special order.
seems okay to install, definitely prefer mineral wool.
in either case, i'm going from an ~r19 ceiling to r50+. so i suppose anything would be an improvement.
You could do the R25 that fits in a 2x8 cavity and just cross them with R38 above? My understanding is the top layer is just lying on top and not in a stud cavity, so you can fit whatever you want there.
yeah dunno how i missed r-25 fiberglass
its over $1000 cheaper than r-30 mineral wool for what i need.
tough decision, though.
cs55,
Make a catwalk for future access.
yes sir, thats also part of the reason for wanting batts. i have a lot of low voltage cabling and other junk in the attic.
half of the house has 8' ceilings, the other half has 10'. this makes blown in + cat walks a little more difficult. but going with batts will make that easier.
2 months later i am now making progress. i built around 64' worth of r-20 catwalk by cutting r-10 foam board into 2x4 pieces, glued them together, then screwed a 2x4 osb to the top to make it easier to carry into attic.
the previous catwalk was about 4' wide, but i will be reducing that to 2' because theres no more hvac equipment in the attic. but there was previously well over 100 sqft of walking space that was only insulated with about r19.
the 2'x4' sizing works out great because i can easily throw an r-30 fiberglass batt of the same size over them as i walk out.