Renovation - Spray foam or Batt Insulation! Advice please! (3 Viewers)

[FullMonte]

He said it’s a wall that has the sun hitting it in the afternoon. If he has a standard 2x4 wall with fiberglass it’s only giving him an R value of slightly over 6. If he has 3 inches of foam applied it’s almost an R value of 20. That’s a huge difference on a wall the sun beats down on. That part will be noticeably warmer due to that. Also, if he’s not upsizing his A/C unit, there is that to think of also and a foam insulated room costs very little in additional AC. There really is no substitute for foam, and certainly if the price is the same. It will be 3x the insulation R value per inch, and the quietness is very different in a foam house.

I learned something about that when we built our house. We used spray foam insulation, and when the A/C guys were installing everything, they asked us how we wanted the vents pointed. I asked him what he recommended, and he said that normally, he recommends pointing them towards the walls because the walls get heated by the sun, and if they are pointed towards the room, it will take longer to cool because of the heat absorbed by the wall. He said that because we were using the spray foam insulation, it made less of a difference because the interior walls wouldn't be as warm.

One thing I'd add about the spray foam/traditional insulation debate. While it isn't relevant to the OP because he was only looking to do the attic....Something that never gets mentioned in the debate is critters. When we had spray foam insulation, we had almost no non-flying critters in the house. We saw no spiders, scorpions, etc. Since the foam seals everything up, there are no pathways for the bugs to get in.

 

[dtc]

He said it’s a wall that has the sun hitting it in the afternoon. If he has a standard 2x4 wall with fiberglass it’s only giving him an R value of slightly over 6. If he has 3 inches of foam applied it’s almost an R value of 20. That’s a huge difference on a wall the sun beats down on. That part will be noticeably warmer due to that. Also, if he’s not upsizing his A/C unit, there is that to think of also and a foam insulated room costs very little in additional AC. There really is no substitute for foam, and certainly if the price is the same. It will be 3x the insulation R value per inch, and the quietness is very different in a foam house.

Where I live and work the code requires r13 minimum in the wall. R13 is standard insulation batt in a 4" stud wall. You can increase it by adding rigid foam to the outside or inside, but R6 is incorrect in a 4" space.


Oh, and this is factually incorrect:

If he has a standard 2x4 wall with fiberglass it’s only giving him an R value of slightly over 6.

If you wanted to increase that one space you could do it with a higher R product and still not need foam or you could use the foam.

If you're worried about the radiant heat from the exterior due to sun exposure, you could also add a radiant barrier by using RB sheathing on the exterior wall.

Also, you said "if he's not upsizing his AC unit" and I don't understand what you're talking about.

You can't just add on 30% to a house and not increase or add ac. Building code won't allow it unless you're in some place where they don't have codes. Further, if you over insulate and then upsize your AC you're likely to cause all sorts of problems with excess moisture and short-cycling your ac.

Bigger AC units and more insulation are not sound building science in a vacuum.

 

[dtc]

I learned something about that when we built our house. We used spray foam insulation, and when the A/C guys were installing everything, they asked us how we wanted the vents pointed. I asked him what he recommended, and he said that normally, he recommends pointing them towards the walls because the walls get heated by the sun, and if they are pointed towards the room, it will take longer to cool because of the heat absorbed by the wall. He said that because we were using the spray foam insulation, it made less of a difference because the interior walls wouldn't be as warm.

One thing I'd add about the spray foam/traditional insulation debate. While it isn't relevant to the OP because he was only looking to do the attic....Something that never gets mentioned in the debate is critters. When we had spray foam insulation, we had almost no non-flying critters in the house. We saw no spiders, scorpions, etc. Since the foam seals everything up, there are no pathways for the bugs to get in.

Excellent point.

The contrary point is there is also such a tight envelope that without a make-up air system you end up with a potential air quality problem. Bugs can't get in. Moisture can't get in or escape. If your ac isn't running long enough you can build up moisture and feel uncomfortable at 70.

Every time you spray cleaner or other chemical in your house, it stays there unless it's flushed out naturally or mechanically. Over time, if you are not cleaning the air in a very tight house it will build up toxic residue from off-gasing building materials or cleaning products.

We build SIP houses as often as our clients will let us. They are as tight as tight can get and without makeup air and filtration it's a recipe for problems.

 

[Maxp]

My house is spray foam on all exterior walls attic ceiling and floors. My bedroom is the hottest room in the house. The ceiling of my bedroom is not insulated because the attic above it has the foam. Would it help to put down insulation between the studs of my bedroom ceiling? I have pretty easy access to that space.

 

[dtc]

My house is spray foam on all exterior walls attic ceiling and floors. My bedroom is the hottest room in the house. The ceiling of my bedroom is not insulated because the attic above it has the foam. Would it help to put down insulation between the studs of my bedroom ceiling? I have pretty easy access to that space.

Shouldn't make any difference and could cause problems.

Buy one of these and use it to determine if your ceiling is hotter than the rest. If it is, then it's likely because the ducts are not balanced because all the attic should be the same temp if foam is done right.

Amazon product ASIN B00837ZGRY

 

[ArchiesGhost]

What climate zone is this project in? This is an important consideration.

Its sounds like you are planning to spray foam the roof structure and make a conditioned attic. This can be a great thing or a very bad thing.

Spray foam applied directly to the underside of the roof sheathing can lead to disaster.

 

[Maxp]

Shouldn't make any difference and could cause problems.

Buy one of these and use it to determine if your ceiling is hotter than the rest. If it is, then it's likely because the ducts are not balanced because all the attic should be the same temp if foam is done right.

Amazon product ASIN B00837ZGRY

I bought that exact laser thermometer on black Friday. The main exterior wall in our bedroom is facing South. I'll shoot all the ceilings. When I get home

 

[dtc]

I bought that exact laser thermometer on black Friday. The main exterior wall in our bedroom is facing South. I'll shoot all the ceilings. When I get home

I've had one of those laser things for about 20 years. The first one I bought cost over 200 and was worth it's weight in gold for finding leaks and temp problems. I use it on the diesels in my boat every time I run it just to see if something is hotter than it should be. Saved me thousands over the years, but for this there's no better tool.

If your ceiling is hotter in one room than the other with foamed attic, then you probably have a duct problem. Restriction somewhere I bet.

 

[zeetes]

what the fork? this a serious thread? blasphemy!