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Too much Insulation??

4488 Views 8 Replies 4 Participants Last post by  Scuba_Dave
We are removing old x90 siding and replacing it with vinyl. Our walls are 2x8 construction. Most siding companies are telling us to add 1" foam board style insulation before putting on the siding. One company (been in business 35 yrs) says he would not recommend adding more insulation. (We are going to typar the house) He's afraid it would create such a tight home, that it wouldn't breathe properly and we could end up with moisture/vapor issues. The house was built in 1982.

I'd appreciate any info on whether we should spend the $5000 on adding the 1" insulation, or save it and put it towards new windows in a few years.
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I'm no expert, but quite honestly, if I was consulting someone who claimed to be an "expert" yet didn't know where potential vapor barrier / moisture issues might arise, I would steer clear of that person.

I would recommend you pickup up Taunton's "Insulate and Weatherize: Expert Advice from Start to Finish".

Education is key. Linky
Location is the difference. Read about location, rigid foam, and vapor barriers here:

Be safe, G
Too Much Insulation

The house was built in 1982 and has the pink fibreglass in the 2x8 walls. I've talked to a few other people who say the return on the investment when I already have 8 inch thick walls would be nominal and take years to pay for itself. Plus, as far as cutting down on street noise, I've been told by a few people that changing my older double-pane wood casement windows to triple argon would not only help with any heat loss, but also cut down on street noise more than 1" of foam-board insulation.

The guy who suggested not adding the insulation has been in the siding business for 35 years. He'd love to charge me more labor to put on insulation, but he said a home builder he works with advised him years ago not to waste money on additional insulation on 2x8 walls, unless it is a very old home with poor insulation to begin with. (2x4 or 2x6 definitely wouldn't hurt to add some.) Too much insulation could prevent the house from breathing properly and lead to moisture/vapor problems. Plus, he advised the return on the investment would take years to pay for itself.

I'm leaning towards not adding the insulation, and putting that money towards windows in a couple of years.

I'd appreciate anyone's input. thanks for the links. I'm going to ready them and see what it says.
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Too Much Insulation??? was a good read. It says if you are using extruded polystyrene insulation, use less than 1". Since it is a very good moisture barrier, using more than that could prevent vapor from inside the house during cold periods (when moisture moves from the warmer inside to the colder outside through openings in the walls) from escaping the house. Deduction leads me to believe that if the polystyrene is too thick, this vapor from the inside of house could get trapped behind it and lead to moisture/vapor problems in the wall.

I live in Saskatchewan Canada, where our winters do get very cold (moisture vapors travel from the warm inside to the colder outside through the walls). Since thicker walls take longer for vapor to travel through, I don't think I want to add anything that will keep the vapor inside the wall any longer than necessary since my walls are 2x8. Typar wrap should keep outside moisture out, but also allow inside vapors through.

Thoughts anyone???
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8" walls hopefully means 8" of insulation
But they could have stuck R13 in there
So you need to know the RValue of the existing insulation
The house is a bilevel (only 4 ft in the ground). When in my laundry room in the basement, the main floor floor joice is not drywalled and is above groundlevel. I removed the plastic moisture barrier and pulled out the pink fibreglass insulation. It was between 6 and 7 inches thick (2x8 wall means the wall studs are 7 inches wide, since lumber is never the actual size, ie: 2x4 is actually 1.5 by 3.5 in real measurement).

I'm also rebuilding the deck at the same time. The headerboard was lag-bolted to the house over the x90, so it is all being torn apart. While the x90 is off, before I put the new headerboard on, I'm going to remove a small area of the sheathing and check the insulation behind it.

Our home was built by a contractor for himself to live in, so I doubt if he would scrimp on the insulation, but better safe than sorry.

If the walls have 7 inches of pink fibreglass insulation, would I really need more on the outside???
2x6 walls have R19 = R18 - slightly compressed
2x8 could have the same (but R19)

2x8 options
R21= R21

I would say no, better to put the $$ towards new windows
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