Bonus Room Above Garage Insulation – SE Michigan
I know this has been talked about a lot but my bonus room above the garage is freezing in the winter especially the floor. The room has knee walls also. I am looking to get the room spray foamed and have had 2 guys come out with 2 different options and its a bit confusing. Options are below:
Current status – I ripped out the garage ceiling dry wall and pulled all the insulation down. There was differently a gap between insulation and the floor in spots and just not insulated good in my opinion.
Option 1
Garage ceiling (bonus room floor) – Open cell spray foam about 6- 8in. He wanted do to open cell to get the R value and with the thickness its also an air barrier.
Knee wall – Closed cell
Option 2
Garage ceiling (bonus room floor) – closed cell 2 in and then I would fill the rest in with Roxul to achieve the proper R value. Is adding Roxul correct?
Knee wall – I would leave the faced batt insulation and then put proper sheathing behind it. Right now there is no sheathing behind it. What is proper sheathing for a knee wall? Pink foam board or foil faced foam board? I would then tape all seams and air seal cracks with great stuff.
Cost is a big factor as I am still waiting for the quote for Option 1. But as far as the foam goes what would you do for the floor/garage ceiling? Open or Closed Cell? Closed cell your only going to get 2in of foam…is that enough? Thats why I was thinking I would have to add Roxul also.
I was also going to add blocking in the joists where the knee wall is to block/prevent air flow from the attic area of the knee wall.
This is kind of confusing because alot of stuff I read says something different…etc. Any input would be great. Thanks
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Replies
S.W.,
Either approach can work. If you decide to simply add an exterior air barrier on your kneewalls, it's important that this layer be installed with attention to airtightness. Some type of rigid foam is the best material for this purpose (because it provides R-value as well as the opportunity to create an airtight layer).
In general, open-cell spray foam usually has a more environmentally benign blowing agent than closed-cell spray foam (and it is therefore usually preferred by green builders). For more information on blowing agents for spray foam, see this article: Next Generation Spray Foams Trickle into the Market.
Whatever you decide to do, remember that air sealing efforts are at least as important as new insulation.
For more information on these issues, see these articles:
How to Insulate a Cold Floor
Two ways to insulate attic kneewalls
Great Thanks! Yes I will definitely be adding solid blocking beneath the knee wall, as there is non currently which was probably one of the main culprits for a cold floor in the winter. I will go with rigid foam board behind the knee wall to seal off the attic area and seal up any gaps and seams. Great articles. It will definitely be cheaper to add the rigid foam board then get the knee walls spray foamed.
I had a situation very similar to yours, similar climate. Cold air flowing directly underneath the floor boards is definitely a main culprit. Blocking is a must.
Have you considered dense pack cellulose? It's greener and could be cheaper. Cellulose will fully fill all cavities and the improvement in my case (packed 2x10 joists) is night and day better. With access from the garage below, it should also be pretty straightforward. Also consider adding a layer of rigid foam under the joists before putting the drywall back up.
How are the sloped ceiling and upper attic above the bonus room insulated? Those aspects are harder to retrofit with insulation. It's possible though.
For rigid foam, go with foil faced polyiso. Use a 3in or 4in drywall taping knife to cut through the foam. The knife will become your best friend. Martin likes cap nails to hang the foam. I find that screws with the plastic cap washers (buy a big box of cap washers on Amazon and make your own cap screws) are easier for novices / non-roofers and are forgiving if you want to take foam off and re-do.
The best approach would be to spray open cell onto the garage ceiling/bonus room floor. 6" would be good because that gives efficient thermal resistance while giving an air seal and sound dampening.
If the knee wall areas are also sprayed with open cell you get the same benefits. The air seal is the most important.
- RetroFoam of Michigan
S.W.,
It certainly looks like the last piece of advice posted on this thread (Comment #4) came from a spray foam contractor, so keep that fact in mind when assessing the value of the advice.