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NickL
Insulation adice on a cape built in 1943
NickL

I am insulating a cape built in 1943. When originally built ,I do not think the second floor was finished. So the exterior walls and roof rafters were insulated. in the mid 80's the second floor was finished(I being kind). Some or the rafter insulation was loosened from the rafters and layed on the new ceiling of the second floor. Its old wool insulation which has disintegrated. I removed all the roof rafter insulation I can get to. But there is a 4ft area behind the angled portion of the second floor ceiling where the old insulation remains. I put R13 on all the vertical knee walls and plan to blow in R49 above the rooms as well as behind the knee walls. I have 2 questions . 1- Should I install provent attic vents to keep air flowing from behind the knee wall area to the upper ceiling. There is wool insulation there, but very thin. there are no soffet vents. 2- Is there anything else I should do, without removing the sheetrock on the angled ceiling. There are 2 14x16 vents on either side of the attic. and i plan on adding a static roof vent in the roof area behind the knee wall if necessary. I appreciate any advice you can give..

keith3267
Re: Insulation adice on a cape built in 1943
keith3267

A lot depends on some construction details and the scope of your plans. Do you plan on removing the sheetrock from the angled ceiling? from the flat ceiling? What are the dimensions of the rafters? Do you have dormers?

You will do the best job if you do remove the sheetrock from the angled ceiling. It is doable without removing the sheetrock though.

Personally I would not have insulated the kneewalls but that is how it is most commonly done. My preference would be faced batts between the rafters, stapled to the edges of the raters and not the inside faces from the soffit to the flat ceilings. I would use batts that are 2" less than the width of the rafters, i.e 6" batts for 2x8 rafters or 3.5" batts for 2x6 rafters.

With the wings (flanges) of the batts stapled to the edges of the rafters, it will hold the batts away from the underside of the roof decking and air will be able to flow from the soffits to the attic. The advantage of this is that you can use the kneewall for storage, the disadvantage is that you will need a continuous soffit vent, or a soffit vent for each cavity as you may not have a continuous soffit cavity. If you do have a soffit cavity, then you don't need as many separate vents.

When you insulate the kneewall cavity the normal way, the kneewall cavity will distribute air to each rafter cavity in the slanted ceiling area. Here is a website showing both types of kneewall insulation.

http://www.finehomebuilding.com/how-to/departments/energy-smart-details/two-ways-to-insulate-attic-kneewalls.aspx

If you have dormers, you do insulate the walls and ceiling like a typical house.

If your rafters are 2x6 or smaller, you will have another problem if you live in cold or snow country. The r-value through a 2x6 pine rafter is about 5.5. That is a lot less than the insulation you will put in the cavity. In winter you will see snow melt or frost melt lines on the roof where each rafter is located under the roof in the slanted ceiling portion. Even 2x8 rafters can and will do this. One solution is to use inch thick 4x8 foam panels under the rafters. Put up the foam panels and then the sheetrock. You will need the longer sheet rock screws, preferably 2.5" screws. I would not use nails for this. Use closed cell foam. In this case, if you use faced batts, cut some slits in the faced material so they can breath and tape all the seams of the foam panes.

Now for venting. You will need soffit vents if you insulate the kneewall cavity my way. If you do it the traditional way, the way you are planning, you can use small gabler type vents at each end of the kneewall cavity. If you have two or more dormers, you will still need soffit vents between the dormers.

The gable end vents in the attic are fine as long as you have at least one square foot of vent for every 300 sq ft of attic floor. A 1/150 ratio would be better but 1/300 is the minimum. Most roofers today are recommending ridge vents in place of the gable vents and in theory that seem better. I am starting to see a lot of issues people posting here are having with the ridge vents now so I am not so sure. I had my roof reshingled last year and I had a ridge vent installed but if I were doing it today, I think I would have skipped the ridge vent.

Generally attic insulation is put in without a vapor barrier because attics are very well ventilated. Walls on the other hand are not well ventilated and so a vapor barrier is usually needed. A vapor barrier will not hurt, it just isn't necessary like in a wall.

NickL
Re: Insulation adice on a cape built in 1943
NickL

Thanks, Since I already started. I think i will continue have to continue with my current plan, in order not to redue work. Not as good as the way you suggested but I'll continue and see how it works out. I'll add the gable vents as you suggested.
Thanks Again

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