insulating dormer knee walls

ravendude

Hi there,
Have posted about spray foam in the past for insulating the crawl space and ruled it out. Some options were advised but are non runners fir me on cost. Have looked at cellulose also but cant seem to get a supplier to do it for me (got the run around and cant have my time wasted on it any longer)

So we have currently got 75mm of fibre on the backs of the knee walls, and greater than the current regulations on the crawl space floor (ovrr the ceiling below)

Im thinking of putting an additional layer of foil blanket (knauf space blanket or similar) on the back of the knee walls,- basically covering the existing 75mm fibre wool, and stapling the foil stuff to the joists on the knee wall. I could leave some of the knee wall joist exposed. The benefit would be that it would be another layer and also protect a bit from the wind etc in the crawl space.
Condensation might be the concern, but its not the roof, just the knee walls.

Any thoughts?

ravendude

Should I be worried about condensation with this? that would be my main concern. ..

MicktheMan

Is there an air tightness barrier installed?

ravendude

Sorry about being so long in getting back on this, - have been offline for a while.

So, house is 15 years old and I would expect there is no airtightness barrier.
Its a "cold" roof. ie. no breathable felt and there is a gap along the eaves.

I'm more interested in a bit of wind tightness for the knee walls rather than full on air tightness per se. This would be a DIY job.
On the knee walls, there is is 75mm approx of fibreglass wool stuff packed in between the timbers.
At times of high wind, - there can be a fair breeze blowing through the crawl space. I'm thinking of covering the knee walls (including the timbers on the knee wall) with foil - like the stuff in the link below, - basically running it along the knee wall and stapling it to the knee wall timbers:
www.diy.com/nav/build/insulation/constructional-insulation/B-and-Q-Value-Aluminium-Foil-Loft-Insulation-12848829?skuId=13359535

I'm presuming that foil isn't breathable.
However, I plan to apply it a bit loosely so some air can still get in behind the foil to the knee wall timbers, - certainly not sealed with tape etc. The idea is more for windproofing than any kind of airtightness. My understanding is that the breeze in the crawl space makes the knee wall fibreglass wool insulation much less effective (by blowing out the warmth), so I'm thinking a cover/shield like this would help.

My question is, - whether this is a no-no in terms of condensation?
- or should I be OK if its applied a bit loosely and not taped airtight? There would certainly be less air though getting the the knee wall timbers than before.
ie. Should it be fine, - or is it a case of "just don't do it"

MicktheMan

If "a bit of wind tightness for the knee walls" is what you're after then I wouldn't bother. You will be introducing a potential risk of condensation into your roof structure without any reward, in that, any improvement in the insulation effectiveness will still be negated by heatloss anyway caused by the probable lack of airtightness.
You haven't said what problem you are trying to resolve so I will presume it is to reduce the heat loss through the dormer roof. In this case, get a proper survey done by an experienced independent professional to highlight the main heat loss mechanism(s) and outline potential solutions.

ravendude

Thanks. Might give it a miss so and look into getting a survey done.

I'm thinking of beefing up the fibre insulation on the knee walls anyway, - presume this would be same enough as its breathable?