Insulating Loft Rafters

pavb2

Hi All

I'm thinking about insulating in between the roof rafters in our loft, it is a traditional pitched roof, four sloping sides.

So what material should I use and size I was thinking of Kingspan sheets rather than rockwool.

How do I retain the material within the joists or do I just make a tight fit.

I was thinking of measuring each individual rafter, cutting the material to size in a workshop and then fitting.

Any help much appreciated.

I dont know about the kingspan but normally you would fill the gap between up to the top and then do another layer perpendicular to the to cover the tops of the joists.
I think 300mm is the recommended amount now.
Need to do a bit myself soon

Lumen

I'm no expert on this, but in my experience use a product that makes your life as easy as possible, e.g. friction-fitting batts in the correct size for your rafter spacing.

I spent a day cutting dense woodfibre boards (free leftovers from another project) to millimetric precision for two-thirds of a small sloped roof, and after I ran out I bought some Gutex Thermoflex and friction fitted them straight out of the wrapping in about 5 minutes.

The cutting is harder than it looks because the distances between rafters tend to wander a bit over the length.

You can get a dense rockwool-type product similar to the Thermoflex (it's not the same as the stuff that goes on attic floors!), I just like woodfibre even though it's expensive.

Also, make sure you leave 50mm above the insulation for the rafters to breathe, particularly if you're using non-breathable insulation like Kingspan.

10-10-20

I have to agree. The rigid boards are not suited to be tight-fitted between rafters especially if there is any variation of the width over the length. Plus your options to seal gaps is limited. I'd stick to rockwool flexi batts or rolls.
If you were to go with the boards, you would also need supporting battons to maintain the gap between the insulation and the roof membrane.

Lumen

Oh btw, you haven't said why you're insulating the rafters.

Is it a heated space? If not then you're better off concentrating on insulating the floor and making the ceilings air tight.

pavb2

Just trying to keep the heat in, it is an old stone old cottage 100 years old. The attic isn’t a heated space. The loft floor is insulated and boarded out. The ceilings below the loft could do with taking down, plaster board fitted and plastering but this is a massive dirty job and one I’m trying to avoid.

FlipperThePriest

You mention both joists and rafters, which caused a bit of confusion about what you're trying to do. Joists are above the ceiling, rafters are above the attic space, at the roof. You don't want to insulate at the rafters unless your attic is a living space - in the thermal envelope of the house and heated.

I don't think yours is. So it would be pointless insulating at the roof, because you will want the attic ventilated at the gable ends and eaves. So to have air circulating through the attic, and then insulating above it doesn't make sense. Insulate above the ceiling, between the joists and above them. If the attic floor is boarded, was this done directly on top of joists and just for easy access in the attic and for storage? If so, you don't really know what kind of insulation is under the boards, if any, and how old it is?

For good insulation, you'd probably at least need to remove the flooring and check what's there. If it's fine, add a second layer perpendicular to the joists and first layer of insulation, bringing the total insulation up to 300mm. I would also advise improving the airtightness at ceiling level.. any entry points, such as electrical cables, tv cables, lights... a little bit of silicone to stop air being sucked out through these holes. If you have recessed lights, they need covered and sealed using proper covers.

pavb2

Thanks Zippy and others, no the attic isn't a living space.

I'm probably going to go back to basics, lift the attic floor boards as you suggested and put more insulation in there, the existing is about 20 years old.

My thinking is this would have the same effect as removing the existing wooden ceilings and fitting plaster board which as stated I don't really want to do.

FlipperThePriest

I wouldn't know anything about ceilings, but it sounds like a big job! I only know a bit about insulating the attic and the importance of improving airtightness just from reading about it online.

If you do plan on ripping up the attic floor, that would defo be the time to tackle airtightness, before burying an easily improved problem under a foot of insulation. Just make a note of every entry point from below (hotpress pipes, lights, alarm sensors, etc) and go around above with a silicone gun. Attic hatch should also be draught stripped and a bit of rigid insulation glued to the back of it or something.

The insulation in our place was fairly old, probably 50mm max, none at all in places. I didn't bother to remove it, just added 100mm along the joists on top of it, then 200 mm across them.

The more you insulate and improve airtightness, the more you then need to worry about ventilation above and below the insulation. You're effectively trying to minimize air exchange so that the air you've trapped retains heat (and moisture as a result). This wasn't a problem in older leaky houses, that didn't bother with either insulation or airtightness. So like I did, you may have little or no attic or room ventilation... this will have to be addressed if so. You'll also have to insulate water pipes and tanks in the attic.

Google and Youtube is your friend. I noticed a big difference though, well worth it.