How to get started with attic insulation?

johnnyboy08

Hi all,

I live in a dormer built in 2000-2001 and as with most older dormers it leaks warmth for sport in the upstairs rooms.

The main issue is cold air coming in through the soffit vents and I think is getting in between the downstairs celing and upstairs floor.

The attic and crawl space has had blown insulation installed in the past and the back of the vertical stud parallel to the wall plate and dormer roof are insulated with 100mm foil back rigid board but I'm not convinced that they're up to scratch especially the blown insulation glass wool.

I'm looking for some advice on best approaches. I was thinking of having thermal imaging survey done for starters just to see what the state of play with the whole house is.

For the attic I think options are:
1. at least 100mm spray foam between rafters, all the way from wall plate to top of roof. That would stop air infiltration from the soffit vents penetrating the attic space, allowing the air to move through the 50mm gap left between the felt and the top of the foam to remove condensation. I've no idea how the dormer part of the roof would be done though and what the u-value would be comparable to other options

2. Re-lay at least 300mm of rock/glass/mineral wool or PIR in attic and crawl space floors between the joists. I'd still have to seal the gap between every single joist in the roof between attic floor and downstairs ceiling and I'm not sure how viable that is in a small crawl space.

3. Install PIR between the rafters which would mean a warm attic space (attic is only used for christmas decoration storage). Again would have seal at wall plate with some kind of material. I presume 50mm gap would still be needed between top of PIR and roof felt? How would PIR compare to spray foam for u-value?

Apologies for lack of building/roofing terminology, I'm just trying to get a handle on the options at the minute!

Thanks.

MicktheMan

First port of call would be to have a full heat loss survey done on the house. This survey will use, in addition to thermal imaging, air tightness testing, which in the case of dormers is the no.1 reason for heat loss. After the survey, you will be in no doubt about where the issues are and what will / won't fix the issues. Proper advise on ventilation should also be a part of the survey.

Remember air tightness addresses convection heat losses whereas insulation addresses conductive heat loss. This is why, eventhough houses can be very well insulated, they suffer rapid heat loss due to draughts (air leakage) so messing about with the "insulation" will have little effect.

johnnyboy08

MicktheMan wrote: »

First port of call would be to have a full heat loss survey done on the house. This survey will use, in addition to thermal imaging, air tightness testing, which in the case of dormers is the no.1 reason for heat loss. After the survey, you will be in no doubt about where the issues are and what will / won't fix the issues. Proper advise on ventilation should also be a part of the survey.

Remember air tightness addresses convection heat losses whereas insulation addresses conductive heat loss. This is why, eventhough houses can be very well insulated, they suffer rapid heat loss due to draughts (air leakage) so messing about with the "insulation" will have little effect.

Thanks for this information Mick. It makes sense to tackle the main cause of the heat loss first before preparing for additional insulation. I've no doubt at all that the air tightness test will find significant air penetration from the crawl spaces.

Once verified what is the usual port of call after that? Does a builder or energy company who specialise in air tightness come on board? Finally, can sealing works usually be done within the crawl space or is more major work like removing the studs required?

MicktheMan

johnnyboy08 wrote: »

1. Once verified what is the usual port of call after that?
2. Does a builder or energy company who specialise in air tightness come on board?
3. Finally, can sealing works usually be done within the crawl space or is more major work like removing the studs required?

The following is based on achieving a reasonable / noticeable improvement in air tightness only. If passive or close to levels are required then a different approach would be needed.

1. Identify / tackle the major issues first (biggest bang for buck) and understand that perhaps not all issues can be economically addressed.
2. Either a tradesman who a) has experienced the air tightness test, and b) pays attention to detail or often the home owner with basic diy skills
3. Depends very much on access in crawl space but generally there should be no need to remove studs etc.

johnnyboy08

MicktheMan wrote: »

The following is based on achieving a reasonable / noticeable improvement in air tightness only. If passive or close to levels are required then a different approach would be needed.

1. Identify / tackle the major issues first (biggest bang for buck) and understand that perhaps not all issues can be economically addressed.
2. Either a tradesman who a) has experienced the air tightness test, and b) pays attention to detail or often the home owner with basic diy skills
3. Depends very much on access in crawl space but generally there should be no need to remove studs etc.

Going biggest bang for buck is exactly the plan. Sealing the dormer in itself will be a big win, the draughts that come in there on a windy evening can be felt coming in under the skirting boards and across the floor surface. After that it'll replacing the existing blown insulation with something better.

Thanks for the information again. Have an air tightness test booked in for October.